摘要:

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

ISSN:0300-9599    

DOI:10.1039/F198884FX037

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications. Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers.In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987).A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications.Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)

ISSN:0300-9599    

DOI:10.1039/F198884BX039

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

Contents 3 1 13 Extraframework Aluminium in Steam- and SiC1,-dealuminated Y Zeolite. A 27Al and 29Si Nuclear Magnetic Resonance Study J. Sanz, V. FornCs and A. Corma Acidic Properties of Vanadium Oxide on Titania H. Miyata, K. Fujii and T. Ono Influence of Organic Solutes on the Self-diffusion of Water as studied by Nuclear Magnetic Resonance Spectroscopy P-0. Eriksson, G. Lindblom, E. E. Burnell and G. J. T. Tiddy Excess Enthalpies and Cross-term Second Virial Coefficients for Mixtures containing Water Vapour C. J. Wormald and N. M. Lancaster Excess Molar Enthalpies of (xH,O + (1 - x)C,H,,}(g) up to 698.2 K and 14.0 MPa N. M. Lancaster and C. J. Wormald Nature of the 8-Phase of Bismuth Molybdate M. M. El Jamal, M. Forissier and A. Auroux Interactions between Metal Cations and the Ionophore Lasalocid.Part 5.-A Potentiometric, Polarographic and Electron Spin Resonance Study of Cop- per(I1)-Laslocid Equilibria in Methanol P. Laubry, G. Mousset, P. Martinet, M. Tissier, C. Tissier and J. Juillard 3121 3129 3 141 3159 3169 3 175Contents 3 1 13 Extraframework Aluminium in Steam- and SiC1,-dealuminated Y Zeolite. A 27Al and 29Si Nuclear Magnetic Resonance Study J. Sanz, V. FornCs and A. Corma Acidic Properties of Vanadium Oxide on Titania H. Miyata, K. Fujii and T. Ono Influence of Organic Solutes on the Self-diffusion of Water as studied by Nuclear Magnetic Resonance Spectroscopy P-0. Eriksson, G. Lindblom, E. E. Burnell and G. J. T. Tiddy Excess Enthalpies and Cross-term Second Virial Coefficients for Mixtures containing Water Vapour C.J. Wormald and N. M. Lancaster Excess Molar Enthalpies of (xH,O + (1 - x)C,H,,}(g) up to 698.2 K and 14.0 MPa N. M. Lancaster and C. J. Wormald Nature of the 8-Phase of Bismuth Molybdate M. M. El Jamal, M. Forissier and A. Auroux Interactions between Metal Cations and the Ionophore Lasalocid. Part 5.-A Potentiometric, Polarographic and Electron Spin Resonance Study of Cop- per(I1)-Laslocid Equilibria in Methanol P. Laubry, G. Mousset, P. Martinet, M. Tissier, C. Tissier and J. Juillard 3121 3129 3 141 3159 3169 3 175ISSN 0300-9599 JCFTAR 84(10) 31 87-3647 (1 988) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 3187 3207 3215 3223 3233 3243 3249 3257 3263 3267 3275 3279 3293 3307 3319 3331 3341 3347 3359 ~~ ~ ~~ CONTENTS The first 19 papers of this issue were given at the 15th Annual International E.S.R.Conference at Cardiff on 21st-25th March 1988, including the third Bruker Lecture by Professor H. Fisher. Kinetic Electron Spin Resonance Spectroscopy of Complex Reaction Systems. Solvent Effects on the Self-termination of 2,6-Di-t-butylphenoxyl Radicals D. Ruegge and H. Fischer Crystalline Heterocyclic Radical Cation Salts as Stable Intermediates of New Redox-mediating Systems A. Schulz, W. Kaim and H.-D. Hausen Electron Spin Resonance Study of Azoalkane and Imine Radical Cations C. J. Rhodes Substituent Effects in Anthrasemiquinones Electron Spin Resonance Studies of Cycles and Bicycles F. MacCorquodale and J. C. Walton Is Gram-negative Shock a Free-radical-mediated Condition? S.K. Jackson, J. M. Stark, C. C. Rowlands and J. C. Evans Radicals formed by Ultraviolet Irradiation of Substituted 4-Chlorophenols J. C. Evans, C. C. Rowlands, L. A. Turkson and M. D. Barratt Room- temperature Powder ENDOR Spectra of /3-Protons in Some Organic n-Radicals N. M. Atherton and C. E. Oliver Electron Spin Resonance and ENDOR Study of the Photochemical Decomposition of Substituted Quinoxaline Bis-N-oxides T. C. Christidis and F. W. Heineken Electron Spin Resonance and ENDOR Reinvestigation of Iminoxyl Radicals from 1-Halogenofluorenone Oximes Two-dimensional ENDOR Imaging based on Differences in Oxygen Concentration Electron Spin Resonance and ENDOR Investigation of the Ion Pairs of 4,4'-Dicyanobenzophenone Ketyl with Alkali-metal Cations M.Barzaghi, A. Gamba, C. Oliva, M. Branca and A. Saba ENDOR Studies of Flavins and Flavoproteins H. Kurreck, N. H. Bretz, N. Helle, N. Henzel and E. Weilbacher Electron Spin Resonance Studies of the Reactions of Aluminium Atoms with Some Alkenes in a Rotating Cryostat J. A. Howard, H. A. Joly, B. Mile, M. Histed and H. Morris Generation and Reactions of the Chlorine Atom in Aqueous Solution B. C. Gilbert, J. K. Stell, W. J. Peet and K. J. Radford Magnetic Resonance Reinvestigation of Mn"-S'ATP Equilibria in Solution C. Rossi, F. Laschi, R. Pogni, E. Tiezzi, R. Basosi and L. Pogliani Electron Spin Resonance and ENDOR Spectra of Radicals formed by the addition of Superoxide Ions to Dimethylformamide P. J. Boon, M. T. Olm and M. C. R. Symons Spin Trapping with Thiocarbonyl Compounds A.Alberti, H. Benaglia, B. F. Bonini and G. F. Pedulli Electron Spin Resonance Identification of Irradiated Strawberries J. J. Raffi, J.-P. L. Agnel, L. A. Buscarlet and C. C. Martin J. A. Pedersen B. Kirste, K. Grothe and H. Kurreck Y. Kotake, U. M. Oehler and E. G. Janzen3363 3377 3389 340 1 341 3 3423 3435 3445 3459 3475 3487 3501 351 1 3517 3529 3539 3547 3567 3575 3587 3597 Contents Activity Measurements and Spectroscopic Studies on the Catalytic Oxidation of Toulene over Vanadium Oxides supported on Silica-Alumina COK84 B. Jonson, B. Rebenstorf, R. Larsson and S. L. T. Andersson ' Crystal-like ' Structure of Colloidal Spheres in an Alternating Electric Field T. Okubo Brarnsted Relationships for Heterogeneous Proton Transfer at Electrode Interfaces B.E. Conway and D. P. Wilkinson Electron-addition and Electron-loss Pathways for Cyanoalkanes H. Chandra and M. C. R. Symons Kinetic Aspects of the Bray-Liebhafsky Oscillatory Reaction S. Anid. and L. Z. Kolar-Anid. Aspects of the Oxidation of Naphthazarin as studied by Pulse Radiolysis T. Mukherjee, E. J. Land, A. J. Swallow, J. M. Bruce, P. C. Beaumont and B. J. Parsons Enthalpies of Interaction of Some Alkali-metal Halide Salts with Formamide in Water at 25 "C Surface-enhanced Resonance Raman Spectroscopy of Iron(I1) and Ruthenium(I1) Bipyridyl Complexes adsorbed on Silver Sols T. J. Dines and R. D. Peacock Spectroscopic Studies of the Solvation of N,N-Dimethyl Amides in Pure and Mixed Solvents Micellar Effect on the Photosensitized Debromination of 2'3-Dibromo-3- phenylpropionic Acid.Control of Forward and Back Electron Transfers K. Takagi, N. Miyake, E. Nakamura, H. Usami, Y. Sawaki and H. Iwamura Local Motions of Counter-ions in Polyelectrolute Solutions without Added Salts. A Neutron Quasielastic Scattering Study T. Kanaya, K. Kaji, R. Kitamaru, B. GabryS and J. S. Higgins X-Ray Photoelectron Spectra of Defective Nickel Oxide M. Tomellini 'H Nuclear Magnetic Resonance Study of Solute-Solvent Interactions of 1-Methyl-2-pyrrolidone and some Substituted Benzenes H. A. Zainel, S. F. Al-Azzawi and H. I. Swellem Transference-number Measurements of Silver Nitrate in Pure and Mixed Solvents using the Electromotive Force Method D. S. Gill and M. S. Bakshi Reversible and Steady Photogeneration of 4,4'-Bipyridinium Radical Cations via the Excitation of Ion-pair Charge-transfer Complexes between 4,4'- Bipyridinium and Tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate in Organic Solutions Thermodynamic Parameters for the Complexation Process between Metal(1) Cations and Dibenzocryptand 222 in Dipolar, Aprotic Solvents.Linear Correlation between Entropies of Complexation and Entropies of Solvation of Cations Activity Measurements and Spectroscopic Studies of the Catalytic Oxidation of Toluene over Vanadium Oxides supported on Titania B. Jonson, B. Rebenstorf, R. Larsson and S. L. T. Andersson Conductance Stopped-flow Study of the Association Reaction of Colloidal Spheres with Poly(vinylpyrro1idone) T. Okubo Solubilities and Vapour Pressures in the Quinquinary System NaC1-KC1- MgC1,-CaC1,-H,O.Part 1 .-Predictions and Measurements at 25 "C Y. Marcus and N. Soffer Excess Enthalpies of (x(CH,),CO + (1 - x)C6HI4) in the Supercritical Region C. J. Wormald, N. Al-Bizreh and T. K. Yerlett Theoretical Interpretation of the Heats of Immersion of Lower n-Alcohols on Faujasites and Pentads W.-D. Einicke, U. Messow, R. Schollner and G. Zahn P. J. Cheek, M. A. Gallardo-Jimenez and T. H. Lilley G. Eaton and M. C. R. Symons T. Nagamura and K. Sakai A. F. Danil de Namor and F. Fernandez SalazarCon tents 3605 3615 3625 3633 3641 Infrared Study of Ammonia-Carbon Monoxide Reactions on Silica-supported Iron Catalysts Infrared Study of the Adsorption of Ammonia, Pyridine and Hydrogen Chloride on Barium Sulphate Infrared and Gravimetric Studies of the Adsorption of Water on Barium Sulphate W. Neagle and C. H. Rochester Temperature-programmed Desorption of p-Xylene from ZSM-5, ZSM- 1 I and Theta-1 Infrared Study of the Adsorption of C,,E, on Silica immersed in Carbon Tetrachloride C. Johnston, N. Jorgensen and C. H. Rochester W. Neagle and C. H. Rochester L-f. Chen and L. V. C. Rees S. Keith and C. H. RochesterCon tents 3605 3615 3625 3633 3641 Infrared Study of Ammonia-Carbon Monoxide Reactions on Silica-supported Iron Catalysts Infrared Study of the Adsorption of Ammonia, Pyridine and Hydrogen Chloride on Barium Sulphate Infrared and Gravimetric Studies of the Adsorption of Water on Barium Sulphate W. Neagle and C. H. Rochester Temperature-programmed Desorption of p-Xylene from ZSM-5, ZSM- 1 I and Theta-1 Infrared Study of the Adsorption of C,,E, on Silica immersed in Carbon Tetrachloride C. Johnston, N. Jorgensen and C. H. Rochester W. Neagle and C. H. Rochester L-f. Chen and L. V. C. Rees S. Keith and C. H. Rochester

ISSN:0300-9599    

DOI:10.1039/F198884FP137

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

1645 1655 1669 1683 1697 1709 1721 1727 1743 1749 1759 1767 71 1783 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions II, Issue 10,1988 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I7 the contents list of Faraday Transactions 117 Issue 10, is reproduced below. Degree of Binary Association in Electrolyte Solutions. The Roles of the Short- range Interionic Potential and of the Friction Coefficient L. Degrkve Librations of Hydrogen in Stage I1 Caesium Graphite, C,,Cs W. J. Stead, P. Meehan and J. W. White Tunnelling of Hydrogen in Alkali-metal-Graphite Intercalation Compounds. A Systematic Study of C,,Rb(H,), and its Structural Consequences W. J. Stead, I. P. Jackson, J. McCaffrey and J. W. White Solvent Influence on Dipole Moment and Charge-transfer Effects in n-Electron Systems.N7N-Dimethyl-p-nitrosoaniline 2. Pawelka NO, Kinetic Studies using Laser-induced Fluorescence C. Anastasi and D. U. Hancock Molecular Electronic Structure of Aluminium Trihalide Dimers and their Quadrupole Couplings by the SCF-MS Molecular-orbital Method E. M. Berksoy and M. A. Whitehead Temperature Profiles in Weak Thermal Explosions L. Thang, A. Moise and H. 0. Pritchard Generation of Sodium and Copper Atoms in the Gas Phase by Microwave- induced Plasma Afterglows C. Vinckier, A. Dumoulin, J. Corthouts and s. De Jaegere Correlation of Subsystems at the Onset of a Centre Manifold Organization A. Fernandez Solvation Dynamics in Methanol and n-Butanol. Comparison between Temperature-dependent Fluorescence and Dielectric Data E.Venuti, N. Camaioni and F. Baigelletti Investigations of Interactions between Radicals and Molecules using the Austin Model 1 Molecular-orbital Method. Part 1 .-Carbon-centred Radicals S. Brumby Classical Trajectory Studies of the Reagent Rotational Energy Dependence for the Reactions X+ICH,+XI+CH, (X = Na and F) M. MenCndez, L. Baiiares, A. Gonzalez Ureiia and J. C. Whitehead The following papers were accepted during July 1988. Heat of Solution and Electrical Water-Tetrahydrofuran Mixtures Bald, 2. Kozlowski and A. Szejgis (0 for publication in Faraday Transactions I Conductivity of Some Electrolytes in S. Taniewska-Osinska, B. Nowicka, A.7/1985 7/2229 8 /00745D 8/0087 1 J 8/00922H 8/010121 8/01013G 8/0 1078A 8/01 11 1G 8/01 167B 8/01275J 8/01287C 8/01 3821 8/01473F 8/0 1620H 8/01623B General Phenomenological Treatment of Activation, Diffusion and Pseudodiffusion Control of Biomolecular Reactions in Solution J.F. Garst Photosensitised Oxidation of Water by CdS-based Suspensions A. Mills and G. Williams Spectroscopic Characterization and Photochemical Behaviour of Titanium Hydroxyperoxo Compounds Minuera, G., Gonzalez-Elipe, A. R., Fernandez, A., Malet, P. and Espinos, J. P. Cationic Lead(I1) Halide Complexes in Molten Alkali-metal Nitrate. Part 1 .-A Thermodynamic Investigation of the Fluoride System Bengtsson, L. and Holmberg, B. Cationic Lead(I1) Halide Complexes in Molten Alkali-metal Nitrate. Part 2.-A Thermodynamic Investigation of the Chloride, Bromide and Iodide Systems The Interaction of Oxygen with Isolated Silver Particles of ca.70 nm Supported on Alpha-Alumina. Part 1 .--Oxygen Sorption and T.P.D. Measurements Meima, G. R., Vis, R. J., Van Leur, M. G. J., Van Dillen, A. J., Van Buren, F. R. and Geus, J. W. The Interaction of Oxygen with Isolated Silver Particles of ca. 70 nm Supported on Alpha- Alumina. Part 2.-Co-oxidation Measurements Meima, G. R., Knijff, L. M., Van Dillen, A. J., Van Buren, F. R. and Geus, J. W. Retention of Rod-like Solutes in Non-ideal Multicomponent Mixed Solvent Borowko, M. Tin Oxide Surfaces. Part 18.-An Infrared Study of the Adsorption of Very Low Levels (2&50 ppm) of Carbon Monoxide in Air on to Tin(1v) Oxide Gel Harrison, P. G. and Guest, A. Cobalt Manganese Oxide Water Gas Shift Catalysts: A Kinetic and Mechanistic Study Hutchings, G.J., Gottschalk, F., Hunter, R. and Orchard, S. W. Study of Ultramicroporous Carbons by High-pressure Sorption. Part 1.-N,, CO,, 0, and He Isotherms. Koresh, J. E., Kim, T. H. and Koros, W. J. Isotope Effects in the NMR Spectra of Mixed Aqua Complexes of A~(III). Akitt, J. W. and Howarth, 0. W. Stability and Structure of Formamide and Urea Dimers in Aqueous Solution. A Theoretical Study Cristinziano, P., Lelj, F., Amodeo, P., Barone, G. and Barone, V. Determination of the Kinetics of Facilitated Ion-transfer Reactions Across a Micro-Ities Campbell, J. A., Stewart, A. A. and Girault, H. H. Effect of Form on the Surface Reactivity of Differently Prepared Zinc Oxides A Room- temperature ENDOR Study of X-Irradiated Pyridoxine Hydrochloride Single Crystal Bengtsson, L.and Holmberg, B. Bolis, V., Fubini, B., Giamello, E. and Reller, A. Masiakowski, J. T. and Lund, A. (ii)8/01 6251 8/0 16901 8/01715H 8/0 1 720D 8/01735B 8/01 766B 8/01 824C 8/01838C 8/01944D 8/02145G 8/02240B 8/02538 J Infrared Study of the Adsorption of Ethanoic Acid and Trifluoroethanoic Acid on Barium Sulphate Neagle, W. and Rochester, C. H. Studies of Electrical Transport Processes in Polyelectrolyte Solutions Vink, H. Characterization of Iron Oxide-dispersed Activated Carbon Fibres with Fe K-edge XANES and EXAFS and Water Adsorption Kaneko, K., Kosugi, N. and Kuroda, H. Infrared Study of the Adsorption of Ethyl Ethanoate on Barium Sulphate Rochester, C. H. and Neagle, W. Fluorescence Decays in a Wide Variety of van der Waals Complexes of 9-Cyanoanthracene Studied in Supersonic Free Jets Hirayama, S., Adachi, A., Tanaka, F., Shobatake, K.and Jung, K-H. A Computer Simulation of the Solvation of a Solvatochromic Pyridinium Betaine Beckett, M. A. and Dawber, J. G. Transfer Chemical Potentials, Solubilities and Reactivities in Binary Aqueous Mixtures of two Complex Iron(I1) Cations : Tris[(3-meth- oxyphenyl)iminophenyl-2-pyridylmethane] iron(I1) and 1,8-bis[(2-qui- nolyl-methylene)amino]-3,6-diazaoctane iron(I1) Blandamer, M. J., Burgess, J., Guardado, P. and Hubbard, C. D. Characterization of the Mixed Perovskite BaSn,-, Sb,O, by Electrolyte Electroreflectance, Diffuse Reflectance and XPS Larramona, G., Gutierrez, C., Pereira, I., Nunes, M. A. and Da Costa Fernanda, M. A. Elastic Modulus of Crystal-like Structures of Deionized Colloidal Spheres in Sedimentation Equilibrium as Studied by the Reflection Spectrum Method Okubo, T.Rotating-Disc Electrodes and the Theory of CE Processes : Arbitrary Rate Constants and Diffusion Coefficients Compton, R. G. and Harland, R. G. Electrochemical Behaviour of Polyaniline in Weak Acid Solutions Hirai, T., Kuwabata, S. and Yoneyama, H. Estimated Mean Activity Coefficients of Aqueous BeCl, and Properties of Solution Mixtures Containing the Be2+ Ion Clegg, S. L. and Brimblecombe, P. (iii)8/01 6251 8/0 16901 8/01715H 8/0 1 720D 8/01735B 8/01 766B 8/01 824C 8/01838C 8/01944D 8/02145G 8/02240B 8/02538 J Infrared Study of the Adsorption of Ethanoic Acid and Trifluoroethanoic Acid on Barium Sulphate Neagle, W. and Rochester, C.H. Studies of Electrical Transport Processes in Polyelectrolyte Solutions Vink, H. Characterization of Iron Oxide-dispersed Activated Carbon Fibres with Fe K-edge XANES and EXAFS and Water Adsorption Kaneko, K., Kosugi, N. and Kuroda, H. Infrared Study of the Adsorption of Ethyl Ethanoate on Barium Sulphate Rochester, C. H. and Neagle, W. Fluorescence Decays in a Wide Variety of van der Waals Complexes of 9-Cyanoanthracene Studied in Supersonic Free Jets Hirayama, S., Adachi, A., Tanaka, F., Shobatake, K. and Jung, K-H. A Computer Simulation of the Solvation of a Solvatochromic Pyridinium Betaine Beckett, M. A. and Dawber, J. G. Transfer Chemical Potentials, Solubilities and Reactivities in Binary Aqueous Mixtures of two Complex Iron(I1) Cations : Tris[(3-meth- oxyphenyl)iminophenyl-2-pyridylmethane] iron(I1) and 1,8-bis[(2-qui- nolyl-methylene)amino]-3,6-diazaoctane iron(I1) Blandamer, M.J., Burgess, J., Guardado, P. and Hubbard, C. D. Characterization of the Mixed Perovskite BaSn,-, Sb,O, by Electrolyte Electroreflectance, Diffuse Reflectance and XPS Larramona, G., Gutierrez, C., Pereira, I., Nunes, M. A. and Da Costa Fernanda, M. A. Elastic Modulus of Crystal-like Structures of Deionized Colloidal Spheres in Sedimentation Equilibrium as Studied by the Reflection Spectrum Method Okubo, T. Rotating-Disc Electrodes and the Theory of CE Processes : Arbitrary Rate Constants and Diffusion Coefficients Compton, R. G. and Harland, R. G. Electrochemical Behaviour of Polyaniline in Weak Acid Solutions Hirai, T., Kuwabata, S.and Yoneyama, H. Estimated Mean Activity Coefficients of Aqueous BeCl, and Properties of Solution Mixtures Containing the Be2+ Ion Clegg, S. L. and Brimblecombe, P. (iii)Cumulative Author Index 1988 Abdel-Kader, M. H., 2241 Abe, H., 511 Abraham, M. H., 175, 865, 1985 Abraham, R. J., 1911 Adachi, H., 1091 Agnel, J-P. L., 3359 Ahluwalia, J. C., 2651 Aicart, E., 1603 Al-Azzawi, S. F., 3511 Alberti, A., 3347 Al-Bizreh, N., 3587 Al-Haidary, Y. K., 3027, 3043 Allen, G. C., 165, 355 Amorelli, A., 1723 Anazawa, I., 275 Anderson, S. L. T., 1897, 3363, Anid, S., 3413 Anpo, M., 751, 2771 Antonini, A. C. R., 1889 Aoi, H., 2421 Aoyama, T., 2209 Aracil, J., 539 Archer, G. P., 2499 Arora, K. S., 1729 Asdkura, K., 1329, 2445, 2457 Atherton, N. M., 3257 Auroux, A,, 3169 Aveyard, R., 675 Ayyoob, M., 2377 Baba, K., 459 Back, D.M., 2585 Bagchi, S., 1501 Baglioni, P., 467 Bakshi, M. S., 3517 Baldini, G., 979 Barna, T., 229 Barone, G., 1919 Barouch, E., 3093 Barratt, M. D., 3249 Barzaghi, M., 3279 Basosi, R., 3331 Basumallick, I. N., 2697 Baulch, D. L., 1575 Bazsa, G., 215, 229 Beaumont, P. C., 3423 Benaglia, H., 3347 Benmouna, M., 1563 Benoit, H., 1563 Berei, K., 367 Berroa de Ponce, H., 255, 1671 Berry, F. J., 2783 Bertoldi, M., 1405 Beyer, H. K., 1447 Bhat, R., 2651 Binks, B. P., 675 3547 Birch, G. G., 2635 Blandamer, A. H., 1889 Blandamer, M. J., 1243, 1889, 2703, 2906 Blesa, M. A., 9 Blinov, N. N., 1075 Bloor, D. M., 2087 Bonini, B. F., 3347 Bonnefoy, J., 941 Boon, P. J., 3341 Borbtly, G., 1447 Borckmans, P., 1013 Borgarello, E., 261 Borowko, M., 1961 Bourdillon, C., 941 Branca, M., 3279 Brandreth, B.J., 1741 Breen, J., 293 Bretz, N. H., 3293 Briggs, B., 1243, 2703 Brown, M. E., 57, 1349 Brown, P., 3059 Bruce, J. M., 2855, 3423 Brustolon, M., 2875 Brydson, R., 617, 631 Bulow, M., 2247, 3001 Burgess, J., 1243, 1889, 2703 Burget, U., 885 Burnell, E. E., 3129 Busca, G., 237, 1405, 1423 Buscarlet, L. A., 3359 Buxton, G. V., 1101, 11 13 Caballero, A., 2369 Caceres, M., 539 Caceres-Alonso, M., 1603 Carbone. A. I., 207 Caro, J., 2347 Carr, N. J., 1357 Castronuovo, G., 1919 Cavani, F., 237 Cavasino, F. P., 207 Celik, F., 2305 Centi, G., 237 Cesaro, A., 2573 Chagas, A. P., 1065 Chandrd, H., 609, 3401 Chatterjee, J. P., 2697 Che, M., 751, 2771 Cheek, P. J., 1927, 3435 Chen, L. F., 3641 Cheng, V. K.W., 899 Chien, J. C. W., 1123 Chinchen, G. C., 2135 Chirico, G., 979 Christensen, P. A., 2795 Christidis, T. C., 3263 Chudek, .I. A., 1145, 1737 Clarke, J. K. A., 251 1 Clarke, R. J., 365 Clint, J. H., 675 Coates, J. H., 365 Coles, B. A., 2357 Coller, B. A. W., 899 Coluccia, S., 751 Compton, R. G., 473, 483, 2013, 2057, 2155, 2357 Contarini, S., 2335 Conway, B. E., 3389 Cook, A., 1691 Corma, A., 31 I3 Costas, M., 1603 Covington, A. K., 1393 Crowther, N. J., 121 1 Dadok, J., 2595 Daldrup, N., 2553 Danil de Namor, A. F., 255, 1671, 2441, 3539 Das, S., 1057 Dash, A. C., 75, 2387 Dash, N., 75 Davydov, A., 37 Dawber, J. C., 41 Dawber, J. G., 41, 713 Day, M. J., 2013 de Bleijser, J., 293 Delben, F., 2573 Del Vecchio, P., 1919 Derylo-Marczewska, A., 295 1 Diaz Peiia, M., 539 Dickinson, E., 871 Dines, T.J., 3445 Disdier, J., 261 Domen, K., 511 Dong, S., 2979 Dougal, J. C., 657 Duarte, M. Y., 97, 367 Duce, P. P., 865 Duckworth, R. M., 1223 Duplitre, G., 2831 Dyster, S., 11 13 Eagland, D., 121 1 Eaton, G., 2181, 3459 Egawa, C., 321 Einfeldt, J., 931 Einicke, W-D.. 3597 Ekechukwu, A. D., 1871 Eley, D. D., 2069 Elia, V., 1919 El Jamal, M. M., 3169 Elliot, A. J., 1101 Elvidge, D., 2703 Engel, W., 617, 631AUTHOR INDEX Eriksson, P-O., 3129 Eszterle, M., 575 Evans, J. C., 1723, 3243, 3249 Everett, D. H., 1455 Eyears, J. M., 1437, 3097 Fernandez, A., 1543 Fernandez-Pineda, C., 647 Fiedler, K., 3001 Finter, C. K., 2735 Fischer, H., 3187 Flanagan, T. B., 459 Fletcher, P. D. I., 1131 Foresti, E., 237 Foresti, M. L., 97 Forissier, M., 3169 FornCs, V., 3 I 13 Forni, L., 2397, 2477 Forster, H., 491 Foster, R., 1145, 1737 Fraenkel, D., 1817, 1835 Franklin, K.R., 687, 2755 Franks, F., 2595 Fubini, B., 1405 Fujihira, M., 2667 Fujii, K., 3121 Funabiki, T., 2987 Furedi-Milhofer, H., 1301 Gal, D., 1075 Gabrail, S., 41 Gabryi,. B., 3487 Gaffney, S. H., 2545 Gallardo-Jimenez, M. A., 3435 Galwey, A. K., 57, 729, 1349, Gamba, A., 3279 Gans, P., 657 Gardner, P. J., 1879 Garrone, E., 2843 Geblewicz, G., 561 Geertsen, S., 1101 Georges, V., 1531 Giamello, E., 1405 Gilbert, B. C., 3319 Gilbert, R. G., 3107 Gill, D. S., 1729, 3517 Gill, J. B., 657 Gilot, B., 801 Girault, H. H., 2147 Giuliacci, M. E., 2311 Goldfarb, D., 2335 Gopalakrishnan, R., 365 Grabielle-Madelmont, C., 2609 Grampp, G., 366 Gratzel, M., 197, 1703 Gray, A. C., 1509 Gray, P., 993 Green, P., 2109 Green, S.I. E., 41 Green, W. A., 2109 Grepstad, J. K., 1863 Griffiths, J. F., 1575 Grigera, J. R., 2603 Grigo, M., 931 Grimson, M. J., 1563 1357 Gritzner, G., 1047 Grothe, K., 3267 Grzybkowski, W., I55 I Guardado, P., 1243, 2703 Guarini, G. G. T., 331 Guarino, G., 2279 Guglielminotti, E., 2195 Guidelli, R., 97, 367 Gupta, D. Das, 1057 Guyan, P. M., 2855 Hadjiivanov, K., 37 Hakin, A. W., 1889, 2703 Hall, D. G., 773, 2087, 2215, 2227, 3059 Hall, N. D., 1889 Halle, B., 1033 Hamada, K., 1267 Hanawa, T., 1587 Handreck, G. P., 1847 Hanson, G. R., 1475 Harrer, W., 366 Harriman, A., 2109, 2795, 2821 Hasebe, T., 187 Hashimoto, K., 87 Haslam, E., 2545 Hatayama, F., 2465 Hausen, H-D., 3207 Hayashi, K., 2209 Hazra, D.K., 1057 Heatley, F., 343 Hegarty, B. F., 251 1 Hegde, M. S., 2377 Heineken, F. W., 3263 Helle, N., 3293 Henzel, N., 3293 Herley, P. J., 729 Herrmann, J-M., 261 Hertz, H. G., 2735 Hey, M. J., 2069 Heyward, M. P., 815 Hidalgo, M. del V., 9 Higgins, J. S., 3487 Hill, A., 255 Histed, M., 3307 Hoare; 1. C . , 3071 Holzwarth, J. F., 2807 Homer, J., 2959 Hoshino, K., 2667 House, W. A., 2723 Howard, J. A., 3307 Howson, M. R., 2723 Hubbard, C. D., 1243, 2703 Hudson, B. D., 1911 Huis, D., 293 Hunter, R., 1311 Hurst, H. J., 3071 Hutchings, G. J., 131 1 Ichikawa, K., 3015 Ige, J., I Ikeda, S., 151 Irnai, H., 923 Imamura, H., 765 Imanaka, T., 851, 2173 Inoue, A., 1195 Irinyi, G., 1075 Ishiguro, S., 2409 Ishikawa, T., 1941 Isobe, T., 1199 Ito, D., 1375 Ittah, B., 1835 Iwamoto, E., 1679 Iwamura, H., 3475 Iwasawa, Y., 321, 1329, 2445, Iyer, R.M., 2047 Jackson, S. D., 1741 Jackson, S. K., 3243 Jaeger, N. I., 1751 Jaenicke, W., 366 Janzen, E. G., 3275 Jaroniec, M., 2951 Jeminet, G., 951 Jens, K-J., 1863 Jin, T., 3015 Johnson, G. R. A., 501 Johnson, I., 551 Johnston, C., 309, 2001, 3605 Joly, H. A., 3307 Jonasson, R. G., 231 1 Jones, A. R., 2914 Jonson, R., 1897, 3363, 3547 Jorge, R. A., 1065 Jorgensen, N., 309, 2001, 3605 Jozwiak, M., 2077 Juillard, J., 951, 959, 969, 1713, Kaim, W., 3207 Kaizu, Y., 1517 Kaji, K., 3487 Kakei, K., 1795 Kanaya, T., 3487 Kane, H., 851 Kaneko, K., 1795 Kanno, T., 281, 2099 Kasahara, S., 765 Kato, C., 2677 Kato, S., 151 Katz, N. E., 9 Kawasaki, Y., 1083 Kay, R. L., 2595 Keeble, D. J., 609 Keith, S., 3633 Keller, A., 2904 Kemp, T.J., 2027 Kermode, M. W., 1911 Kevan, L., 467, 2335 Kimura, T., 2099 Kinnaird, S., 2135 Kirby, C., 355 Kiricsi, I., 491 Kirste, B., 3267 Kishore, N., 2651 Kiss, I., 367 Kitamaru, R., 3487 Kiwi, .I., 1703 Klinowski, J., 2902 Klinszporn, L., 1551 Klissurski, D., 37 2457 3175Kobayashi, A., 1795 Kobayashi, H., 1517 Kobayashi, M., 281, 2099 Koiiiik, M., 2247 KoEiik, M., 3001 Koda, S., 1267 KodejS, Z . , 2885 Koksal, F., 2305 Kolar-Anic, L. Z., 3413 Komatsu, H., 2537 Kondo, J., 51 1 Kondo, M., 2771 Kondo, S., 1941 Kondo, Y., 11 1 Konishi, Y., 281 Kordulis, C., 1593 Kornhauser, I., 785, 801 Kosugi, N., 1795 Kotake, Y., 3275 Kowalak, S., 2035 Kraehenbuehl, F., 1973 Krausz, E., 827 Krebs, P., 2241 Kristyan, S., 917 Kubelkova, L., 1447 Kubokawa, Y., 751, 2129, 2771 Kumamaru, T., 1679 Kurimura, Y., 841, 1025 Kuroda, H., 1329, 1795 Kuroda, K., 2677 Kuroda, Y., 2421 Kurreck, H., 3267, 3293 Kusabayashi, S., 11 1 Kuwabata, S., 1587, 2317 Lahy, N., 1475 Laing, M.E., 2013 Lajtar, L., 19 Lambi, J. N., 1 Lancaster, N. M., 3141, 3159 Land, E. J., 2855, 3423 Larsson, R., 1897, 3363, 3547 Laschi, F., 3331 Laubry, P., 969, 3175 Laval, J-M., 941 Lawrence, K. G., 175 Lea, J. S., I181 Leaist, D. G., 581 Lefever, R., 1013 Lefferts, L., 1491 Lengyel, I . , 229 Levine, H., 2619 Levy, A., 1817 Levy, M., 1835 Lewis, T. J., 1531 Leyendekkers, J. V., 397, 1653 Leyte, J. C.. 293 Lhermet, C., 2567 Lilley, T. H., 1927, 2545, 3435 Lincoln, S. F., 365 Lindblom, G., 3129 Lindner, Th., 631 Lips, A., 1223 Llewellyn, J.P., 153 1 Logan, S. R., 1259 AUTHOR INDEX Louis, C., 2771 Lu, Z . , 2979 Lycourghiotis, A., 1593 MacCorquodale, F., 3233 Machida, K., 2537 MacKay, R. L., 1145, 1737 Mackley, M., 2910 Maezawa, A., 851 Malanga, C . , 97 Malet, P., 2369 Mandel, M., 2483 Maniero, A. L., 2875 Marcandalli, B., 2807 Marcus, Y., 175, 1465, 3575 Marczewski, A. W., 295 1 Markovic, M., 1301 Maroto, A. J. G., 9 Marsden, A., 2519 Martin, C. C., 3359 Martin, R. R., 231 1 Martinet, P., 3175 Martins, L. J. A., 2027 Maruya, K., 511 Mason, D., 473, 483, 2057 Mathlouthi, M., 2641 Matsumoto, T., 1375 Matsumura, Y., 87 Matsuoka, K., 1277 Matteoli, E., 1985 Maxwell, I. A., 3107 Mayagoitia, V., 785, 801 McAleer, J. F., 441 McMurray, N., 379 Mead, J., 675 Medda, K., 1501 Mehta, G., 2297 Mensch, C . T.J., 65 Merkin, J. H., 993 Messow, U., 3597 Meunier, F., 1973 Meyerstein, D., 2933 Mile, B., 3307 Mills, A., 379, 1691 Mines, J. R., 1911 Mintchev, L., 1423 Mirti, P., 29 Mitsushima, I., 851 Miura, K., 2421 Miyagawa, S., 2129 Miyajima, K., 2537 Miyakawa, K., 1517 Miyake, N., 3475 Miyanaga, T., 2173 Miyata, H., 2129, 2465, 2677, Mohamed, M. A-A., 57, 729, Mohammadi, M. S., 2959 Moiroux, J., 941 Moller, K., 1751 Morel, J-P., 2567 Morel-Desrosiers, N., 2567 Morimoto, T., 2421 Morris, H., 3307 3121 1349 Morris, J. J., 865 Morterra, C., 1617 Morton, J. R., 413 Moseley, P. T., 441 Mosseri, S., 2821 Mousset, G., 969, 3175 Muhler, M., 631 Mukai, T., 2465 Mukherjee, T., 2855, 3423 Murray, A., 2783 Murray, B. S., 871 Nagamura, T., 3529 Nagao, M., 1277 Nahor, G. S., 2821 Nakagaki, M., 2537 Nakagawa, Y., 2129 Nakamura, E., 3475 Nakamura, T., 1287 Nakamura, Y., 1 11 Nakao, N., 665 Nakayama, N., 665 Nandan, D., 2047 Napper, D.H., 3107 Narayanan, S., 521 Nazhat, N. B., 501 Neagle, W., 3615, 3625 Neta, P., 2109 Newman, K. E., 1387, I393 Nicolis, G., 1013 Nishihara, C., 433 Nishikawa, S., 665 Nishio, E., 1639 Nisi, M., 2279 Nomura, H., 151, 1267 Norris, J. 0. W., 441 Northing, R. J., 2013 Noszticzius, Z . , 575 Nucci, L., 97 Oehler, U. M., 3275 Ohno, T., 2465 Ohshima, K., 1639 Ohtaki, H., 2409 Ohtani, S., 187 Okabayashi, H., 1639 Okamoto, K., 2317 Okamoto, Y., 851 Okubo, T., 703, 1163, 1171, 1949, 3377, 3567 Oliva, C., 2397, 2477, 3279 Oliver, C. E., 3257 Oliver, S. W., 1475 Ollivon, M., 2609 Olm, M. T., 3341 Olofsson, G., 551 Ommen, J. G.van, 1491 Onishi, T., 51 1 Ono, T., 2465, 3121 Ono, Y., 1091 Oosawa, Y., 197 Overbeek, J. Th. G., 3079 Ozeki, S., 1795 Ozutsumi, K., 2409 Page, F. M., 1145 Painter, D. M., 773, 2087 (vii)AUTHOR INDEX Pal, M., 1501 Pan, C-f., 1341 Pandey, J. D., 1853 Pandey, P. C., 2259 Pang, P., 1879 Paoletti, S., 2573 Pappin, A. J., 1575 Parrott, D., 1131 Parsons, B. J., 3423 Passelaigue, E., 17 13 Patil, K., 2297 Patterson, D., 1603 Peacock, R. D., 3445 Pedatsur, N., 2821 Pedersen, J. A., 3223 Pedulli, G. F., 3347 Peet, W. J., 3319 Pelizzetti, E., 261 Pena-Nuiiez, A. S., 2181 Penar, J., 739 Penman, J. I., 2013 Perutz, R. N., 2901 Pethybridge, A. D., 2723 Pezzatini, G., 367 Pfeifer, H., 2347 Piccini, S., 331 Pichat, P., 261 Pickering, I. J., 2795 Pickl, W., 1311 Piekarski,: H., 529, 591 Pilarczyk, M., 1551 Pilbrow, J.R., 1475 Pilkington, M. B. G., 2155 Plath, P. J., 1751 Pogliani, L., 3331 Pogni, R., 3331 Pointud, Y., 959, 1713 Polavarapu, P. L., 2585 Pota, G., 215 Pradhan, J., 2387 Preston, K. F., 413 Price, W. E., 2431 Prior, D. V., 865 Pushpa, K. K., 2047 Quinquenet, S., 2609 Quist, P-O., 1033 Radford, K. J., 3319 Radulovic, S., 1243, 2703 Raffi, J. J., 3359 Rai, R. D., 1853 Rajam, S., 1349 Rajaram, R. R., 391 Rao, B. G., 1773, 1779 Rao, K. J., 1773, 1779 Rao, K. M., 2195 Rayment, T., 2915 Rebenstorf, B., 1897, 3363, 3547 Rebuscini, C., 2397 Rees, L. V. C., 291 1, 3641 Reller, A., 2327 Renuncio, J. A. R., 539 Rhodes, C. J., 1187, 3215 Richardson, N. V., 2909 Richardson, S. M., 2909 Richoux, M-C., 2109 Riis, T., 1863 Ruegge, D., 3187 Riva, A., 1423 Robson, B., 2519 Rochester, C.H., 309, 2001, 3605, 3615, 3625. 3633 Rojas, F., 785, 801, 1455 Rooney, J. J., 251 1 Ross, J. R. H., 1491 Rossi, C., 3331 Rowlands, C. C., 1723, 3243, Rubio, R. G., 539 Saadalla-Nazhat, R. A., 501 Saba, A., 3279 Sacchetto, G. A., 2885 Saito, M., 1025 Saito, Y., 275 Saji, T., 2667 Sakai, K., 3529 Sakaiya, H., 1941 Sakamoto, Y., 459 Sakata, Y., 511 Salazar, F. F., 3539 Saleh, J. M., 3027, 3043 Salvagno, S., 1531 Sanz, J., 31 13 Sarkany, A., 2267 Sartorio, R., 2279 Sato, T., 275 Sauer, H., 617 Savile, G., 2907 Sawabe, K., 321 Sawaki, Y., 3475 Sayari, A., 41 3 Sbriziolo, C., 207 Scarano, D., 2327 Schelly, Z. A., 575 Schiffrin, D. J., 561 Schiller, R. L., 365 Schlenoff, J. B., 1123 Schlogl, R., 631 Schmelzer, N., 931 Schollner, R., 3597 Schonert, H., 2553 Schulz, A., 3207 Schulz, R.A., 865 Schwarz, H. A., 2933 Schwarz, W., 1703 Scott, S. K., 993, 2904, 2908 Seidl, V., 1447 Sellers, R. M., 355 Senna, M., 1199 Senoda, Y., 1091 Sermon, P. A., 391 Serpelloni, M., 2609 Serpone, N., 261 Seuvre, A-M., 2641 Shamil, S., 2635 Sheppard, N., 2913 Shindo, H., 433 Shukla, A. K., 1853 Shukla, R. K., 1853 3249 Sidahmed, I. M., 1153 Simmons, R. F., 1871 Sinclair, G. R., 1475 Singh, B., 1729 Singh, P. P., 1807 s’Jacob, K. J., 1509 Slade, L., 2619 Smith, E. R., 899 Smith, T. D., 1475, 1847 Soffer, N., 3575 Sokolowski, S., 19, 739 Somsen, G., 529 Soriyan, 0. O., 1 Speight, J. M., 2069 Spoto, G., 2195 Stainsby, G., 871 Stange, G., 2807 Stark, J. M., 3243 Stead, K., 2905 Stearn, G. M., 2155, 2357 Steel, A.T., 2783 Stell, J. K., 3319 Stevens, J. C. H., 165 Stirling, C. J. M., 1531 Stocker, M., 1863 Stoeckli, F., 1973 Stone, F. S., 2843 Stone, W. E. E., 117 Stramel, R. D., 1287 Struve, P., 2247, 3001 Stuart, W. I., 3071 Subba Rao, M., 1703 Sudol, E. D., 3107 Suga, K., 2667 Sugahara, Y., 2677 Sun, L-M., 1973 Suzuki, T., 1795 Swallow, A. J., 2855, 3423 Swellem, H. I., 351 1 Sykes, A. F., 1575 Symons, M. C. R., 609, 1181, 1187, 2181, 2499, 3341, 3401, 3459 Szamosi, J., 91 7 Taga, K., 1639 Taga, T., 2537 Tagawa, T., 923 Takada, T., 765 Takagi, K., 3475 Takagi, Y., 1025 Takaishi, T., 2967 Takato, K., 841 Takisawa, N., 2087, 3059 Takriti, S., 2831 Tamaki, J., 2173 Tanaka, F., 1083 Tanaka, K., 601, 2895 Tanaka, K-i., 601 Tanaka, T., 2987 Taniewska-Osinska, S., 2077 Tardajos, G., 1603 Taylor, D.M., 1531 Taylor, P. J., 865 Tazaki, K., 231 1 (viii)AUTHOR INDEX Tewari, J., 1729 Thampi, K. R., 1703 Theocharis, C. R., 1509 Thomas, J. K., 1287 Thomas, J. M., 617, 63 1, 2795, Thompson, J. S., 2519 Tiddy, G. J. T., 2900, 3129 Tiezzi, E., 3331 Tissier, C., 951, 969, 3175 Tissier, M., 3175 Tofield, B. C., 441 Tomellini, M., 3501 Torres-Sanchez, R-M., 117 Townsend, R. P., 687, 2755 Tra, H. V., 1603 Trifiro, F., 237, 1405, 1423 Tschirch, G., 2247 Tsuchitani, R., 2987 Tsuchiya, S., 765 Tsukamoto, K., 1639 Tummalapalli, C. M., 2585 Turkson, L. A., 3249 Turner, D. J., 2683 Twiselton, D. R., 1145 Uematsu, R., 1 I 1 Uma, K., 521 Unwin, P. R., 473, 483, 2057 Usami, H., 3475 Vaccari, A., 1405, 1423 2915 van Rensburg, L. J., 131 1 van Veen, J.A. R., 65 van Wingerden, R., 65 Varani, G., 979 Vasriros, L., 367 Vazquez-Gonzilez, M. I., 647 Viddczy, T., 1075 Viguria, E. C . , 255 Vink, H., 133 Viswanathan, B., 365 Vogel, V., 1531 Vordonis, L., 1593 Walker, R. A. C., 255 Waller, A. M., 2013, 2357 Walton, J. C., 3233 Wang, E., 2289 Ward, J., 713 Ward, T. R., 2545 Webb, G., 2135 Weilbacher, E., 3293 Wells, C. F., 815, 1153 Welsh, M. R., 1259 White, T. J., 3071 Wijmenga, S. S., 2483 Wilkinson, D. P., 3389 Williams, B. G., 617, 631 Williams, C., 2915 Williams, D. E., 441 Williams, R. A., 713 Winstanley, D., 1741 Wong, J., 1773, 1779 Wood, N. D., 1113 Wormald, C. J . , 1437, 2912, 3097, 3141, 3159, 3587 Wurzburger, S., 2279 Wyn-Jones, E., 773, 2087, 3059 Yamada, M., 2457 Yamada, Y., 751 Yamamoto, Y., 2209 Yamane, T., 2173 Yamasaki, S., 1679 Yamashita, H., 2987 Yamashita, S., 1083 Yao, S., 1375 Yasugi, E., 2421 Yerlett, T.K., 3587 Yoffe, A. D., 2899 Yoneyama, H., 1587, 2317 Yoshida, S., 87, 2987 Yuqing, L., 2289 Zahn, G., 3597 Zainel, H. A., 3511 Zecchina, A., 751, 2195, 2327, Zeitler, E., 617, 631 Zelano, V., 29 Zibrowius, B., 2347 Zielinski, R., 151 Zundel, G., 885 2843THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 24 Orientation and Polarization Effects in Reactive Collisions To be held at the Physikzentrum, Bad Honnef, West Germany, 12-14 December 1988 Organising Committee: Dr S. Stolte Professor R.A. Levine Dr K. Burnett Professor R.N. Dixon Professor J. P. Simons Dr H. Loesch The Symposium will focus on the study of vector properties in reaction dynamics and photodissoci- ation rather than the more traditional scalar quantities such as energy disposal, integral cross-sec- tions and branching ratios.Experimental and theoretical advances have now reached the stage where studies of Dynamical Stereochemistry can begin to map the anisotropy of chemical interac- tions. The Symposium will provide an impetus to the development of 3-D theories of reaction dyna- mics and assess the quality and scope of the experiments that are providing this impetus. The following areas will be covered: (A) Collisions of oriented or rotationally aligned molecular reagents (B) Collisions of orbitally aligned atomic reagents (C) Photoinitiated ’collisions’ in van der Waals complexes (D) Polarisation of the products of full- and half-collisional complexes.The programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 87 Ca ta I ys is by We I I Character i sed Mate r ia Is University of Liverpool, 11-13 April 1989 Organising Committee: Professor R. W. Joyner (Chairman) Professor A. K. Cheetham Professor F. S. Stone The understanding of heterogeneous catalysis is an important academic activity, which complements industry’s continuing search for novel and more efficient catalytic processes. The emergence of rele- vant, in particular in situ techniques and new developments of well established experimental ap- proaches to catalyst characterisation are making a very significant impact on our knowledge of catalyst composition, structure, morphology and their inter-relationships. Well characterised cata- lysts, which will be the subject of the Faraday Discussion, include single-crystal surfaces, whether of metals, oxides or sulphides; crystalline microporous solids, such as zeolites and clays, and ap- propriate industrial catalysts.The elucidation of structure/function relationships and catalytic mech- anism will be important aspects of the scientific programme. Contributions describing novel methods for synthesising well characterised catalysts and also reporting important advances in characterisa- tion techniques will also be included. The preliminary programme may be obtained from: M r s Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN. Dr K. C. Waugh Professor P. B. WellsFARADAY DIVISION INFORMAL AND GROUP MEETINGS Division jointly with Dalton Division inorganic Solids and their Surfaces (including the Nyholm Lecture by R. Hoffmann) To be held at the Scientific Societies' Lecture Theatre, London on 22 November 1988 Further information from Mrs Y A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Polymer Physics Group jointly with Physical Crystallography Group Diffraction from Polymers To be held at the Geological Society, London on 30 November 1988 Further information from Dr M. J.Richardson, Division of Materials, National Physical Laboratory, Queens Road, Teddington, Middlesex i w 1 1 OLW Polar Solids Group with the Applied Solid State Chemistry Group Computer Modelling of inorganic Solid Structures To be held at the Scientific Societies' Lecture Theatre, London on 2 December 1988 Further information from Dr A.E. Comyns, R & D Department, Laporte lndusties Ltd., Moorfield Road, Wdnes WA8 O Q J Theoretical Chemistry Group Beyond the Born-Oppenheimer Approximation To be held at Trent Polytechnic, Nottingham on 14 December 1988 Further information from Dr R. G. Woolley, Department of Physical Sciences, Trent Polytechnic, Clifton Lane, Nottingham NG118NS Electrochemistry Group New ideas In Electrochemistry To be held at the University of Cambridge on 15-16 December 1988 Further information from Dr S. P. TyfieM, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB ~ ~~~~ Colloid and Interface Science Group Aggregation in Colloidal Systems To be held at the Scientific Societies' Lecture Theatre, London on 16 December 1988 Further information from Dr R.Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runcom, Cheshire WA7 4QE High Res o h tion Spectroscopy Group High Resolution Molecular Spectroscopy To be held at the University of Birmingham on 1920 December 1988 Further information from Dr M. N. R. Ashfold, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS Neutron Scattering Group Muon Spectroscopy To be held at the University of Nottingham on 2@22 December 1988 Further information from Dr S. Cox, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 OQX Electrochemistry Group with the Electrotechnology Group of the SCI Battery Works hop To be held at the University of Oxford on 3-4 January 1989 Further information from Dr S.P. Tyfield, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB Electrochemistry Group with the Organic Reaction Mechanisms Group Electron Transfer Reactions To be held in London on 5 January 1989 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB Gas Kinetics Group Reactions of Ions and Free Radicals To be held at the University of Warwid on 6 January 1989 Further information from Professor R. G. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJNeutron Scattering Group Neutron and X-ray Scattering: Complementary Techniques To be held at the University of Kent at Canterbury on 2930 March 1989 Further information from Dr R.J. Newport, Physics Laboratory, University of Kent, Canterbury CT2 7NR Division jointly with the Colloid and Interface Science Group Annual Congress: Surfactant Interactions in Colloidal Systems To be held at the University of Hull on 4-7 April 1989 Further information from Dr J. F. Gibson, The Royal society of Chemistry, brlington House, London W1 V OBN Electrochemistry Group Spring Informal Meeting To be held at the University of Warwick on 1@12 April 1989 Further information from Dr S. P. Tyfieki, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB Electrochemistry Group with the Electroanalytical Group Electroanalytical Biennial Meeting To be held at Loughborough University of Technology on 12-14 April 1989 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB Industrial Physical Chemistry Group with the Thin Films and Surfaces Group of the IOP Materials for Non-linear and Electro-optics To be held at Girton College, Cambridge on 4-7 July 1989 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX Polymer Physics Group Biennial Meeting To be held at the University of Reading on 131 5 September 1989 Further information from Dr M. J. Richardson, Division of Materials, National Physical Laboratory, Queens Road, Teddington, Middlesex TW11 OLW Division with the lnstitute of Physics Sensors and their Applications To be held at the University of Kent at Canterbury on 1922 September 1989 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX ~ ~~ ~ Division with the Deutsche Bunsen Gesellschaff, Division de Chimie Physique of the SocietcS Franqaise de Chimie and Associazione ltaliana di Chimica Fisica Transport Processes in Fluids and Mobile Phases To be held at the Physikalische Institiit, Aachen, West Germany on 2528 September 1989 Further information from Professor G. Luckhurst, Department of Chemistry, University of Southampton, Southampton SO9 5NH Division Autumn Meeting: Chemistry at Interfaces To be held at Loughborough University of Technology on 26-28 September 1989 Further information from Professor F. Wilkinson, Department of Chemistry, Loughborough University of Technology, Loughborough LE11 3TU (xii)

ISSN:0300-9599    

DOI:10.1039/F198884BP143

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1988, 84(10), 3187-3205 Kinetic Electron Spin Resonance Spectroscopy of Complex Reaction Systems Solvent Effects on the Self-termination of 2,6-Di-t-butylphenoxyl Radicals? Daniel Ruegge and Hanns Fischer” Physikalisch-Chernisches Institut der Universitat, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland Kinetic electron spin resonance spectroscopy with intermittent radical generation has been used to obtain the rate constants of various simultaneous reactions in systems containing 2,6-di-t-butylphenoxyl or 2,6- di-t-butyl-4-methylphenoxyl radicals, the latter terminating reversibly. The radicals were generated in various solvents by photolysis of di-t-butyl peroxide in presence of the parent phenols. A data-analysis procedure is presented for reaction systems also involving species not detectable by e.s.r. The results show that self-termination rate constants of the phenoxyl radicals are up to two orders of magnitude lower than expected for diffusion control. This is usually attributed to steric hindrance due to the bulky t- butyl groups, but the activation energies found for most solvents contradict this interpretation. An explanation is presented which involves an intermediate in the termination reaction and an extra stabilization of the free radicals by dipole-induced-dipole interactions with the solvents, the latter being characterized by their molar polarization._ _ ~ In recent years recombination and disproportionation reactions of phenoxyl radicals in solution have been studied inten~ivelyl-~ because these radicals are important intermediates in antioxidant processes and because their terminations are unusually slow.Self-termination reactions of many small radicals in liquids are diffusion- c o n t r ~ l l e d , ~ ~ ~ i.e. the rate constant 2k, is given by the classical von Smoluchowski7 formula : 2kS 2k, = 1 + 2kS/2k, where 2k, = a8n1000-1N,Dp. With the spin statistical factor 0 = 1/4 accounting for non-reactive radical pairs in triplet states, 2k, $- 2k, for high reactivity, and the customary estimate of D via the Stokes-Einstein relation eqn (1) reduces to the Debye formula For most phenoxyl radicals6 deviations of second-order termination rate constants from (2) are up to two orders of magnitude, and different explanations have been given.An analysis on the basis of eqn (2) led to the conclusion1i3 that several phenoxyl radicals undergo ‘diffusion enhanced ’ reaction. The rate constant for self-termination is proportional to T/q, but is modified by an effective steric factor 0 < feff < 1. Radicals approaching each other in unfavourable orientations cannot react without previous reorientation. If the time required for such reorientation exceeds the time of contact the radicals diffuse apart without reaction. feff is estimated using geometric models for the radicals of unreactive ‘white’ spheres with a reactive ‘black’ surface spot. The latter is 2k, = RT/ 150011. (2) t Third Bruker Lecture of the Royal Society of Chemistry given at the Fifteenth Annual International E.S.R. Conference held at University College, Cardiff, 21-25 March, 1988.31873188 Kinetic E.S.R. of Complex Reaction Systems assumed to be limited by a circle characterized by the polar angle 0. Different theoretical treatments are discussed in ref. (3) and involve rotational diffusion, cage effects, coordination spheres, stepwise diffusion and other effects. They all yield similar relations betweenf,,, and 0 forfefr 2 lo-,. A second model assumes the formation of short-lived intermediates or chemical complexes in the reaction from radicals to products. Such complexes have first been invoked by Williams and Kreilick,* who investigated the equilibrium between 2,6-di-t-butyl-4-R-phenoxyl radicals and their dimers by n.m.r. Intermediate complexes are also discussed in ref. (3), (4), (9Hll), and enthalpies of reaction from radicals to complex were estimated to be -4 to - 12 kJ m ~ l - ~ .' Finally, an influence of solvent polarities on 2k, has been found for phenyl-substituted phenoxyl radicals.12 A V-shaped correlation resulted in plots of log(2kJ us. the empirical Dimroth-Reichardt solvent parameter "Q" R2 ___t CH3 pz 4 D2 Scheme 1. In this work the reactions of 2,6-di-t-butylphenoxyl (R,) and 2,6-di-t-butyl-4- methylphenoxyl radicals (R,) in different solvents were investigated (scheme 1). The tail- to-tail dimerization of R, to the semiquinone D,, followed by enolization to the di- phenol P, is irreversible and has been investigated by Weiner and Mahoney.14 Radical R, dimerizes in a head-to-tail reaction reversibly to an isolatable dimer D,.15 D, reacts on a long timescale (z > 1 min) to final products P,.After the pioneering work of Land and Porter1' the kinetics of radicals R, and R, have been investigated mainly by Weiner and Mahoney14* l7 and by Khudyakov, Levin, Burshtein, Kuzmin and co-workers1 using either optical or e.s.r. detection. The former reported a strong dependence of 2k, on solvent nature and on pH. In ref. (1H3) and (10) the reactions of R, and R, have been stated to be diffusion-enhanced, i.e. controlled by diffusion and by steric hindrance due to the bulky t-butyl groups. An activation energy for the overall rate constant equal to the activation energy of the diffusion coefficient D is then e~pected.~ However, the same authors reported activation energies which were significantly lower than the activation energy of the solvent viscosity.Moreover Griller and co-workers found diffusion control for the self-termination of the 2,4,6-tri-t-butylbenzyl radical, i.e. a radical with similar steric hindrance as R, and R,." Interpretations of 2kt in terms of intermediate complexes were also discussed in ref. (3), (4), (9) and (10) but excluded for R, in ref. (1 1). Influences of the solvent polarity on 2kt were excluded for RI2* 3* lo and have not yet been analysed for radical R,.D. Riiegge and H . Fischer 3189 In view of the previous discussions we decided to re-examine the termination rate constants of R, and R, in different solvents using kinetic e.s.r., a method which has been refined continuously in our group over the past de~ade.~ Recently, we have shown that rate constants of competing reactions in complex reaction systems with two different radicals can be obtained by time-resolved e.s.r.19 The method is straightforward. One observes and analyses the individual concentration us.time profiles of all species [R,] of a reaction system undergoing intermittent photolysis. In this work, analyses based on simple second-order kinetics could be applied for the reactions of radical R, in all solvents with the exception of isopropyl alcohol. In the latter phenoxyl and 2- hydroxyprop-2-yl radicals were generated simultaneously, and a two-radical reaction scheme was applied. A kinetically complex situation arises for R, owing to the back- reaction of the dimer D,. Here both species, R, and D,, had to be taken into account in the kinetic analyses, but only one, the radical, was detectable by e.s.r.In addition, their concentrations were inhomogeneous over the sample under conditions of our experiments which further complicated matters. Therefore, a new method of data analysis was developed. It allows the simultaneous determination of all kinetic constants in a general complex reaction system involving at least one detectable radical. Experiment a1 The experimental set-up for the stationary and kinetic e.s.r. experiments was mainly the same as in earlier papers of this ~eries.l~-~l A new sector unit with an Estan GfmO 8/4 d.c. motor reduced the width of the on+ff and the off-on transition region from ca. 4" to 2" in units of angular degrees,lg still providing high-frequency stability ( < 0.5 & over 1 h) and a range for the chopper frequency from 10 to 300 Hz.All chemicals were used in the purest available forms; 3-methylpentan-3-01 was distilled twice, 2,6-di-t-butylphenol was resublimed from the liquid at T = 50 "C and p = 10 mbar. Solutions contained 1.37 mol dm-3 di-t-butyl peroxide and 0.1-1 .O mol dm-3 of the phenol. They were photolysed in a flat cell of 0.4 mm optical pathlength using an XeHg high-pressure lamp. The light was filtered to a range of 305 < A/nm < 330, leading to negligible inhomogeneity effects due to absorption.,, The flow rates of 0.33-0.83 cm3 min-l corresponded to dwell times of 5.7-2.3 s in the reaction zone and led to conversion of phenols lower than ca. 5 %. All solutions were freed from oxygen prior to use by purging with helium.Temperatures given below are accurate to f 1 K. For the kinetic measurements the following procedure was applied. The concentration us. time profiles of the radicals were sampled during intermittent photolysis in the transient recorder with the spectrometer constant set to 19 ,us, which was lower than the channel width of the recorder (ca. 65 ps). After transfer of the data to the NAS AS/XL V60 computer of the Rechenzentrum of the University of Zurich they were analysed according to the analysis procedure described in the next section. Measurements on the kinetics of the phenoxyl radicals were preceded by experiments involving photolysis of acetone in isopropyl alcohol. From the concentration us. time profile of the 2-hydroxyprop-2-yl radical the parameters describing the light intensity as a function of time were determined.lg These parameters were then kept fixed for the subsequent measurements and had to be redetermined only after a change in the optical arrangement.For the conversion of signal voltage to radical concentration the spectrometer had to be calibrated. To this end double integrals of the phenoxyl radicals were obtained during intermittent photolysis with a high-spectrometer time constant, which averages out the concentration variation due to the modulated light intensity. The double integrals were related to those of t-butyl radicals obtained by steady-state photolysis of di-t- butylketone in tetradecane at 300 K. As in ref. (19) and (20), they were calibrated in terms of absolute concentrations by time-resolved experiments on the self-termination3190 Kinetic E.S.R.of Complex Reaction Systems of t-butyl in tetradecane at 300 K23 from the second-order lifetime with the known rate constant 2k, = 3.3 x lo9 dm3 mol-' s-l. Changes in spectrometer sensitivity were taken into account by referring to a s~bstandard,~~ the Curie correction was applied for temperature changes, and the signals were corrected for errors due to saturation. We estimate statistic errors of f 2 0 % for all the kinetic constants given below, caused mainly by uncertainties in the spectrometer calibration. 23 The diffusion coefficients of the parent phenols which served as model substances for the radicals were measured by a chromatographic-broadening technique as described in ref.(24) and (25) in the same solvents and over the same temperature range as in the kinetic experiments with the corresponding radicals. Data Analysis As mentioned above, all kinetic constants of a complex radical reaction system can be obtained simultaneously from the concentration us. time profiles of the detectable radicals. This is done by a multiparameter fit of the numerically integrated rate laws to the data for all species. For a general radical reaction system involving self- and cross- terminations and first-order processes of all radicals Ri the kinetic equations are -- d[Ri(t)l - I i ( t ) - 2k:[Rt(t)I2 - C e [ R i ( t ) ] [R,(t)] - C k"[R,(t)] + C kii[Ri(t)]. (3) d t i f i i f i i f i They are integrated numerically using a Runge-Kutta-Fehlberg 7/8 26 algorithm and the result is compared in an iterative fitting procedure with the experimental signal us.time profiles si(n). These are sampled in the kinetic runs as 512 data points in the n channels of the transient recorder. The si(n) obey si(n) = ci[Ri(t)] with t = nz, z being the channel width. The sensitivity constants ci are determined by relating measured radical concentrations to signal amplitudes si(n). Three procedures can be chosen. In the firstlg. 2o e.s.r. double integrals are recorded during steady-state photolysis under the same conditions as in the kinetic runs and converted to steady-state concentrations [RT] with the known spectrometer calibration. In the kinetic runs low chopper frequencies are used, so that the signals si(n) reach the steady-state value s? for all radicals at the end of the 'on' period, and ci is given by sy/[RT].Alternatively, one measures average radical concentrations during intermittent photolysis from double integrals obtained with high spectrometer time constant and uses the average of the kinetic signals sr = c,si(n)/512 for the conversion. In the third method, as in the first, steady-state photolysis is applied to obtain [RT]. However, the kinetic runs employ high chopper frequencies, so that the signals at the end of the 'on' period sp do not reach steady-state values. The conversion is then performed by assuming in a first stage that the sp values were equal to s?. After completion of the fitting procedure the integration of eqn (3) is then repeated with a prolonged Ii(t), and the relation between sp and sip" is obtained from the curves thus calculated.The iterative fitting procedure is based on the simplex alg~rithm,~' which systematically varies the parameters of the numerical integration starting from a given set until a minimal reduced x square is obtained. Here N is the number of observed radicals, P is the total number of parameters and xied is expected to be 1 for a correct kinetic model. The standard deviations oi of the signals are calculated from the last points in the ' off' period. The fit is followed by the determination of the one-standard deviations or. uncertainties of all parameters. By definition2* all parameter sets satisfying the relation xiest 2 x2 2 xiest + 1D.Riiegge and H. Fischer 3191 Table 1. Diffusion coefficients cm2 s-') of 2,6-di-t-butylphenol and self-termination rate constants ( lo8 dm3 mol-' s-l) of phenoxyl R, in various solvents T / K D 2k, 1,2-epoxybutane 233 4.1 1.4 235 4.3 1.5 222 3.2 1.1 226 3.5 1.3 233 4.1 1.7 240 4.8 2.1 247 5.6 2.1 253 6.4 2.4 260 7.3 2.9 267 8.4 2.8 275 9.6 3.3 281 10.6 3.3 287 11.7 3.9 294 13.0 3.8 222 229 236 242 249 254 26 1 268 273 280 287 293 300 30 1 n-heptane 3.9 4.7 5.4 6.3 7.1 7.9 9.0 10.1 11.1 12.4 13.8 15.0 16.6 16.7 1.3 1.6 1.8 2.1 2.3 2.6 2.9 3.1 3.5 4.1 4.3 4.6 4.6 4.8 T / K D 2k, acetoni trile 251 6.8 6.9 255 7.4 7.7 262 8.4 9.0 269 9.4 8.3 273 10.1 8.6 282 11.6 9.3 289 12.8 11.5 295 14.0 9.6 t- butyl benzene 230 1.3 0.58 236 1.5 0.79 242 1.8 0.93 249 2.2 1.1 255 2.6 1.3 261 3.0 1.5 267 3.5 1.7 273 4.0 1.9 273 4.0 2.0 280 4.6 2.3 287 5.3 2.4 294 6.1 2.7 3-methylpentan-3-01 255 0.76 0.59 259 0.90 0.67 264 1.1 0.83 270 1.4 1.0 276 1.8 1.2 277 1.8 1.3 283 2.3 1.5 289 2.8 1.8 290 2.9 1.8 296 3.6 2.0 302 4.3 2.3 302 4.3 2.2 309 5.3 2.5 315 6.4 2.8 lie in the confidence interval, x2 given by ~i~~(N512-P).To estimate the errors of the parameters, it is customary28 to expand x2 or the fitting function sialcd parabolically as a function of the parameters. Here, we search for the confidence intervals in a more straightforward manner. This is done for one parameter p k of interest by keeping its value fixed on pEest + Apk and running the simplex algorithm again, optimizing all other free parameters. Apk is varied until xE,,,(Apk) = &st(a:) = x;,,, + 1.a! is calculated in an analogous way. The confidence interval for p k is given by pEest - af < pk < pEest + a: and reflects a maximum standard deviation not lowered by compensation effects due to parameter correlations. In this way it is assured that parameter values lying outside the3192 Kinetic E.S.R. of Complex Reaction Systems confidence intervals lead to remarkably wrong fits [examples are given in ref. ( 5 ) ] . Parameters having only small influence on the signal profiles, like kinetic constants of side-reactions for instance, will have correspondingly large confidence intervals. The set of parameters contains first the rate constants of the underlying reaction system and secondly parameters entering the rates of initiation.These are given by IpfT@), where @ = tf,,,,, n is the sector position in rad and A@) is 0 in the ' off ', 1 in the ' on ' period and varies in the transition regions where the sector edges pass the light beam. The form of A@) was determined as described earlier,lg so that only the 14 were treated as unknown. Thirdly, as also described earlier, relative radical concentrations or relative sensitivity constants can also be regarded as fitting parameter~l~ and, as demonstrated later on, the procedure can also be applied to cases where certain species are not observable, provided their presence influences the concentration-us. time behaviour of the observable cases. Fourthly, the concentrations [Rt(t = O)] needed for numerical integration and taken from the last signal points in the 'off' period for observable radicals become a parameter for non-observable species.Additionally, a closure condition for the latter is introduced, which forces the fit to hold their concentrations equal at t = 0 and at t = 5122, i.e. at the beginning and the end of the integrated concentration us. time profiles. In the present work this is the case for the dimer D, of scheme 1. Its rate equation was set up and solved numerically together with that of the observable phenoxyl radical, the signal of the latter serving alone as base for the fit. The dimer D, was found to be rather long-lived, it is built up during the dwell time of the solution in the reacting cell. This leads to inhomogeneous concentration profiles along the flow direction.In the kinetic equations this is incorporated in the form of a first- order flow term (see Appendix). Results and Discussion Time-resolved experiments were performed with solutions of 2,4-di-t-butylphenol in acetonitrile, 1 ,2-epoxybutane7 n-heptane, 3-methylpentan-3-01 and t-butylbenzene. The e.s.r. spectra showed the signal of R,. No signals were detected in the absence of either phenol or peroxide. The concentration vs. time profiles were taken at different temperatures and analysed successfully by taking into account only second-order self- terminations and generation. The results are collected in table 1. Fig. 1 shows two traces from runs in 1,2-epoxybutane at the lower and the upper ends of the experimental temperature range. Superimposed are the best fits to integrated second-order rate laws.A similar series of experiments was run in isopropyl alcohol. Here, additionally the lines of 2-hydroxyprop-2-yl radicals .appeared (fig. 2) and the traces of both radicals were recorded using the lines marked by arrows. Analysis proceeded with kinetic equations for the reaction scheme I2 PhOH + ButO' + R, + ButOH Pr'OH + ButO' ---* Pri'OH + ButOH I t 2Pri.0H 5 } non-radical products 2kx J R, + Pr"0H ---* where R,, Pr"0H and ButO' are the phenoxyl, 2-hydroxyprop-2-yl and t-butoxyl radicals, respectively. The results are given in table 2. Traces and superimposed fits forD. Riiegge and H . Fischer 3 193 1 t t 0 c 2 0 30 ’* r/ms 0 x 2 x 2 Fig. 1. Concentration us. time profiles for phenoxyl R, in 1,2-epoxybutane at 233 K (a) and 294 K (b).The lower traces are amplified residuals. A 0 0 0 & O O A I , H 1 mT Fig. 2. E.s.r. spectrum of phenoxyl R, (0) and 2-hydroxyprop-2-yl (A) during photolysis of di-t-butyl peroxide in isopropanol containing 0.01 mol dmn-3 of the parent phenol. T = 225 K. both radicals in isopropyl alcohol at -48 “C are displayed in fig. 3. To test the kinetic model the following side-reactions were added to the above scheme: R, + Pr’OH -+ PhOH + Pr”0H Pri’OH + PhOH + Pr’OH + R, non-radical products. Pr”0H -+3194 Kinetic E.S.R. of Complex Reaction Systems Table 2. Diffusion coefficients ( cm2 s-l) of 2,6-di-t-butylphenol and self- and cross-termina- tion rate constants ( lo8 dm3 mol-1 s-l) of phen- oxyl R, and 2-hydroxyprop-2-yl radicals in isopropyl alcohol T/K D 2kt kx 2k," 223 225 225 249 249 249 263 263 263 275 275 275 0.44 0.90 0.49 1.1 0.49 0.89 1.2 2.1 1.2 2.1 1.2 2.0 1.9 2.5 1.9 2.5 1.9 2.9 2.8 2.9 2.8 3 .O 2.8 3.2 0.84 1.8 0.99 1.8 0.84 3.7 3.0 3.6 3.4 3.3 4.5 4.7 7.9 7.4 7.4 7.2 5.2 5.5 9.1 8.3 8.9 7.8 8.9 7.8 OH A CH, CH3 I - 10 20 30 tlms Fig.3. Concentration us. time profiles for 2-hydroxyprop-2-yl (a) and phenoxyl R, (b). These reactions describe hydrogen abstraction from the solvent and phenol by the radicals and pseudo-first-order decays. Fits to the data with the extended reaction scheme yielded for the additional rate constants values close to zero, i.e. these reactions are negligible. The results for all other rate constants and & were the same as in fits not accounting for side-reactions.Special attempts were made to reproduce the reported acceleration of the self- termination of R, in benzene in the presence of acid. Weiner and Mahoney14 photolysed biacetyl in benzene containing 2,6-di-t-butylphenol in steady-state experiments andD. Riiegge and H . Fischer 3195 0 0 0 A A A O 0 0 t , H I 1 mT Fig. 4. E.s.r. spectrum of phenoxyl-type radicals in benzene containing p-toluene sulphonic acid. (a) Zero or low acid concentration, (b) 26 mmol dm-3 acid. 0, R,; A, secondary phenoxyl, see text. determined 2kt in a relative way from the concentrations of R, and the semidione radical. Addition of 6 mmol dmP3 p-toluene sulphonic acid resulted in a 10 times faster self-termination. We photolysed di-t-butyl peroxide in the presence of 2,6-di-t- butylphenol in benzene with 0, 6 and 26 mmol dmP3 p-toluene sulphonic acid at room temperature.The steady-state spectra for 0 and 6 mmol dmP3 acid were identical, exhibiting the signal of R, only [fig. 4(a)]. In both cases the kinetic traces showed pure second-order reaction and no difference in radical lifetime or kinetic constant [2k, = (2.9k0.5) x 10' dm3 mol-1 s-'1. The spectrum for 26 mmol dmP3 acid is shown in fig. 4(b). Besides the signals of R, a triplet with equal splitting and g factor as for R, appears. We assign it to 2,6-di-t-butyl-4-(3,5-di-t-butyl-4-hydroxyphenyl)phenoxyl radicals. In the presence of acid the enolization reaction from D, to P, is fast enough to build up a sufficiently high concentration of P, during the dwell time. From P, the secondary radical is generated via hydrogen abstraction by ButO'.Consequently, its signal decreased for high flow rates. Analysis of the kinetics of R, revealed a slightly perturbed second-order decay, probably due to reactions of R, with the additional species, but still no appreciable change in the self-termination rate constant was observed. Similar observations were made in solvent mixtures containing di-t-butyl peroxide, t-butyl alcohol, water, phenol and 0-0.1 mol dmP3 HCl. These results are in good agreement with Levin et al.," who found no change in 2kt going from aprotic solvents to acetic acid, but observed a faster enolization reaction D, -+ P, in the acid. Probably, the results of Weiner and Mahoney are artefacts caused by the dependence of the semidione radical spectrum on PH.~' pH variation strongly influences the lineshapes which, if not fully taken into account, disturbs the determination of 2kt from concentration measurements significantly. A second series of experiments concerned the reactions of 2,6-di-t-butyl-4- methylphenoxyl radicals.Concentration us. time profiles were recorded for different3196 Kinetic E.S.R. of Complex Reaction Systems 0 10 20 30 tlms x 5 x 5 Fig. 5. Concentration us. time profiles for phenoxyl R, in t-butylbenzene at 235 K (a) and 260 K (b). Lower traces are amplified residuals. temperatures in the solvents acetonitrile, n-heptane, 1,2-epoxybutane, t-butylbenzene, 3- methylpentan-3-01 and di-n-butylphthalate. Fig. 5 shows traces obtained in t- butylbenzene at two different temperatures.They reveal a remarkable temperature dependence in shape, which was not observed for radical R, (fig. 1). This is due to the strongly activated back-reaction k , of dimer D, to radicals R,. At high temperatures this leads to a constant, non-zero radical concentration for most of the 'off' period, which reflects the equilibrium between R, and D,. At low temperatures the back-reaction is slow compared to the dwell time of the solutions in the cavity, and thus the radicals exhibit normal second-order decay. Superimposed on the traces in fig. 5 are fits to integrated forms of the rate laws ( 5 ) and (6), d[R21 = Z(t) - 2kt[R,I2 + 2k,[D,] - k,[R,] ( 5 ) dt where kfl is the dwell time. A justification of eqn ( 5 ) and (6) is given in the Appendix. As for radical R,, we tested for additional side-reactions.Pseudo-first-order reactions of either R, or D, and direct disproportionation of R, to phenol and semiquinonel' were found to be kinetically irrelevant under our conditions. The results are collected in table 3. The confidence intervals of all kinetic rate constants listed in tables 1-3 were significantly smaller than the uncertainty given by the spectrometer calibration so anD. Ruegge and H. Fischer 3197 Table 3. Diffusion coefficients ( cm2 s-l) of 2,6-di-t-butyl-4- methylphenol and self-termination (lo8 dm3 mol-’ s-l) and frag- mentation (s-l) rate constants of phenoxyl R, and dimer D, in various solvents T / K D 2k, k, T/K D 2k, k, 253 255 257 259 26 1 263 227 23 1 234 237 240 243 247 255 22 1 225 228 233 236 240 244 247 250 254 acetonitrile 6.9 2.4 1.7 7.1 2.4 2.7 7.5 2.4 3.5 7.8 2.5 4.9 8.1 2.4 6.5 8.4 2.6 8.4 L ,2-epoxybutane 3.8 0.65 0.052 4.2 0.74 0.065 4.4 0.81 0.11 4.8 0.88 0.16 5.1 0.97 0.27 5.5 0.89 0.42 5.9 0.97 0.67 7.0 1.1 2.8 n- heptane 4.3 0.91 4.7 1.1 5.0 1.1 5.6 1.3 5.9 1.3 6.4 1.5 7.0 1.5 7.4 1.6 7.8 1.7 8.3 1.8 0.01 1 0.020 0.043 0.12 0.20 0.36 0.61 1.1 1.5 2.4 3-methylpentan- 3-01 250 0.56 0.61 0.29 252 0.61 0.76 0.55 254 0.67 0.63 0.73 254 0.67 0.81 0.41 255 0.69 0.69 0.59 257 0.77 0.95 1.2 260 0.85 0.89 1.8 260 0.85 0.83 1.4 263 0.97 1.1 2.6 265 1.1 1.3 3.9 t- butylbenzene 232 1.3 0.32 235 1.5 0.38 237 1.6 0.39 241 1.8 0.49 245 2.0 0.55 248 2.2 0.61 252 2.4 0.68 255 2.6 0.71 257 2.8 0.76 260 3.0 0.87 0.049 0.097 0.13 0.26 0.46 0.81 1.4 2.0 3.1 5.1 di-n-butyl phthalate 263 0.55 0.22 1.2 263 0.55 0.22 1.3 263 0.55 0.22 1.3 269 0.69 0.23 2.4 269 0.69 0.25 2.7 269 0.69 0.24 2.6 275 0.87 0.33 9.5 275 0.87 0.32 9.9 275 0.87 0.31 9.8 overall accuracy of & 20 % is estimated.The rate data were analysed in terms of the Arrhenius law, frequency factors and activation energies given in table 4. Fig. 6 shows k, vs. temperature with the Arrhenius curves superimposed. The results for the self-termination reactions of 2-hydroxyprop-2-yl radicals are in excellent agreement with the values found by Lehni and Fischer.,l Since our values were obtained by calibrating the spectrometer to the 2,6-di-t-butylphenoxyl radical only, this cross-check assures the calibration and data-analysis procedure to be accurate and independent of the reaction systems chosen.Comparison of our data with those given in the literature for reactions of R, and R, reveals general agreement within a factor of For comparison of the data with values expected for diffusion-control double logarithmic plots of self-termination rate constants against diffusion coefficients are given in fig. 7 for radical R, and fig. 8 for radical R,. The uppermost lines with slope 1 ca. 2.3,4,9,10,14,16,17,30,313198 Kinetic E.S.R. of Complex Reaction Systems Table 4. Frequency factors and activation energies (kJ mol-l) of diffusion coefficients D (cm2 s-') of the parent phenols, of the self- and cross-termination rate constants 2k,, k, (dm3 mol-' s-l) of the radicals R,, R, and Pr'O' and of the decomposition rate constants k , (s-l) of the dimer D, in various solvents D 2kt kl reaction system radical solvent log A Es logA E, logA E a R,, 2k, acetonitrile -3.08 ( 5 ) 10.0 (3) 9.9 (3) 4.9 (13) - - n- heptane -3.01 ( 5 ) 10.2 (3) 10.2(1) 8.8 (3) - - 3-methylpentan- - 1.3 (1) 23.7 (8) 11.2 (1) 16.2 (6) - - R,+Pri0'2k: isopropyl alcohol -2.2 (1) 17.9 (6) 10.5 (2) 10.5 (9) - - k, isopropyl alcohol - - 12.7 (4) 19.8 (22) - - 2k," isopropyl alcohol - - 11.4(4) 13.3 (2) - - 1,2-epoxybutane -2.99 ( 5 ) 10.7 (3) 10.1 (3) 8.5 ( 5 ) - - 3-01 t-butylbenzene -2.74 (8) 13.9 ( 5 ) 10.7 (1) 12.5 (4) - - R,, 2k, acetoni trile -2.92 ( 5 ) 10.9 (4) 8.9 ( 5 ) 2.7 (26) 16.8 (7) 80 (3) n- heptane -3.03 ( 5 ) 10.0 (3) 10.1 (2) 8.9 ( 5 ) 15.1 (2) 72 (1) 1,2-epoxybutane -2.95 (6) 10.7 (4) 9.6 (3) 7.8 (12) 17.0 (8) 81 ( 5 ) 3-methylpentan- - 1.3 (1) 23.9 (6) 13.0 (9) 25.1 (45) 19 (1) 93 (5) t-butylbenzene -2.69 (8) 14.1 ( 5 ) 11.3 (2) 16.6 (7) 16.8 (7) 80 (3) 3-01 di-n-butyl - 1.8 (3) 22.8 (21) 11.3 ( 5 ) 20 (3) 25 (2) 126 (10) phthalate Errors are given in units of the last digit quoted.7 I 0 - 40 - 30 -20 - 10 0 T/"C Fig. 6. Rate constants for the back reaction of dimer D, us. temperature in various solvents: ., acetonitrile; A, 1,2-epoxybutane ; +, n-heptane : 4,3-methylpentan-3-01; V, t-butylbenzene ; b, di-n-butyl phthalate.D. Ruegge and H. Fischer 3199 0.2 0 . 5 2 5 20 Fig. 7. Rate constants for the self-termination of phenoxyl radical R, us. diffusion coefficients of the parent phenol in various solvents : 0, acetonitrile ; 0, isopropyl alcohol ; A, 1,2-epoxybutane ; 0, n-heptane; 4, 3-methylpentan-3-01; V, t-butylbenzene.D/10-6 cm' s-' indicate diffusion-controlled reaction, the rate constant given by the classical von Smoluchowski limit 2k, = a8d000-'NL Dp. As in previous ''3 20, 21, 23 the reaction distance p = 9.2 x cm has been taken from the diameter of the radicals, which was estimated from geometric increments and the density of liquid 2,6-di-t-butylphenol at 40 "C for both radicals. All rate constants are significantly below the diffusion limit, with the exception of the data for isopropyl alcohol. For diffusion-enhanced reactions3 the data are supposed to form straight lines parallel to the line demarcating the diffusion limit. Obviously this does not hold for the most of the solvents. Quantitatively this can also be seen from table 4.For several cases the activation energies of 2k, are significantly lower than those of D. Low activation energies of self-termination rate constants observed before were tentatively explained3 by a failure in the approximation of D by the Stokes-Einstein relation [eqn (2)]. Since we compare directly with the diffusion coefficient this reasoning is not applicable here. On the other hand, low activation energies can also be explained using models which involve the formation of an intermediate C in the reaction pathway from radical to dimer and vice Then the reaction scheme extends to ks kC kD R + R e C e D i.e. the radicals R react in the first diffusion-controlled step to a short-lived intermediate C, which can be either just a radical pair or a weakly stabilized complex as proposed for benzyl radicals (isoelectronic with phen~xyl).~~' 33 C decays to free radicals and dimer D3200 Kinetic E.S.R.of Complex Reaction Systems 0.2 0-5 2 5 20 D/ cm2 s-' Fig. 8. Rate constants for the self-termination of phenoxyl radical R, us. diffusion coefficients of the parent phenol in various solvents, for symbols see fig. 6. with the rate constants k; and k:, respectively. With a stationary ansatz for the intermediate the observed self-termination rate constant becomes The solid lines in fig. 7 and 8, drawn for selected solvents only, result from a data analysis using eqn (7). 2k, was calculated from experimental diffusion coefficients, and k; and k: were replaced by Arrhenius expressions.Two parameters resulted from these fits, the ratios of A factors, &/A:, and the differences of the activation energies, &-E& The data, collected in table 5, show that A; is larger than A:, thus supporting the model. Further, the positive values of 4 - for most of the solvents yield the energy profile shown in fig. 9. From kJk; and eqn ( 8 ) the activation parameters of the true decomposition rate constant k , of the dimer D, can be calculated from the experimental constants k,. Within the experimental errors the same parameters as for k, resulted for k,, i.e. the observed rate constant k, is close to kD. With the exception of the high values for 3-methylpentan-3-01 and di-n-butyl phthalate solvents (table 4) the frequency factors and activation energies of k , (k,) are reasonable for a bond-breaking process.34 Assuming the exceptionally high values to be caused mainly by error compensation we redetermined the activation energies in restricted fits with the frequency factors kept fixed at the mean value of log(A',eS) = 16.4 from the other solvents.Good fits then resulted for all solvents. A comparison of the resulting data (table 5 ) shows a variationD. Riiegge and H. Fischer 320 1 Table 5. Frequency factors and activation energies (kJ mol-') of the ratio of rate constants k; and k: (see text) and of the decomposition rate constant k , from fits with restricted logA,, and the stabilization energy AE (kJ mol-') in various solvents of molar polarization [PI (cm3 mol-') reaction system k,lk: k , AE us.[PI radical solvent AJA: 4-q logA',"" ET AE [PI ace toni trile 60 (40) 6.8 (18) - isopropyl alcohol 660 (400) 12.9 (20) - 1,2-epoxybutane 28 (7) 2.5 (6) - 3-methylpentan-3-01 210 (60) 9.1 (6) - Rl n-heptane 20 (3) 1.5 (3) - t-butylbenzene 13 (25) 1.6(9) - R2 acetonitrile 650 (400) 9.0 (15) 16.4 1,2-epoxybutane 91 (40) 3.1 (10) 16.4 n-heptane 22 (5) 0.8 (5) 16.4 3-methylpentan-3-01 1 (9) - 1.8 (57) 16.4 di-n-butyl phthalate 32 (40) 3.0 (30) 16.4 t- buty lbenzene 3 (10) -2.7 (7) 16.4 3.2 (28) 14.8 (8) 4.9 (39) 25.4 (16) 8.2 (16) 24.4 (14) 8.7 (13) 37.8 (6) 14.5 (16) 41.4 (24) 12.3 (14) 49.0 (24) 1.9 (25) 14.8 (8) 7.6 (20) 24.4 (14) 8.8 (15) 37.8 (6) 26 (7) 41.4 (24) 16.8 (17) 49.0 (24) 19.7 (40) 84.7 (42) E - reaction coordinate Fig. 9. Energy-level scheme for the radical termination involving an intermediate complex.of E',"' of only 5 O h . The slightly higher activation energies for 3-methylpentan-3-01 and di-n-butyl phthalate contribute to the clearly lower k, values for these (fig. 6 ) . However, for 3-methylpentan-3-01 the low value of k, is partially caused also by the relatively high termination constant (fig. 8) since this also leads to a low k, [eqn (7) and @)I. From the difference in activation energies & - 4 the energy gap AE between the free radicals R in solution and the transition state CDt follows as AE = E, + - 4 (fig. 9). As table 5 shows, AE varies similarly for both radicals with changing solvents. In a search for a solvent parameter which explains this tendency we found that AE correlates with the molar polarization [PI as has been observed for several non-ionic organic ~eacti0ns.l~ For a pure solvent [PI is given by the sum of electronic and atomic p~larization~~ and can be estimated via 1 n k - l M [PI = (1 +fJ----- 1.05n2,+2 p (9)3202 Kinetic E.S.R.of Complex Reaction Systems 0 20 40 60 00 100 [p3/m3 mo~-l Fig. 10. Stabilization energy of radicals R, and R, us. molar polarization of the solvent, for symbols see fig. 6 and 7. from the refractive index n,, the molecular weight M, the density p of the solvent and the contribution of the atomic polarization 0.06 < fA < 0.36.35 The factor 1.05 corrects for the difference between the measurable n, and the required nca, the refractive indices for Na D-line and infinite frequency. For solvent mixtures [PI is approximated by [PI = c x,[P],.A plot of AE us. [PI reveals a straight line with positive slope and an intersection close to zero (fig. 10). The vertical error bars are estimated from statistical errors in the fits; horizontal bars indicate the uncertainty in the estimation off!. We attribute the positive slope of AE us. [PI to a stabilization of the free radicals R in solvents of high polarization, wiiich is higher than that of the transition state CD:. The stabilization is provided by dipole-induced forces, which depend linearly on the molar polarization [PI of the solvent, on the dipole moment of the dissolved species and on their average distances. The dipole moments of R, and R, are estimated from the dipole moment of the similar 4-hydroxylphenoxyl radical36 as p > 4 D,? whereas the dipole moment of the transition state CDf is certainly lower.The deviation of the observed self-termination rate constants from diffusion control therefore results from the extra stabilization of the free radicals. London interactions3' and dipole-dipole interactions may also contribute to the overall interactions between solvent and solute, of which the latter are usually assumed to be important. Yet, as has been demonstrated c~nvincingly,~~ this holds for small molecules only. For large molecules the interaction is small, and in our radicals R, and R, the bulky t-butyl groups provide sufficient separations between the dipole moments of the radic 11s and solvent. In contrast, the dipole-induced-dipole interactions are transferred via tt-e non-polar parts of molecules.Conclusions Self-termination rate constants 2k, of 2,6-di-t-butyl-substituted phenoxyl radicals have been measured using kinetic e.s.r. spectroscopy. The rate constants are up to two orders of magnitude lower than expected for diffusion c o ~ t m l and depend characteristically on the diffusion coefficient of the radical and the solvent polarization. The latter t 1 D = 3.33564 x C m.D. Riiegge and H. Fischer 3203 dependence lends evidence against mechanisms involving diffusion and reorientation effects only. In fact, explanations on the basis of steric hindrance due to the bulky t-butyl groups contradict our observation of diffusion control for the termination of one of the radicals in isopropyl alcohol as well as the diffusion-controlled termination of 2,4,6-tri-t- butylbenzyl.la We propose a two-step process for the overall termination reaction. The first step consists of a diffusion-controlled formation of an intermediate C . In a second step C rearranges via an activated transition state CD' to the final product or cleaves to radicals. In highly polarizable solvents the free radicals are stabilized due to interactions of the radical dipole moment with the solvents with respect to CDx. This leads to a relatively high transition state CDf and to a slowing of the overall termination. Appendix Kinetic equations ( 5 ) and (6) were used in the analyses of the reactions of the 2,6-di-t- butyl-4-methylphenoxyl radical R, and its unstable dimer D,. Besides terms for the generation and the reactions of the species they contain first-order terms to account for the disappearance of the species from the sample cell due to the flow of the solvent, k,' given by the dwell time.Such a treatment is correct for homogeneous concentrations. In our case, however, the dimer D, is built-up during the dwell time. Thus the concentrations of D, and radical R, are inhomogeneously distributed along the cell and eqn ( 5 ) and (6) can be regarded as approximations only. They also do not account for inhomogeneous laminar flow velocities along y or for inhomogeneous spectrometer sensitivity along the same coordinate. A rigorous derivation of correct kinetic equations for the experimentally observed mean concentrations could not be found. To test for the validity of the approximation we performed a simulation of the true situation and checked whether eqn ( 5 ) and (6) are in fact able to cover it.The simulation accounted for the spectrometer geometry, its sensitivity distribution and for a parabolic distribution of the flow velocity along the x coordinate. The time required for a single experiment corresponds to the time for one 'on-off' period (fig. 1, 3, 5 ) and is typically z~~~ z 33 ms, i.e. much shorter than the dwell time (2.3-5.7 s). This allows splitting of the cell into a large number of small subcells, in which the concentrations are practically homogeneously distributed. As shown in fig. 11, the cell is divided along x into layers of equal thickness with parabolically distributed flow speeds. The individual layers are split along y into subcells.The lengths of these are chosen so that the dwell time in all subcells is equal to the time z~~~ for one experiment. Depending on the flow rate and the number of layers chosen, a total of up to 3000 subcells resulted for typical conditions. For a given set of kinetic constants all the signal us. time profiles from the individual subcells were calculated by numerical integration of rate equations for homogeneous concentrations followed by a multiplication with the spectrometer sensitivity cos2 (ay), with a known from the geometry of the spectrometer cavity. The concentrations of R, and D, at the beginning of the experiment in an individual subcell were set equal to the concentrations calculated for the preceding subcell at the end of the experiment.This means that experiments occurring parallel were in fact calculated serially, starting with zero concentration for the first subcell in every layer. The passing of concentrations from one subcell to the next physically corresponds to a pulsed flow. The large number of subcells yields a correspondingly large number of pulses in quick succession, i.e. it describes properly the continuous flow situation. Finally all simulated signal us. time profiles of the subcells were added to give one profile and were passed to the program normally used for kinetic evaluations. The kinetic rate constants k,, k,, and I" were fitted as described above to the profiles using the kinetic equations ( 5 ) and (6). For the mean concentrations [D,](t = 0) needed for the fits [D,](t = 0) = I0ki1/16 was used, i.e.half the concentration at the upper end of the sample cell, corrected for stoichiometry and the 1 :4 'on-off' ratio. This approach is3204 Kinetic E.S.R. of Complex Reaction Systems 1 y , f [ o w X 0 Fig. 11. Division of the sample cell into layers along the optical axis x and into subcells according to a parabolic flow speed profile along the flow direction y. Side view of the bottom part. correct since [D,] % [R,]. Within 0-25% deviation the constants resulting from the fits were identical to the constants used in the simulation. Thus eqn (5) and (6) seem reasonable. Presumably, they lead to lower errors even than those indicated by the deviations since the effects of convection and depletion which flatten out inhomogeneous concentrations were not taken into account.This work was supported by the Swiss National Foundation for Scientific Research. References 1 E. T. Denisov and I. V. Khudyakov, Chem. Rev., 1987,87, 1313. 2 I. V. Khudyakov and B. I. Yakobson, Rev. Chem. Intermed., 1986,7, 271. 3 A. I. Burshtein, I. V. Khudyakov and B. I. Yakobson, Prog. React. Kinet., 1984, 13, 221. 4 I. V. Khudyakov, P. P. Levin and V. A. Kuzmin, Russ. Chem. Rev., 1980, 49, 982. 5 H. Fischer and H. Paul, Ace. Chem. Res., 1987, 20, 200. 6 Landolt-Bornstein, New Series. Group 11, vol. 13: Radical Reaction Rates in Liquids, ed. H. Fischer 7 M. von Smoluchowski, 2. Phys. Chem., 1971, 92, 129. 8 D. J. Williams and R. Kreilick, J. Am. Chem. Soc., 1968, 90, 2775. 9 P. P. Levin, I.V. Khudyakov and V. A. Kuzmin, Int. J. Chem. Kinet., 1980, 12, 147. (Springer, Berlin, 1984). 10 P. P. Levin, I. V. Khudyakov and V. A. Kuzmin, Izv. Akad. Nauk SSSR, Ser. Khim., 1980,2,255; Engl. 11 I. V. Khudyakov, G. R. H. I. de Jonge and V. A. Kuzmin, Izv. Akad. Nauk SSSR, Ser. Khim., 1978,7, 12 I. V. Khudyakov, P. P. Levin, V. A. Kuzmin and G. R. H. I. de Jonge, Int. J. Chem. Kinet., 1979, 11, 13 C. Reichardt, Solvent Effects in Organic Chemistry, ed. H. F. Ebel (Verlag Chemie, Weinheim, 14 L. R. Mahoney and S. A. Weiner, J. Am. Chem. SOC., 1972, 94, 585. 15 H-D. Becker, J. Org. Chem., 1965, 30, 982. 16 E. J. Land and G. Porter, Trans. Faradzy Soc., 1963, 59, 2016. 17 S. A. Weiner, J. Am. Chem. Soc., 1972, 94, 581. 18 D. Griller, L. R. Barcley and K. U. Ingold. J. Am. Chem. Sac., 1975, 97, 6151. transl. : Bull. Acad. Sci. USSR, Div. Chem. Sci., 1980, 29, 175. 1492; Engl. transl.: Bull. Acad. Sci. USSR, Div. Chem. Sci., 1979, 27, 1304. 357. 1979).D. Riiegge and H. Fischer 3205 19 D. Ruegge and H. Fischer, Znt. J. Chem. Kinet., 1986, 18, 145; 1987, 19, 719. 20 K. Munger and H. Fischer, Znt. J. Chem. Kinet., 1984, 16, 1213. 21 M. Lehni and H. Fischer, Znt. J. Chem. Kinet., 1983, 15, 733. 22 H. Fischer, H. Paul, K. Munger and T. Dschen, J. Chem. Soc., Perkin Trans. 2, 1985, 213. 23 H-H. %huh and H. Fischer, Helv. Chim. Acta, 1978, 61, 2130. 24 C. Huggenberger, J. Lipscher and H. Fischer, Znt. J. Chem. Kinet., 1979, 11, 705. 25 G. Taylor, Proc. R. SOC. London, Ser. A, 1953, 219, 186; 1954, 235, 473. 26 E. Fehlberg, Computing, 1969, 4, 93. 27 M. S. Caceci and W. P. Cacheris, Byte, 1984, 9(5), 340. 28 P. R. Bevington, Data Reduction and Error Analysis (McGraw-Hill, New York, 1969). 29 H. Zeldes and R. Livingston, J. Chem. Phys., 1967, 47, 1465. 30 V. A. Daragan, E. Ae. Ilina and I. V. Khudyakov, Jz. Phiz. Khim., 1986, 60, 572. 31 A. I. Burshtein, I. V. Khudyakov and P. P. Levin, Zzv. Akad. Nauk SSSR, Ser. Khim., 1980, 2, 261; 32 R. Zahradnik and P. Carsky, Prog. Phys. Org. Chem., 1973, 10, 327. 33 H. Langhals and H. Fischer, Chem. Ber., 1978, 111, 543. 34 S. W. Benson, The Foundations of Chemical Kinetics (McGraw-Hill, New York, 1960). 35 C. J. F. Bottcher, Theory of Electric Polarisation (Elsevier, Amsterdam, 1952). 36 R. W. Fessenden, P. M. Carton, H. Paul and H. Shimamori, J. Phys. Chem., 1979, 83, 1676. 37 F. London, 2. Phys., 1930, 63, 222. 38 E. F. Meyer and R. E. Wagner, J. Phys. Chem., 1966, 70, 3162. Engl. transl. : Bull Acczd. Sci. USSR, Div. Chem. Sci., 1980, 29, 261. Paper 8/01551A; Received 18th April, 1988

ISSN:0300-9599    

DOI:10.1039/F19888403187

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chern. SOC., Faraday Trans. I , 1988, 84(10), 3207-3214 Crystalline Heterocyclic Radical Cation Salts as Stable Intermediates of New Redox-mediating Systems Andreas Schulz, Wolfgang Kaim* and Hans-Dieter Hausen Institut fur Anorganische Chemie der Universitat Stuttgart, Pfafenwaldring 55, D- 7000 Stuttgart 80, Federal Republic of Germany N,W-Dialkyl-pyrazinium and -quinoxalinium radical cations have been isolated as stable iodide or tetraphenylborate salts. E.s.r. spectroscopy of these materials reveals delocalization of seven 7~ electrons in the 1,4-diazine ring, but no intermolecular spin-pairing in the solid state, results which are supported by structural data for the pyrazinium radical cation salt. The thermodynamic stability and chemical persistence of these intermediates suggest their use as electron-transfer mediators by analogy with the methyl viologen system.Radical cations of diquaternary N-heterocycles have found wide applications in such different areas as material sciences, lP4 molecular ar~hitecture,~? solar-energy conversion electro-organic synthesis, lo herbicide development l1 and bio(e1ectro)- chemistry.l27 l3 While the intermediate character of such species between the fully oxidized dications and fully reduced neutral molecules in a two-step reversible redox system (I)', +e- +e- (1) M2+ + Me+ A Ered - - M - -e -e leaves them as essential species in electron-transfer processes, their radical character has often been used to construct new ordered materials with special electrical or optical properties. The prototype of this class of compounds is the extensively employed blue methyl viologen radical cation,l4' l5 which exists at negative potentials between -0.7 and -0.2 V us.SCE in acetonitrile; other important species of that kind are the N,N- dialkylphenazinium radical cations,' which are formed in the more positive potential region, between 0.0 and + 1.0 V us. SCE.16 We report here the characterization of a new group of such radical cation intermediates 1-7 which are stable at potentials between -0.6 and +0.6 V us. SCE and thus neither strongly oxidizing or reducing,l'. l8 chemically persistent and isolable as pure and crystalline salts with counterions such as I- or BPh,, and effectively delocalized with seven n electronslg over the six centres of the 1,4-diazine ring system. 1 I R 2 R = M e 3 R = E t 4 R = Me, R' = 6,7-Me2 5 R = Et, R' = 6,7-Me2 6 R = Me, R' = 2,3-Me2 7 R = Me, R' = 2,3,6,7-Me4 32073208 New Redox- Mediating Systems High-resolution e x .data will be presented and discussed for the six radical cations 1-7. The structurally characterized system 1 has also been studied in the solid state. Isolated materials include l(BPhi), 3(1-), 3(BPhJ, 4(BPh,) and S(BPh;). Energy data, such as redox potentials and electronic absorption features of some species, have been reported previously" and will be discussed in detail together with relevant m.0. calculation results in a separate publication.20 Experimental E.s.r. spectra were recorded on a Varian E9 spectrometer in the X-band. g Factors and coupling constants were determined by use of the double-cavity technique with the perylene radical anion in 1,2-dimethoxyethane as reference.21 Simulation of spectra was performed using a program described previously.22 Cyclic volammetry was performed with ca. mol dm-3 solutions of the compounds on a P.A.R. 363/Bank VG 72 system using a glassy carbon working electrode; the electrolyte was 0.1 mol dmP3 tetrabutyl- ammonium perchlorate in acetonitrile. Elemental analyses were obtained using Perkin-Elmer 240 equipment ; C values were sometimes found too small because of boron carbide formation. Crystallographic details concerning the structure of l(BPh,) will be published elsewhere. Dications of the diquaternary salts were prepared according to the procedure of Curphey and PrasadZ3 from the appropriate heterocycles and trialkyloxonium tetrafluoroborates in 1,2-dichloroethane.1 7 9 2o Radical cations for e.s.r. measurements are formed best by dissolution of the dication bis(tetrafluoroborates) in basic solvents of high dielectric constant such as water or alcohols. The use of less polar solvents or of the tetraphenylborate salts was found to cause severe anisotropic line broadening due to ion pairing, thus lowering the attainable e.s.r. resolution. Attempts to isolate tetrafluoroborate salts of these ' solvent-generated ' radical cations were unsuccessful. Only stoichiometric reduction of the dications with iodide proved to yield pure, isolable cation radical salts which could be converted to suitably crystalline tetraphenyl bo rates. N, N'- Die t h y lquinoxalin ium Iodide 3(I-) N,N-Diethylquinoxalinium bis(tetrafluoroborate) (1.90 g, 5.2 mmol) was dissolved in 30 cm3 acetonitrile and mixed with 2.40 g (16 mmol) NaI in 60 cm3 CH3CN.After removal of the solvent under vacuum, the residue was treated with dichloromethane, which dissolved only the radical cation product. Filtration from insoluble sodium tetrafluoroborate, evaporation of the solvent and crystallization from a dark brown acetonitrile solution yielded 0.37 g (22.6%) of the product as dark brown crystals (found: C, 45.31 ; H, 5.10; N, 8.90. C,,Hl,IlN2 requires : C, 45.73; H, 5.12; N, 8.89). N,N'- Diethylpyrazinium Tetraphenylborate 1 ( BPh,) NaI 0.15 g (1 mmol) in 10 cm3 acetonitrile was added dropwise to a solution of 0.71 g (1 mmol) N,N-diethylpyrazinium bis(tetrafluorob0rate) in 10 cm3 CH3CN.The green- brown solution was then added to 0.38 g (1.02 mmol) sodium tetraphenylborate in 10cm3 acetonitrile. Addition of 100 cm3 water gave a dark precipitate which was recrystallized from CH3CN to yield 0.079 g (1 7.8 "/o) of dark green crystals. (Found : C, 81.24; H, 7.46; N, 5.91. C3,H3,B1N, requires: C, 84.02; H, 7.49; N, 6.12.) Similar procedures gave the following compounds.A . Schulz, W. Kaim and H-D. Hausen 3209 N,N'-Diethylquinoxalinium Tetraphenylborate 3(BPh,) Yield 28 '10. (Found: C, 85.73; H, 7.13; N, 5.59. C,,H,,B,N, requires C, 85.20; H, 7.95; N, 5.52.) N,N'-Dimethyl-6,7-dimethylquinoxalinium Tetraphenylborate 4(BPh,) Yield 3%. (Found: C, 85.20; H, 7.12; N, 5.40. C,,H,,B,N, requires: C , 85.20; H, 7.12; N, 5.52.) N, N'- Diethyl-6,7-dimethylquinoxalinium Tetraphenylborate S(BPh,) Yield 6 YO.N, 5.23.) Formation (Found: C, 83.74; H, 7.37; N, 5.62. C,,H,oBlN2 requires: C , 85.22; H, 7.53; Results and Stability of the Radical Cations It has been reported previously'7* ''v 2 3 9 24 that pyrazinium and quinoxalinium dications form radical cations in most solvents except for the least basic ones (acetonitrile, trifluoroacetic acid, concentrated sulphuric acid) by a pathway that is still not well understood. l9 Similar ' base-induced ' radical formation from dications, albeit at much higher pH values, has been observed and d i s c ~ s s e d ~ ~ , ~ ~ for the methyl viologen system. Apparently, a deprotonated dication acts as an effective reductant towards other di~ations.~,.26 Despite the mechanistic uncertainty surrounding this reaction, the e.s.r. spectra of ' reaction solutions ' of the dications, e.g. in water are often very well resolved (fig. l), whereas anisotropic line broadening due to ion pairing27 frequently impairs the e.s.r. resolution in the case of the less water-soluble tetraphenylborate salts. However, attempts to isolate tetrafluoroborate salts of these ' solvent-generated ' radical cations were unsuccessful. The presence of unknown side products as well as a possible fluoride attack at acidic hydrogens may preclude the formation of stable crystalline materials by this route. It is well known that the a protons of alkyl substituents in radical cations can be quite acidic.2s Only rational, stoichiometric reduction of the dications with iodide proved to yield pure, isolable radical cation salts which could be converted to crystalline tetraphenylborates.While a number of stable crystalline radical cation salts could be obtained in this way, the chemical stability, radical concentration and e.s.r. resolutions were low for the two 2,3-dimethyl-substituted quinoxalinium systems 6 and 7. This is seen as an indication of steric interference of four vicinal alkyl groups at the 1,4-diazine ring [see (111), later]. Despite having a planar ring conformation in the energy minimum, the seven n-electron configuration in the 1,4-dihydropyrazine radical cation exhibits a less steep potential well than the six n-electron form and is thus more susceptible to perturbation by steric substituent effects.,' Any deviation from planarity would reduce the amount of stabilization by cyclic n-electron delocalization, an effect that is also reflected by the e.s.r.data. Nevertheless, the stability of the unperturbed seven n-electron radicals is remarkable : they exist quite comfortably in an intermediate redox range around 0 V us. SCE with electrochemical stability constants Kcom exceeding 1 O'O, K~~~ = 10~~/59mv (2) whereas the diamagnetic neighbouring oxidation states with six and eight cyclically conjugating n electrons are often more reactive.". 303210 New Redox- Mediating Sys terns I Fig. 1. E.s.r. spectrum of 1 in water (a) and its computer simulation with 0.015 mT line- width (b). E.S.R. Spectroscopy : Structure and Spin Distribution The high e.s.r.resolution of the radical cations in polar solvents as illustrated in fig. 1 and 2 allows determination of the coupling constants in spite of the large number of theoretical lines (e.g. 5145 for 7). The coupling constants obtained from best-fit computer simulations are summarized in table 1 together with the data for N,N-dimethylpyrazinium cation 824 and the electrochemical stability range, i.e. the redox potentials of the radical cations. Assignments for the quinoxalines have been made in accordance with those of a previous study on N,N'-diprotonated quinoxaline radical anions ;31 earlier studies on a less complete series of quinoxaline radical anion derivatives gave conflicting results.32* 33 The data for the 'heterocyclic' protons lH(CH) are not very different from those of the N-H radical cations, i.e. the diprotonated radical anions.31 This is not unexpected since studies of the pyrazinium system have shown a comparable perturbation effect of a proton and an alkyl group on the spin distribution within the heter~cycle.~~ The overall spin distribution in the quinoxalinium radicals, especially in system 7, resembles that of 1 ,Sdihydroflavin radical cations35 (11), although the latter suffer significant perturbation from the unsymmetrically annelated pyrimidinedione ring.31.35 R = methyl, lumiflavin R = ribityl, riboflavin (vitamin B2)A . Schuiz, W. Kaim and H-D. Hausen 321 1 Fig. 2. Low-field section of the e.s.r. spectrum of 2 in water (a) and its computer simulation with 0.03 mT linewidth (b).Table 1. E.s.r. and electrochemical data of N,W-dialkylpyrazinium and -quinoxalinium radical cationsa - radical a(14N) a,(NR) a,(2, 3) a,(5, 8) a,(6, 7) Ered E o x - 1 850 540 290 - - -0.67 0.36 8d 836 800" 285 3 763 407b 355 99 136 -0.34 0.58 5 732 403b 332 75 167 -0.41 0.48 6 700 580" 330" 52 123 - 0.39 0.38 7 655 563" 362" 46 198' - 0.48 0.29 a E.s.r. results from aqueous solutions, coupling constants a in pT (100 pT = 1 G). Electrochemical potentials in V us. SCE from cyclic voltammetry in acetonitrile. Methylene protons. Methyl protons. Ref. (24), redox potentials not determined. " Peak potential, irreversible process due to adsorption. 23" d d - - 2 742 690" 370 92 142 -O.3Oe 0.60 4 740 69Y 340 61 1 72b -0.44" 0.48 Slight differences between the N-H and N-alkyl quinoxalinium radical cations are nevertheless significant : a(14N-alkyl) is ca.80 pT larger than a(14N-H) and a(2,3) is larger for the protonated species, both effects are well known from the studies of related heterocyclic ~ysterns.~~.~' On the other hand, the absolute value of a(5,8) is larger for the N,N-dialkylated systems in sterically unperturbed cases (2-5), but smaller for the3212 New Redox- Mediating Systems Fig. 3. Molecular structure of 1 in the crystal with BPh; as anion. Average distances in the ring are d(C-N) = 137.1 pm and d(C-C) = 134.0 pm. 1,2,3,4-tetra-alkyl species. The assumption that this different e.s.r. response of the sterically encumbered (7) peri-protons H(5,8) is related to the steric strain in the 1,4- diazine ring is supported by the fact that the more distant nuclei in the 6,7-position show no such effect.The coupling of the a protons of the alkyl group is fairly large, whereas the p protons of the ethyl group could only be resolved for the pyrazinium system (fig. 1). The structure of this cation in the crystal with tetraphenylborate anions shows a conformation (fig. 3) of the CH,CH, substituents which is close to the expected energy- minimum arrangement in solution, i.e. the dihedral angle between the ring plane and the N-C(H,)-C(H,) plane is not far from 90". According to the Heller-McConnell equation for o/n hyperconjugative spin tran~fer,,~ a 60" angle between the C-H(meth- ylene) bond and the spin-bearing n system should lead to a halving of the coupling relative to that in a methyl group with an average angle of 45"; 'R' v a l ~ e s ~ ~ ~ ~ ~ of ca.0.59 in the case of the quinoxalines and of 0.65 for the pyrazinium system indicate a considerable degree of sterically [peri-interactions (111) in quinoxalines !] and electronically restricted rotation.41A . Schulz, W. Kaim and H-D. Hausen 3213 An examination of the interesting ratio a(N)/a(NCH,) reveals ‘ normal ’ values of 1.0-1.136 for sterically unperturbed radicals (2,4,8), but high ratios of ca. 1.2 for the sterically congested N-CH, arrangement in 6 and 7 (5). This points to less efficient a/n hyperconjugation in the latter because of deviations from coplanarity within the ring or between the ring(s) and the methyl C atom, in agreement with the diminished chemical stability, as mentioned earlier.In the unencumbered radical cation system 1 with BPh, as counterion, there is a completely planar six-membered ring which ensures effective delocalization of seven 71 electrons. The average CC and CN bond lengths in this ring (fig. 3) reflect exactly that situation; although the CC distance is a little shorter than the CN bond length because of the higher electronegativity of N versus C,42 the values lie almost exactly between those found for organometal-stabilized 1,4-dihydropyrazines with eight 7~ and those of p ~ r a z i n e , ~ ~ the ‘aromatic’ parent heterocycle with six 7~ electrons. The relative ‘innocence’ of these radical cations, i.e. their low reactivity, is nicely reflected by fact that the crystal structure of 145 revealed no close contacts between the radical cations and the BPh, anions (>360 pm) or between the cations themselves (>740 pm).E.s.r. evidence for the absence of spin pairing in the solid comes from powder measurements, which gave an intense exchange-narrowed signal with 0.4 mT peak-to-peak linewidth at g = 2.0034; in contrast to more reactive and wider delocalized radical cations,2 these systems apparently do not tend towards self-aggregation. In conclusion, this study shows that it is not the extent but the effectivity of cyclic spin delocalization that ensures the stability of a n radical;lg the remarkable stability and facile modification of the systems presented here stimulate the search for their application in the fields mentioned at the beginning.’-l3 Support for this work has come from a grant of Stiftung Volkswagenwerk, from the Fonds der Chemischen Industrie and the Flughafen Frankfurt/Main AG.W. K. was in receipt of a Karl Winnacker Fellowship (1982-1987). We thank Prof. A. L. Rieger (Brown University) for making preprints of her work available to us. References 1 P. J. Nigrey, in Extended Linear Chain Compounds, ed. J. S. Miller (Plenum Press, New York, 1983), 2 H. J. Keller and Z. G. Soos, Top. Curr. Chem., 1985, 127, 169. 3 G. J. Ashwell, Phys. Stat. Sol. (b), 1978, 86, 705. 4 A. Graja, L. Firley and A. Rajchel, Mol. Cryst. Liq. Cryst., 1985, 120, 121. 5 A. J. Blacker, J. Jazwinski and J-M. Lehn, Helv. Chim. Acta, 1987, 70, 1. 6 T. S. Arrhenius, M. Blanchard-Desce, M. Dvolaitzky, J-M.Lehn and J. Malthete, Proc. Nut1 Acad. 7 M. Kirch, J-M. Lehn and J-P. Sauvage, Helv. Chim. Acta, 1979, 62, 1345. 8 J. Kiwi, K. Kalyanasundaram and M. Gratzel, Struct. Bonding (Berlin), 1981, 49, 37. 9 A. Launikonis, A. W. H. Mau, W. H. F. Sasse and L. A. Summers, J . Chem. SOC., Chem. Commun., 10 G. Balavoine, D. H. R. Barton, J. Boivin, A. Gref, N. Ozbalik and H. RiviCre, J. Chem. SOC., Chem. 11 L. A. Summers, The Bipyridinium Herbicides (Academic Press, New York, 1980). 12 R. C. Prince, S. J. G. Linkletter and P. L. Dutton, Biochim. Biophys. Acta, 1981, 635, 132. 13 N. Kito, Y. Ohnishi, M. Kagami and A. Ohno, Chem. Lett., 1974, 353. 14 S. Hunig and H. Berneth, Top. Curr. Chem., 1980, 92, 1. 15 A. L. Rieger and P. H. Rieger, J . Phys. Chem., 1984, 88, 5845. 16 R.F. Nelson, D. W. Leedy, E. T. Seo and R. N. Adams, 2. Anal. Chem., 1967, 224, 184. 17 W. Kaim, Heterocycles, 1985, 23, 1363. 18 Y. Sakamoto, G. Matsubayashi and T. Tanaka, Inorg. Chim. Acta, 1986, 113, 137. 19 W. Kaim, Rev. Chem. Intermed., 1987, 8, 247. 20 A. Schulz and W. Kaim, in preparation. 21 J. R. Bolton, J . Phys. Chem., 1967, 71, 3702. vol. 3, pp. 443458. Sci. USA, 1986, 83, 5355. 1986, 1645. Commun., 1986, 1727. 106 FAR 13214 New Redox-Mediating Systems 22 W. Kaim and H. Bock, Chem. Ber., 1978, 111, 3552; 3585. 23 T. J. Curphey and K. S. Prasad, J . Org. Chem., 1972, 37, 2259. 24 M. K. Ahn and C. S. Johnson Jr, J . Chem. Phys., 1969, 50, 632. 25 H. Weidel and M. Russo, Monatsh. Chem., 1882, 3, 850; A. Calderbank, D. F. Charlton, J. A. 26 A. L.Rieger and J. 0. Edwards, J. Org. Chem., 1988, 53, 1481. 27 J. H. Sharp and M. C. R. Symons, in Ions and Ion Pairs in Organic Reactions, ed. M. Szwarc (Wiley, 28 F. G. Bordwell and M. J. Bausch, J . Am. Chem. Soc., 1986, 108, 2473, and references therein. 29 W. Kaim, J , Mol. Struct. (Theochem.), 1984, 109, 277. 30 W. Kaim, Angew. Chem., 1983, 95, 201; Angew. Chem. Int. Ed, Engl., 1983, 22, 171. 31 W. Kaim, J . Chem. SOC., Perkin Trans. 2, 1984, 1767. 32 P. J. Black and C. A. McDowell, Mol. Phys., 1967, 12, 233. 33 J. A. Pedersen and L. T. Muus, Mol. Phys., 1969, 16, 589. 34 W. Kaim, J. Chem. SOC., Perkin Trans. 2, 1984, 1357. 35 H. Kurreck, M. Bock, N. Bretz, M. Elsner, H. Kraus, W. Lubitz, F. Muller, J. Geissler and P. M. H. 36 K. Schemer and H. Stegmann, Elektronenspinresonanz (Springer, Berlin, 1970), pp. 166 and 174. 37 B. L. Barton and G. K. Frankel, J . Chem. Phys., 1964, 41, 1455. 38 C. Heller and H. McConnell, J. Chem. Phys., 1960, 32, 1535. 39 T. M. McKinney and D. H. Geske, J. Am. Chem. Soc., 1967, 89, 2806. 40 H. Bock and W. Kaim, Ace. Chem. Res., 1982, 15, 9. 41 E.g. H. D. Hausen and W. Kaim, Z. Naturforsch., Teil B, 1988, 43, 82. 42 W. Kaim, J . Chem. SOC., Perkin Trans. 2, 1985, 1633. 43 H. D. Hausen, 0. Mundt and W. Kaim, J. Organomet. Chem., 1985, 296, 321. 44 G. R. Newkome and W. W. Paudler, Contemporary Heterocyclic Chemistry (Wiley, New York, 45 H. D. Hausen, W. Kaim and A. Schulz, Chem. Ber., in press. Farrington and R. James, J. Chem. Soc., Perkin Trans, I , 1972, 68, 138. New York, 1972), vol. 1, p. 177. Kroneck, J . Am. Chem. SOC., 1984, 106, 737. 1982). Paper 8/01529E; Received 18th April, 1988

ISSN:0300-9599    

DOI:10.1039/F19888403207

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1988, 84(10), 3215-3222 Electron Spin Resonance Study of Azoalkane and Imine Radical Cations Christopher J. Rhodes School of Chemistry, Thames Polytechnic, Wellington Street, Woolwich, London SE18 6PF Radical cations of azoalkanes have been observed for the first time in a low- temperature matrix by e.s.r. spectroscopy. The e.s.r. data show that they are n-cations despite photoelectron studies which indicate the HOMO for an azoalkane to be the 0 (n-) orbital. Interestingly, these species appear to be stable, at least up to ca. 160 K, and show no tendency to fragment to form alkyl radicals. This contrasts with the solution-phase behaviour of these species, which are intermediates in the oxidation of azoalkanes, and give rise to carbocations and alkyl radicals.Imines form similar n-radical cations which show a greater tendency to decompose. The structures and stabilities of these cations are discussed. Attempts to observe the corresponding azoalkane anions in solid matrices resulted only in the formation of alkyl radicals, and so we conclude that the anions decompose by fragmentation, although they have been studied in solution under steady-state conditions. Previously derived coupling parameters predict isotropic 14N hyperfine couplings for these species which are larger than those observed. Therefore, new values are calculated, and these fit well with the observed couplings. In this way, spin polarisation parameters are obtained for individual contributions to the couplings, as opposed to previous approaches which treat them collectively.Considerable attention has been focused on radical cation intermediates, particularly in recent years. Direct information regarding the nature of these species has been obtained from electronic,' CIDNP2 and e.~.r.,-~ spectroscopies. E.s.r. may be used to study relatively stable radical cations in the liquid phase,6 but in order to obtain e.s.r. data for more reactive species matrix-isolation techniques must be used. Matrix methods have tended to fall into two categories: (1) isolation of the cation in an inert-gas matrix by co-deposition, following photoionisation of its precursor, which has been used mainly for small molecules;3 and (2), by far the most widely used method, exposure of dilute frozen solutions of cation substrates in various freons to ionising radiation at low temperatures.This approach has proved extremely successful in the study of cations of larger organic and organometallic m01ecules,*~~ and is used in the present work. Azoalkanes are well known as thermal or photochemical sources of alkyl radicals, perhaps the most familiar compound of this type being the radical initiator AIBN [Me,C(CN)-N=N-C(CN)Me2]. In terms of free-radical chemistry of the -N=N bond, a recent example is the study of muonated hydrazyl radicals, R-N-N(Mu)R, observed during the irradiation of a series of azo-compounds with positive muons.' Our present interest in azoalkanes and imines stemmed from our expectation that they would readily form their radical cations on irradiation in CFCl,.These cations are interesting since they could potentially exist in either 0 or n-states, and it is possible that the n-states might adopt twisted structures, as proposed for some alkene cation^.^ We are not aware of any previous e.s.r. work on these species other than our own 3215 106-23216 Azoalkane and Imine Radical Cations Table 1. E.s.r. data for azoalkane and imine radical cations and related radicals radical coupling constants/Ga T / K Et-N=N-Eta+ MeCH=NBu"('+) (2H) 20, (14N,,) 24, (14N1) 0 (2H) 16.0, (14N,,) 19 (4H) 21, (14N,,) 24, (14NI) 0 (4H) 17.6 (4H) 21, (14N,,) 24, (l4NL) 0 (4H) 17.8 (1H) 9, (2H) 21, (l4Nl,) 30, (14N*) 0 (4H) 12, (2H) 22, (14N,,) 32, (14N1) 0 (4H) 12.8, (14Niso) 7.75b (2H) 9.73, (14Niso) 8.0b (3H) 27, (2H) 22 (6H) 24, (1H) 22 (2H) 35, (2H) 22 (2H) 36, (2H) 22 77 I50 77 150 77 150 77 77 - 77 77 77 77 a 1 G = T.From ref. (19). All other data are from this work. preliminary report of azoalkane cations,* although studies have been made of reactions believed to involve azoalkane and imine radical cation^,^. lo and well defined photoelectron spectra have been obtained from both azoalkanesl' and imines.12 Experiment a1 The azoalkanes and imines were kindly provided by Dr P. W. F. Louwrier (Amsterdam). Dilute solutions (ca. 0.1 YO by wt) of these compounds were prepared in CFCl,, and reduced to powders by pipetting into liquid nitrogen. The powders were y-irradiated to a dose of ca. 1 Mrad at 77 K and the e.s.r. spectra were recorded using a Varian El09 spectrometer. The samples were annealed either by decanting the liquid nitrogen from the insert Dewar and allowing the sample to warm; refreezing when significant spectral changes occurred, or using a Varian variable- temperature device when specific temperatures were required.Results and Discussion The e.s.r. data for azoalkane and imine radical cations are summarised in table 1; for comparison, data for related radicals, taken from the literature, are also included. Azoalkane Cations The radical cations of azoethane, azo-n-propane and azo-isopropane all show parallel 14N couplings of 24 G to two equivalent nitrogen nuclei at 77 K. Typical spectra are shown in fig. 1 and 2. This implies a 2B term13 of ca. 16 G, and so the p-orbital spin density on each nitrogen atom is ca. 0.5. We therefore assign the SOMO as being the N-N n-bonding orbital (au).The appearance of the spectra shows that the two 14N hyperfine tensors are parallel and so the cations are planar rather than twisted, in contrast with certain alkene cation^.^ For significant twisting a more complex ('x, y , 2') pattern would be expected.C. J . Rhodes 3217 gain x 20 I I II III 1111 11111 1111 III II I ' f N , , + 2 +2.1 +2.10 -2.10-1 .2.10-1-2 .101-2 0 1 . 2 -1-2 - 2 Fig. 1. E.s.r. spectrum of Et-N=N-Et" radical cations recorded following y-irradiation of the parent azoalkane in a CFCl, matrix at 77 K. 0 Fig. 2. E.s.r. spectrum of Pr'--N=N-Pr'('+) radical cations at 77 K. In both Et-N=N-Et+' and Pr"-N=N-Pr"('+) cations, a(H) couplings of 21 G to four equivalent protons were observed at 77 K; however, on annealing to 150 K, the couplings decreased to 18 G.This indicates a preferred conformation of the alkyl groups, as in (I), similar to that found for primary alkyl radicals,l* which maximises 0-7c conjugation, since this is believed to be more effective for C-H bonds than for C-C bonds.15 The rotational barriers must be low, since these changes in coupling constants over a range of ca. 70 K are fairly large. Interestingly, the conformation (I) was not adopted by the ethyl groups in the Et,N'+ radical cation,16 but this is probably because of steric interactions between the alkyl groups, as is believed to be the case in tertiary alkyl radi~a1s.l~ The proton coupling is lower in the isopropyl derivative, most likely as a result of a steric effect which3218 Azoalkane and Imine Radical Cations I I I I 0 0 14N,, +1 +1 +l +10 o+' 0 0-1 -1 -1 -1 Fig.3. E.s.r. spectrum showing features assigned to PhCH=NBu"('+) radical cations recorded following y-irradiation of the parent imine in a CFCl, matrix at 77 K. disfavours conformations such as (I). The actual conformation (on average) appears to be a compromise which maximises hyperconjugation and minimises steric repulsion. This average conformation is still that with a dihedral angle between the C-H bond and the density axis of the SOMO of less than 45", since the coupling is reduced by raising the temperature. ' H 8-6\ a" In all cases, a decrease in the ',Nil coupling was observed during annealing as a result of increased librational motion of the cations.This would also account for the improved resolution obtained at higher temperatures, particularly for the n-propyl derivative. Imine Cations From the 14Nll couplings in MeCH=NBu"('+) and PhCH=NBu"('+) cations, we calculate 2p spin densities on nitrogen of 0.64 and 0.60, respectively. The spectrum of the latter cation is shown in fig. 3. Since the perpendicular features show zero splitting, the isotropic coupling must be low (ca. 10 G) giving an N 2s contribution of ca. 2 O/O, and so the p/s ratio is ca. 30, showing that these are also 71-cations. The results show that a high proportion of the spin resides on the exocyclic group in the PhCH=NBu"('+) cation, by comparison with the data for other benzene cations with unsaturated substituents.17 If the 9 G (1H) coupling is due to the para-proton, then the SOMO is still essentially the styrene-type 71, orbital (11), but with some distortion which increases the coefficient on nitrogen.However, since the spin density on nitrogen is so large, and is very similar to that in the MeCH=NBu"('+) cation, it is possible that the 9 G coupling is due to the a- proton and so essentially all the spin is on the C=N group; however, such a small degree of delocalisation onto the phenyl ring would appear surprising, and so we favour the former assignment. As with the azoalkane cations, there is no evidence that these cations are significantly twisted.C. J . Rhodes 3219 Cation Stabilities Photoelectron spectra of azoalkanesll show that the non-bonding (a,) orbitals on nitrogen are of higher energy than the n-orbitals (au), yet it is clearly the n-cations that are observed under the present conditions.Our suggestion is that the initial electron loss is from the (a,) orbital, giving a 0-cation which then converts to the n-state, since this will be stabilised by hyperconjugation (enhanced by the positive charge). We can argue similarly for the imines, since photoelectron studies12 also indicate that the initial state should be 0, although the 0-n energy difference is less than that for azoalkanes. Studies of the behaviour of azoalkanes with single-electron oxidants in solution9~ lo indicate a facile fragmentation of the azoalkane radical cation R-N=N-R'+ --+ R' + N, + R+ (1) followed by a rapid oxidation of the alkyl radical, giving products compatible with predominant carbocation chemistry.Under our conditions, the azoalkane cations were stable; even on annealing to the melting point of the matrix (ca. 160 K) we were unable to detect the presence of alkyl radicals. Probably, attack on the cation by solvent molecules is required to induce the decomposition, although we note that unimolecular fragmentation of other organic cations has been observed previously in a freon matrix.la The imine radical cations appeared to be less stable. Following irradiation of PrnCH=NPri in CFC1, only alkyl radicals were observed: the spectrum could be ascribed mainly to isopropyl radicals, but features were also present which indicated the formation of lesser amounts of n-propyl radicals. The PhCH=NBu"('+) and MeCH=NBu"('+) cations were stable at 77 K, but, on annealing, features assigned to n-butyl radicals were observed.We suggest the fragmentation R'CH=N-R'+ R'CZNH+ + R' (2) to account for this, in which the relatively stable protonated nitriles are formed. The greater stability of the isopropyl radical probably accounts for the greater tendency of Pr"CH=NPr'('+) to fragment. We suggest that the reaction + Pr"CH=NPr'('+) 4 Pr"' + HCFNPr' (3) also occurs to a lesser extent. Azoalkane Radical Anions Azoalkane radical anions have recently been observed by e.s.r. during continuous photolysis of THF solutions containing the azoalkanes and crotylpotassium ;19 however,3220 Azoalkane and Imine Radical Cations I ' I Fig. 4. E.s.r. spectrum assigned to ethyl radicals formed by y-irradiation of pure azoethane at 77 K.we have been unable to observe these species in the solid state. y-Irradiation of pure materials of relatively high electron affinities at low temperatures often gives rise to the corresponding radical anions ; however, on irradiating the pure azoalkanes, we observed well defined spectra of only the substituent alkyl radicals (fig. 4). Since we know that the cdtions are stable at 77 K [even concentrated solutions (ca. 20% in CFCl,) show only the parent cations, so these do not react with neutral R-N=N-R molecules at 77 K] we suggest that the alkyl radicals may be formed partly via dissociation of the radical anions : (4) although high-energy electron return to a cation in the pure solid could also give alkyl radicals, via an excited state of R-N=N-R.Irradiation of dilute solutions of the azoalkanes in CD,OD, which is a good radiolytic source of electrons in which only anions or anion-derived radicals are formed, gave rise to spectra with wing features assignable to the same alkyl radicals. Simulations obtained by taking Sustmann's isotropic valueslg and estimating the anisotropic 14N hyperfine couplings did not fit the observed features. This supports our conclusion that anion fragmentation takes place. The proton couplings in the azoalkane anions in solution are markedly lower than those in the corresponding cations; this may be due partly to the higher temperatures at which the anion spectra were recorded, resulting in an increased weighting of conformations with low couplings, but mainly to the expectedly weaker hyper- conjugation compared with the cations.R-N=N-R'- + R' + R-N, We were unable to obtain well defined spectra assignable to imine radical anions. Coupling Parameters for 14N A quantitative theoretical treatment was made by Karplus and Fraenke120 for carbon- centred n-radicals which evaluated the various contributions to the isotropic 13C hyperfine couplings; these are symbolised by the S and Q parameters in the equationC. J . Rhodes 322 1 Table 2. Coupling parameters (G)" for 14N and 13C nitrogenb carbonc Q L W , 7.2 - - - 12.7 - s ;s -2.1 SN 2.9 QL 8.0 Q& 14.4 s r", Q;LN 2.7 Q",-, - 13.9 Q;;, 0.0 * 1 G = lop4 T. This work. ' Ref. (20). where ac is the isotropic 13C coupling, Q&x reflects the polarisation of the C-X bonding electrons by 71-spin density on the carbon atom (pg), Q$+ reflects the polarisation of the C-X bond by 71-spin density (p;) on the substituents X, and Sc represents the exchange polarisation of the carbon 1 s2 core electrons.This approach has predicted very successfully the 13C couplings in a variety of carbon-centred 71-radicals. Similar expressions such as have been proposed to account for 14N couplings.2'* 22 The parameters have usually been empirically derived owing to the necessary exchange integrals not being available for all the polarisations. In eqn (6), SN contains both the ls2 and lone-pair terms. From the data for a series of heteroaromatic radical ions it was not possible to achieve a separation of the SN and terms22 and so one parameter QN was used which reflects all contributions to the coupling from 71-spin density on the nitrogen atom itself, leading to the simplified expression : Expressions of this form have been applied successfully to a number of aromatic radical anions21.22 and, more recently, to neutral nitrogen-containing radicals.23 However, the parameters derived from various systems to fit eqn (7) predict 14N couplings for R-N=N-R radical anions and cations which are 60-80 % larger than those observed. This, and the conceptual desirability of separating out the individual contributions to the 14N couplings, prompted us to try to obtain values for all the coupling parameters in the fuller equation, where Sys reflects the ls2 core polarisation and Syp reflects polarisation of the lone pair.By a consideration of the isotropic couplings for nitrogen NH225 and NH326 radicals, with different numbers of lone pairs and bonds, and Sustmann's value for the 14N coupling in R-N=N-R'- radical anions,19 we obtained the parameters in table 2. From eqn (8) and our derived parameters (taking QZPN = 0) we predict an isotropic coupling of 10.15 G for the hydrazine radical cation H2N-NH;+. This compares very well with the experimental value of 11.5 G;27 however, it has been shown that in the case of 1,4-dihydropyrazine cations28 there is a small positive QEeN term of 2.62 G. We can + .3222 Azoalkane and Imine Radical Cations therefore calculate a nearly equal value of 2.7 G for the hydrazine cation. From our data, we calculate 14N couplings of 9.3 G in R-N-N-R" cations.This is close to the experimental values of 8 G, although the broad e.s.r. lines put an error of _+2 G on these. The derived S and Q values show clearly that the difference in the isotropic couplings for the hydrazine and azoalkane radical cations is due to the smaller contribution resulting from polarisation of the lone pair than of the N-H bonding electrons. We note that the term due to polarisation of the 1s' core is negative, as in the case of 13C, although it is much smaller. In contrast with the (QC-,) parameter for carbon, which is negative, the QE.-, parameter for nitrogen is positive and cannot therefore be due to the bond excitation 0N-N -+ ogPN. Similar conclusions were drawn previously from work on aromatic nitrogen-containing anions.21 From the anisotropic 14N coupling in the imine cation, MeCH=NBu"('+), we obtain a 2p orbital density on nitrogen of 0.64.From eqn (8), we predict an isotropic coupling of 10.7 G, which compares excellently with the experimental value of 10.7 G obtained by taking AN, = 0. This suggests that the derived S and Q parameters are also applicable to imine cations. I thank Prof. M. C . R. Symons for access to e.s.r. facilities, and Dr P. W. F. Louwrier for providing samples of the imines and azoalkanes. References 1 T. Shida and S. Iwata, J. Am. Chem. SOC., 1973, 95, 3473. 2 H. D. Roth, Acc. Chem. Res., 1987, 20, 343, 3 L. B. Knight, B. W. Gregory, S. T. Cobranchi, F. Williams and X-Z. Qin, J . Am. Chem. SOC., 1988, 4 M. C. R. Symons, Chem. SOC. Rev., 1984, 13, 393.5 M. Shiotani, Magn. Reson. Rev., 1987, 12, 333. 6 H. Bock and W. Kaim, Acc. Chem. Res., 1982, 15, 9. 7 P. W. F. Louwrier, G. A. Brinkmann, C. N. M. Bakker and E. Roduner, Hyperfine Int., 1986, 32, 8 C. J. Rhodes and P. W. F. Louwrier, J . Chem. Res. ( S ) , 1988, 38. 9 D. H. Bae, P. S. Engel, A. K. M. Mansural Hoque, D. E. Keys, W-K. Lee, R. W. Shaw and H. J. 110, 327, and references therein. 753. Shine, J . Am. Chem. SOC., 1985, 107, 2561. 10 W. Adam and M. Dorr, J . Am. Chem. SOC., 1987, 109, 1570. 11 K. N. Houk, Y-M. Chang and P. S. Engel, J . Am. Chem. SOC., 1975, 97, 1824. 12 C . Sandorfy, J . Photochem., 1981, 17, 297. 13 M. C. R. Symons, Chemical and Biochemical Aspects of Electron Spin-Resonance Spectroscopy (Van Nostrand Reinhold, New York, 1978). 14 J. K. Kochi, Adv. Free-Radical Chem., 1975, 5, 189. 15 D. D. M. Wayner and D. R. Arnold, Can. J . Chem., 1985, 63, 2378. 16 G. W. Eastland, D. N. Ramakrishna-Rao and M. C. R. Symons, J . Chem. SOC., Perkin Trans. 2, 1984, 17 D. N. Ramakrishna-Rao and M. C. R. Symons, J . Chem. Soc., Perkin Trans. 2, 1985, 991. 18 C. J. Rhodes and M. C. R. Symons, J. Chem. Soc., Chem. Commun., 1987, 258. 19 R. Sustmann and R. Sauer, J . Chem. SOC., Chem. Commun., 1985, 1248. 20 M. Karplus and G. K. Fraenkel, J . Chem. Phys., 1961, 35, 1312. 21 J. C. M. Henning, J . Chem. Phys., 1966, 44, 2139, and references therein. 22 C. L. Talcott and R. L. Myers, Mol. Phys., 1967, 12, 549, and references therein. 23 R. A. Jackson and C. J. Rhodes, J . Chem. SOC., Perkin Trans. 2, 1985, 121. 24 P. W. Atkins and M. C. R. Symons, The Structure of Inorganic Radicals (Elsevier, Amsterdam, 25 D. W. Pratt, J. J. Dillon, R. V. Lloyd and D. E. Wood, J . Phys. Chem., 1971, 75, 3486. 26 T. Cole, J. Chem. Phys., 1961, 35, 1 169. 27 J. Q. Adams and J. R. Thomas, J. Chem. Phys., 1963, 39, 1904. 28 B. L. Barton and G. K. Fraenkel, J . Chem. Phys., 1964, 41, 1455. 1551. 1967). Paper 8/01524D; Received 18th April, 1988

ISSN:0300-9599    

DOI:10.1039/F19888403215

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1988, 84(10), 3223-3231 Substituent Effects in Anthrasemiquinones Jens A. Pedersen Department of Chemistry, Aarhus University, 140 Langelandsgade, DK-8000 Aarhus C, Denmark Electron spin resonance spectra have been obtained from series of 2- substituted anthrasemiquinones and of 3-substituted 1 &dihydroxyanthra- semiquinones. The proton splittings are consistently assigned by means of linear correlation plots between splitting constants and a substituent- dependent parameter. All lines in the plots of the two series of compounds obey the following linear equation a; = Ai xR + a: where aF and a: are the splitting constants at position i before and after the substituent has been added. A, is the slope of the line for the splittings from position i and xR is a constant characteristic of the substituent R.xR is comparable to the Hammett CT parameter. Electron-donating substituents at C-2 are shown to increase the spin densities at the positions 1 > 6 > 8, and to decrease them at the positions 4 > 3 > 7 > 5, with the strength of the effect indicated. Electron-withdrawing substituents have the opposite effect. Preliminary Hiickel molecular-orbital calculations qualitatively predict the observed correlations, solely by changing the parameter for the resonance integral. Anthraquinones comprise the largest group of naturally occurring quinones, and practically all of them have non-symmetric substitution patterns.' Their e.s.r. spectra are complex, with many overlapping lines, in certain cases derived from more than eight different splitting constants.The constants range from a few mG to some G. The rules governing the spin distributions are largely unknown, and reliable assignments have been given only for few structures, mainly those with a symmetric substitution pat tern. Recently, we published spectra and data of a large number of anthrasemiquinones. The present study is a continuation of this work, originally started as a search for correlations between e.s.r. splitting constants and other molecular properties. We have been especially interested in examining related series of anthrasemiquinones in which a single substituent is varied in order to obtain information about the redistributions of spin densities within the three-ring radical skeleton.In addition, we hoped to obtain rules for obtaining unique assignments and for predicting spectra of previously unstudied compounds. In this paper we present the results we have obtained from considering 11 suitable model compounds, partly a series of monosubstituted anthraquinones, partly a series containing in addition the 1,8-dihydroxy grouping. We have focused attention on the effects from substituents added at /3-positions with no neighbours. This is in order to avoid complications and unknown factors due to steric hindrance, as observed for substituents at a-po~itions.~ Experimental The anthrasemiquinone radicals were generated in alkaline aqueous ethanol at room temperature by reducing the corresponding anthraquinones with sodium dithionite. The 32233224 Suhstituent Efects in Anthraserniquinones Table 1.E.s.r. proton splitting constants for some 3-substituted 1,8-dihydroxyanthrasemiquinones R XR O 2 4 5 6 7 0- -0.670 -0.52 0.556 3.093 1.663 0.623 2.240 Me -0.096 -0.17 1.414 1.435 1.261 0.937 1.680 H 0 0 1.577 1.192 1.192 0.980 1.577 CO, 0.143 0.13 1.768 0.812 1.085 1.049 1.428 For numbering positions see fig. 1 . 0 parameters are from ref. (7); for the xR parameters see text. A splitting constant of 0.996 G is observed from the methyl-group protons. solvent composition was ca. 64 YO alcohol (v/v). 1 -[2H]anthraquinone was synthesized electrolytically from 1 - bromoanthraquinone. The spectra were recorded on a Bruker ER 200 spectrometer with a modulation frequency of 12.5 or 25 kHz and a modulation amplitude of 25 mG or less. The microwave power was kept below 0.3 mW to avoid saturation effects.All digitized spectra (4 k, with a resolution of 2.5 mG) were simulated on a Vax 11 /780 computer, and ' best-fit ' parameters were obtained by an iterative optimization procedure as described previously. Results and Discussion A crucial point in following how unpaired spin is redistributed owing to the addition of a substituent is to have available reference compounds with spectra correctly interpreted and splitting constants uniquely assigned. For anthrasemiquinones this is not a trivial problem because of the many inherent splitting constants (often of similar magnitude) for each individual compound. In order to assign the two splittings of anthrasemiquinone, we have synthesized l-[2H]anthraq~in~ne. We obtain the unambiguous result 0.550 and 0.960 G for the a- and Q-splittings, respectively.1,8-Dihydroxyanthrasemiquinone is a key compound in the present study. Of the four splitting constants observed the one from the hydroxyl protons is revealed immediately by deuterium exchange. For the remaining splittings (0.980, 1.192, 1.577 G) the published assignments are ~onflicting.~. From the literature and the available evidence there seems no doubt that the larger splitting has to be assigned to the proton at C-2, with an identical one derived from C-7. An assignment by additivity indicated the result a, > a3 > a4, under the assumption that introduction of a hydroxyl group at an a-position had the same effect, whether or not the quinonoid oxygens already participated in internal hydrogen bond^.^ From what follows this assignment turns out to be incorrect and in obvious conflict with the result from 1,8-dihydroxy-3-methyl- anthrasemiquinone, viz.the methyl group at C-3 yields a proton splitting of 0.996 G, comparable to a splitting of 0.980 G from the unsubstituted position of the parent semiquinone (table 1). We here use the assumption, borne out by many experiments, that methyl-group protons give rise to a splitting of comparable magnitude to the splitting of the C-H proton at the site of substitution. Thus, we shall assume the assignment a, > a, > as, cf. ref. (6). 3-Substituted 1,8-Dihydroxyanthrasemiquinones We shall begin discussing how the correlation plot of fig.1 is obtained. It is obvious that splitting constants change considerably, and are dependent on the applied substituentJ. A . Pedersen 3225 0- OH 6 HO ’9 \ R L \ ited Fig. 1. The change in the ring-proton splitting constants of 3-substiti semiquinones us. the substituent parameter xR. For a definition of xR see text. The-opensymbols on the lines have a size of 30 mG. -Table 2. E.s.r. hydroxyl-proton splitting constants (G) for some 3-substituted 1,8-dihydroxyanthrasemiquinones OH 6 HO I l l 0- 0.138 0.156 Me 0.185 0.185 0.195 0.195 CO; 0.219 0.213 R H 6\ 7 c 3 q 0- (table 1). The question is whether simple expressions exist, which correlate the observed behaviour. Attempts to find parameters for the various substituents correlating our data led to plots with scattered points, exhibiting no clear trends.The only plot of some interest is a ‘Hammett plot’ of splitting constants us. CJ parameters.’ In short we have followed a different approach. A first step is to make out where the spin density increases and where it decreases when3226 Substituent Eflects in Anthraserniquinones Table 3. Parameters obtained in the linear-regression analysis of the data from table 1 (cf: fig. 1) 2 1.500 1.562 1.577 1 .ooo 13 7 20 4 - 2.822 1.194 1.192 -1.000 27 15 43 5 - 0.708 1.190 1.192 -1.000 4 2 6 6 0.528 0.980 0.980 0.999 7 4 12 7 - 0.994 1.576 1.577 - 1.000 7 4 12 a? = A,xR+aH. At, a? and a: are in G, the standard deviations in mG. The unsubstituted constants a: are shown for comparison. Table 4. E.s.r. proton splitting constants for some 2-substituted anthrasemiquinones R X R t7 1 4 5 8 3 6 7 0- -0.670 -0.52 1.959 0.147 0.271 1.030 0.537 1.615 0.610 NH, -0.430 -0.66 1.443 0.248 0.401 0.897 0.556 1.411 0.679 Et -0.067 -0.15 0.698 0.507 0.509 0.591 0.853 1.034 0.930 OMe -0.287 -0.27 1.071 0.307 0.335 0.778 0.731 1.226 0.781 H 0 0 0.550 0.550 0.550 0.550 0.960 0.960 0.960 c1 0.096 0.23 0.353 0.609 0.561 0.487 1.070 0.839 0.978 co, 0.143 0.13 0.259 0.633 0.611 0.445 1.095 0.824 1.012 For numbering positions see fig.3 and 4. CT parameters are from ref. (7); for the xR parameters, see text. Additional splitting constants (G) are as follows: NH, (a, = 0.337, aNH, = 0.073); Et (acH, = 0.705). a substituent is added, say, at C-3 of 1,8-dihydroxyanthrasemiquinone. Conclusive evidence comes from examining the data of table 2, showing the hydroxyl-proton splittings of four representative compounds.The splittings are definitely derived from the hydroxyl protons, as substantiated by deuterium exchange in heavy water. Considering the individual splittings we observe that electron-donating groups, 0- and Me, cause a decrease in the magnitude of the splitting constants, i.e. a decrease at positions 1 and 8. For an electron-withdrawing group, CO,, we observe the opposite effect. 0- has the strongest effect, and Me and CO, have effects of similar strength, but of opposite sign. These trends have obvious similarities to the Hammett cr parameter^,^ and have led us to define a substituent parameter xR as follows: XR = 0 ; XR > 0; XR < 0 ; x0- = -0.67; arbitrarily chosen.for R = H for electron-withdrawing groups for electron-donating groups To fix new xR parameters for other substituents we begin by making a plot of the splitting constants of the compounds with R = H and R = 0- us. their xR parameters defined above. The two sets of points are placed on vertical lines above xH = 0 and xo- = -0.67. We then connect the three and five points with five straight lines under the assumption that a linear relation exists between splitting constants and the xR parameters. Since we do not know the assignment for the R = 0--substituted compound, a set of lines can be drawn in 30 different ways. However, we can obtain the ‘correct’ set if we add to vertical lines the five splittings of the COi-substituted compound andJ .A . Pedersen 3227 Fig. 2. The e.s.r. spectrum of 2-aminoanthrasemiquinone. The outermost 1 : 2 : 1 triplets are from the NH, protons. The spectrum was simulated with the data of table 4 and a linewidth of 0.074 G. those of the Me-substituted analogue and move the points in the positive and the negative directions from xH = 0, respectively. We then find that all points plotted fall on straight lines in one case only out of the 30 possible. This case is shown in fig. 1. We have obtained a linear correlation plot with four points on each line. Since we know the assignments of the splitting constants for the unsubstituted compound we have assigned all splitting constants of table 1, with the exception of distinguishing between the ,!?-protons 2 and 7, and a-protons 4 and 5.However, taking recourse to a study of the non-s ymmetric 1 - h ydroxy-6-me t hoxy- and 1 - hydroxy- 7-methoxy- anthrasemiquinones and other methoxy-substituted anthrasemiquinones, the assignment of table 1 turns out to be the only consistent one. These results will be published in a forthcoming paper. The lines in the correlation plot can be represented by the following expression: a: = A i x R + a r where a: and a? are the splitting constants from position i of l&dihydroxyanthra- semiquinone, with and without the substituent R at C-3. Ai, the slope of line i, is a sole function of the proton position. Concerning the xR parameters determined above, we observe them to be comparable to the Hammett 0 parameters in the scale we have chosen (table 1).From a regression analysis we obtain the results of table 3. The extreme linearities are reflected in the correlation coefficients, with 2 0.999 throughout, and in the standard deviations shown in mG. The experimental uncertainties in the splitting constants are estimated to be less than 20 mG. 2-Substituted Anthrasemiquinones Turning to the monosubstituted anthrasemiquinones, we have performed a similar analysis for seven compounds, the data of which are shown in table 4. Most monosubstituted anthrasemiquinones have very complex e.s.r. spectra. Fig. 2 shows as3228 Substituent Eflects in Anthrasemiquinones \r2 V I I l l , 1 1 1 I t 1 1 0 0.5 XR - 0.5 Fig. 3. The change in the a-proton splittings of 2-substituted anthrasemiquinones us. the substituent parameter xR, cf.fig. 1. an example the spectrum of 2-aminoanthrasemiquinone, which may be analysed in terms of nine splitting constants, ranging from 0.073 to 1.443 G. The 11 52 lines are confined within 6.455 G. Plots of the a- and /?-proton splitting constants us. xR are shown in fig. 3 and 4, respectively. The figures have been made by first plotting the data for the three semiquinones with no substituent, with the 0-, and with the CO, substituent, using the previously obtained xR values. In order to determine new substituent parameters we add in turn the data of the four remaining semiquinones, using the same procedure as described in obtaining fig. 1 . We obtain best agreement by using the same xR value in both figures. Furthermore, all eight xR values (tables 1 and 4) compare well with the Hammett 0 parameters.From fig. 3 and 4 an excellent linearity for the proton splittings us. xR for the positions 1, 6, 7 and 8 appears, and a reasonable linearity for the positions 3, 4 and 5. The data points are somewhat scattered for the latter three positions, but clear trends with positive slopes are obvious for all three matching lines. The results of the regression analysis are shown in table 5. The correlation coefficients and the standard deviations stress the excellent agreement to linearity, for nearly all seven lines. A TRIPLE resonance study on 2-hydroxyanthrasemiquinone revealed a negative spin density for one of the carbon positions, yielding a splitting constant of +0.147 G from the corresponding p r ~ t o n . ~ Since we have applied numerical values to all splitting constants in the present work, this particular splitting has been given the value -0.147 G in fig.2. The opposite sign for the splitting would give a much betterJ. A . Pedersen 3229 I I I I l l I l l 1 I I I I I > -0.5 0 0.5 XR Fig. 4. The change in the P-proton splittings of 2-substituted anthrasemiquinones us. the substituent parameter xR, cf. fig. 1. fit for line 4, cf. the comment to table 5. However, we still obtain a consistent assignment for all splittings of the seven compounds studied and have been unable to check the result of negative spin density owing to lack of 2-hydroxyanthraquinone. We can generalize the total results for both series as follows: - I - 0- 0 + I + 0- R electron - donating R electron - withdrawing Here + and - mean the spin densities increase and decrease, respectively, when R is substituted at C-2.The relative power of a particular substituent to redistribute the unpaired spin is expressed through its xR value, whereas the magnitude and direction of change of unpaired spin at a particular position is given by the slopes Ai, cf. tables 3 and 5. As mentioned earlier, we have used throughout the incorrect but customary practice of considering proton splitting constants as positive for positive spins. In choosing the opposite convention the signs of the Ai parameters should be changed accordingly. Molecular-orbital Calculations In order to obtain some insight into the trends observed for the two series of anthrasemiquinones studied, we have performed preliminary calculations on 2-3230 Subst ituen t Eflects in An thraserniquinones Table 5.Parameters obtained in the linear-regression analysis of the data of table 4 (cf fig. 3 and 4) 1 - 2.07 1 0.545 0.550 -0.999 34 15 46 4" 0.906 0.544 0.550 0.979 62 28 84 5 0.406 0.533 0.550 0.95 1 43 19 59 8 -0.740 0.554 0.550 -0.998 16 7 22 3 0.743 0.958 0.960 0.970 61 27 83 6 - 0.999 0.957 0.960 -0.998 19 8 25 7 0.527 0.94 1 0.960 0.990 24 1 1 33 " For an explanation see text and table 3. The figures for position 4 are based on the data of table 4 with one negative constant (-0.147). If +0.147 is used we obtain the following figures shown in the same order as above: (0.635, 0.539, 0.550, 0.990, 29, 13, 40). substituted anthrasemiquinones, using the simple Huckel molecular-orbital (HMO) method.For the quinonoid oxygens we have used the parameters h, = 2.1 for the Coulomb integral and k,, = 1.85 for the resonance integral. High values are chosen in order to incorporate the influences from the polar solvent on the effective electro- negativity of the oxygen atoms. We have not at this stage attempted to determine 'best values ' for these parameters, since the crudity of the HMO method only allow us to use it in a qualitative manner. A calculation for anthrasemiquinone yields the spin densities p1 = 0.0300 and p2 = 0.0452, corresponding to a, = 0.59 G and u2 = 0.90 G, with a Q value (numerical) of 19.8 G applied in the McConnell equation. a, and a, compare well with the experimental a, = 0.55 G and a, = 0.96 G (table 4) for the unsubstituted compound.We next add a hypothetical substituent R at C-2, contributing two n-electrons. Gendell et ul.* have put forward a simple model based on the HMO method, accounting for the variation of splitting constants with solvent in the semiquinone ions. In their calculations they used a larger h, parameter from the Coulomb integral in polar and protic solvents than in less polar and aprotic solvents. Along with this, and since we are dealing with only one polar solvent system, we have chosen a large h, and kept it constant on the value h, = h, = 2.1. The changes of substituents are then reflected solely by a change in the parameter for the resonance integral for R. We have performed eight calculations, varying k,, from 0.4 to 1.8 in steps of 0.2.A plot (not shown) of the spin densities in the eight calculations for the seven actual carbon positions us. the k,, parameters yields seven nearly linear curves with positive and negative slopes, in agreement with the slopes of the lines of fig. 3 and 4 to the left of xH = 0. Using the HMO energy for the single occupied orbital instead of the k,, parameter in the plot led to better linearities for the seven lines, uiz. correlation coefficients of Irl > 0.970, apart from line 7 which had r = 0.955. The actual slopes obtained, using MO energies on the x-axis, are as follows with the first figure being from table 5 and the second from the calculations: 1, (-2.071, -2.071); 4, (0.906, 0.343); 5, (0.406, 0.207); 8, (-0.740, -0.163) 3, (0.743, 1.030); 6, (-0.999, -0.347); 7, (0.527, 0.043).The two sets of data have been scaled to make the slope for line 1 identical. It makes little sense to try to obtain detailed agreement by varying the Huckel parameters owing to the approximations inherent the HMO method. However, since we are dealing with molecules of a very similar type, it may not be unreasonable to expectJ. A . Pedersen 323 1 that the calculations indicate the main features of the variation in ring proton splittings from a change of substituent. Conclusions E.s.r. measurements on two series of anthrasemiquinones have shown that substituents added at C-2 can be arranged in an order which reflects that they perturb the spin distributions for all other ring positions in a linear fashion. The linear correlations of splitting constants us.constants characteristic of the substituents show strong resemblance to correlations obeyed by the Hammett eqaation. If we consider benzene derivatives of the type C,H,XY, where Y is a proton or a side-chain being subject to the perturbation from the variable substituent X, the Hammett equation takes the form log k = pa + log k? Here k and ko are the rate constants of some specified reaction for the substituted and the unsubstituted compound, respectively. a is a constant characteristic of a given substituent X and p is a constant characteristic of the 'position' of side-chain Y and the same for all substituents X. The equation from the present e.s.r. study takes the form ai = A , x + a ; where Ai has the same meaning as p and x has the same meaning as CT.ai and a; are the splitting constants from position i after and before substitution. In the application of the Hammett equation different a values are required for different molecular positions (the para/meta-ortho problem), whereas in the e.s.r. equation only one set of x values for the various substituents is required to correlate the behaviour of splittings for all seven ring positions of P-substituted anthrasemiquinones. A natural question to ask is whether the e.s.r. correlation equation is applicable to other pertinent radical systems e.g. naphthosemiquinones substituted at C-6. Consideration of three systems we have available (R, = Me, 0-, H),2 indicates this to be the case, however, with different slopes, A,, as one would expect, but unchanged x" values. Looking at table 4 and comparing x and a values, a change in the order appears for the couples O-/NH2 and for CI/CO,. Since 0- and CO, are the only substituents in the table with a formal charge, different counter-cations and/or different pH values in the experimental set-up for the present study and for the ones furnishing the a values might lead to a simple explanation of the discrepancies. References 1 R. H. Thomson, Naturally Occurring Quinones (Academic Press, New York, 1971). 2 J. A. Pedersen, Handbook of EPR Spectra from Quinones and Quinols (CRC Press, Boca Raton, 3 J. A. Pedersen and R. H. Thomson, J . Magn. Reson., 1981, 43, 373. 4 V. Axelsen and J. A. Pedersen, J . Chem. Soc., Faraday Trans. 1, 1987, 83, 107. S J. Gendell, W. R. Miller Jr and G. K. Fraenkel, J . Am. Chem. Soc., 1969, 91, 4369. 6 D. Lunney, J. Bailes and J. D. Memory, J . Chem. Phys., 1970, 53, 3387. 7 C. D. Johnsen, The Hammett Equation (Cambridge University Press, Cambridge, 1973); G. W. Klumpp, Reactivity in Organic Chemistry (J. Wiley, New York, 1982). 8 J. Gendell, J. H. Freed and G. K. Fraenkel, J. Chem. Phys., 1962, 37, 2832. 1985). Paper 8/015301; Received 18th April, 1988

ISSN:0300-9599    

DOI:10.1039/F19888403223

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Furuduy Trans. 1, 1988, 84(10), 3233-3242 Electron Spin Resonance Studies of Cycles and Bicycles Finlay MacCorquodale and John C. Walton" University of St. Andrews, Department of Chemistry, St. Andrews, Fife KY16 9ST The e.s.r. spectra of cyclohexylmethyl radicals have shown that two conformations, one with the CH; group equatorial and one with the CH; group axial, can be distinguished. The H, h.f.s. of the axial conformer is large because rotation about the C,-C; bond is hindered by axial hydrogens at C(3) and C(5) in the ring. This proved to be a very useful property enabling the conformations and ring-inversion barriers of cyclohexanes, cyclohexenes and related radicals to be studied by e.s.r. spectroscopy. In addition the various conformers of larger rings (up to 15- membered) also show different spectra.Their preferred conformations and the dynamics of 'corner migration' in the medium rings have been investigated. The e.s.r. spectra of cycloheptenylmethyl radicals showed the presence of a minor conformer which suggested that transannular cyclisation might be important. Product analysis confirmed that bi- cyclo[3.2. lloctane can be obtained in good yield. The stabilisation energy of cyclopropylmethyl radicals was determined from exchange-broadened spectra ; several cyclic homoallyl-type radicals were shown to have essentially zero stabilisation. Hydrogen abstraction from small strained bicycloalkanes, including bicyclo[n . 1 . Olalkanes, spiro[2. nlalkanes and spiro[3. nlalkanes yields the corresponding strained bicycloalkyl radicals, and their rearrangements have been followed by e.s.r.spectroscopy. Bicyclo[ 1 . 1 . 1 Jpentane and bi- cyclo[2.1. llhexane are unusual in that bridgehead radicals are formed. Bicyclo[2.2. Olhexane also shows significant bridgehead reactivity and provides the first example of an SH2 reaction involving a four-membered ring. Bicyclo[3.2. Olheptane, in which four- and five-membered rings are fused together, does not undergo this SH2 reaction with halogens. The rates of rearrangement of several cycloalkyl and bicycloal kyl radicals have been determined by kinetic e.s.r. spectroscopy. The chemical properties that alicyclic rings can show are inherent in their molecular structure and shape. The conformational options open to a particular ring system are specially important in determining the type of chemistry it can display.N.m.r. spectroscopy has been widely exploited in the study of cyclohexanes and medium ring~,l-~ but the closely related technique of e.s.r. spectroscopy has been far less used. The main reason for this lies in the fact that most cyclic radicals so far studied contain the planar radical centre in the ring. This drastically alters the ring's conformation in comparison with that of the parent alicyclic compound, so that the results are of very limited significance for the general chemistry of stable cyclic molecules. This applies, for example, to the e.s.r. work on nitroxide conformations4 and that on mono- and bi-cyclic semidiones. A cycloalkylmethyl radical (1) contains the small, non-polar CH; group which causes minimal perturbation of the adjacent ring.Study of these radicals enables the e.s.r. 32333234 E.S.R. of Cycles and Bicycles method to be successfully applied to conformational analysis. Effectively, the CH; group acts as a ‘spin probe’ of both the ring conformation and the dynamic processes in which the ring takes part. Because the CH; group has such a minor influence itself, the results are of direct interest to all chemists working with alicyclic rings. Six- and Seven-membered Rings The starting point in this e.s.r. approach to conformational analysis is the cyclohexylmethyl radical. At low temperatures only one radical can be observed,‘ with (at 140 K) a(2H,) = 21.5 G, a(H,) = 30.4 G. This is the chair conformer (2) with the CH; group in the equatorial position.At higher temperatures a second conformer can be o b s e r ~ e d , ~ . ~ which has (at 182 K) a(2H,) = 21.5 G, together with a much larger H, h.f.s. of 41.2 G. That this is the axial conformer (3) was confirmed in two ways. First, the radical derived from trans-4-t-butylcyclohexylmethyl bromide, which must be the equatorial conformer, had an H, h.f.s. very similar to that of (2), whereas the radical derived from cis-4-t-butylcyclohexylmethyl bromide (4) in which the But group necessarily occupies the equatorial position, had an e.s.r. spectrum very similar to that of (3) and with a(H,) = 41.9 G at 140 K.’, * Secondly, the 2-adamantylmethyl radical (5), in which the rigid structure ensures that the CH; group is axial in one ring while the H, is equatorial in this same ring, had an H, h.f.s.of 41.1 G at 140 K. (5 1 The surprisingly large difference in the H, h.f.s. of the axial and equatorial radicals has its origin in the different rotational energy functions of the C a C ; bonds. The equatorial radicals have a(H,) values similar to those of open-chain analogues such as isobutyl [a(H,) = 31.7 G at 140 K].‘ In the axial radicals the barrier is much higher because of steric hindrance from the syn-axial hydrogens at C(3) and C(5) [see (6)]. It is well known that axial substituents at C( 1) experience steric repulsion from syn-axial hydrogen^,^ but the effect on the rotation of the substituent at C(l) had not previously been discovered.F. MacCorquodale and J. C. Walton 3235 The axial radicals showed six-line multiplets from long-range splittings at low temperatures.As the temperature was increased the lines broadened, coalesced at ca. 210 K and finally sharpened into an eight-line pattern at 280 K. We attribute this exchange-broadening to restricted rotation about the C,-C; bond, and a barrier of ca. 6 kcal mol-1 can be estimated from the coalescence temperature.? This is a surprisingly high barrier and it highlights the importance of this kind of syn-axial interaction. In cycloalkylmethyl radicals it has the important consequence of making axial and equatorial conformers distinguishable, and hence enables the e.s.r. method to be extended to a wide range of ring systems. The equatorial preference of the CH; group was quantified by measurements of the concentrations of the two conformers at a series of temperatures.The conformational free-energy difference of the CH; group (- AGioo) was found to be 0.71 kcal mol-'.f This value gives a rough measure of the 'size' of the group and thus CH; is 'larger' than OH and OMe, for which -AGioO values of 0.52 and 0.60 kcal mol-' have been determined,ll but 'smaller' than CH, (-AGioo = 1.74 kcal mo1-')l2 or other alkyl groups.13 Two cyclohexenylmethyl radicals have been studied by e.s.r. spectroscopy. The quasi- equatorial (7) and quasi-axial conformers (8) of the cyclohex-2-enylmethyl radical can be distinguished by their H, h.f.s., which were 30.6 and 32.6 G, respectively, at 140 K.14 (7 1 (8 ) The conformational free-energy difference ( - AGioo) was found to be 0.17 kcal mol-', i.e.much less than in cyclohexylmethyl radicals, as would be expected. The spectra showed exchange broadening in the range 170-280 K, from which the cyclohexene ring inversion barrier was found to be ca. 5.6 kcal mol-', i.e. in reasonable agreement with the free energy of inversion as measured by n.m.r. for cyclohexene, uiz:'5*16 AGS = 5.3-5.4 kcal mol-'. Cyclohex-3-enylmethyl radicals were also observed in two conformations, the quasi- equatorial (9) and quasi-axial (10) which had H, h.f.s. of 28.8 and 32.3 G, respectively, at 140 K.'* This pair of radicals existed as an equimolar mixture over a wide temperature b (10) f Assuming log(A/s-') = 13.0 and Au = 0.8 G together with the usual formula for the exchange rate $ I cal = 4.18 J.constant. lo3236 E.S.R. of Cycles and Bicycles range, i.e. -AG:oo = 0 to within the experimental error. In both types of cyclo- hexenylmethyl radical the CH; group experiences steric repulsion from only one axial (or quasi-axial) hydrogen at C(5), and hence the axial and equatorial conformers are closer in energy. The lower H, h.f.s. for both (8) and (lo), as compared with axial cyclohexylmethyl radicals, are due to the same cause. The cycloheptylmethyl radical shows only one conformer even at very low temperatures. Since pseudorotation is very rapid in the cycloheptane ring,l this is presumably the fast-exchange average spectrum over all possible ring conformations. The cyclohept-4-enylmethyl radical showed the presence of two conformers, the major one (80%) having a(H& = 33.7 and the minor one (20%) having a(H,) = 42.9 G at 200 K.17 The major conformer can be identified as the equatorial chair form (lla).The H, h.f.s. of the minor radical suggests that it is an axial conformer, but there are three main candidates, i.e. [(ll b H l l d)], because both the boat and twist-boat conformations are believed to be stable conformations not greatly above the chair in energy.18. l9 Chair conformers interconvert via the boat and twist-boat conformers and the fast equilib- rium20 ensures that this radical has access to the axial boat form (llc). In this conformation the radical centre is immediately above the double bond in a very favourable position for transannular cyclisation. Reaction of the bromide precursor with ButSnH gave good yields of bicyclo[3.2.lloctane (lz).’’ Similarly, good yields of cyclised products were obtained from cyclo-oct-4-enylniethyl bromide. Medium and Large Alicyclic Rings For these larger rings n.m.r. spectroscopy and X-ray diffraction rarely give unambiguous results, and new methods, such as that provided by the CH; ‘spin probe’, are very desirable. We found that cycloalkylmethyl radicals having nine- to fifteen-membered rings (excluding cyclotetradecyl) show only two conformers which are readily distinguishable by e.s.r. spectroscopy. We label the conformer with the smaller H, h.f.s. quasi-equatorial (QE) and the conformer with the larger HB h.f.s. quasi-axial (QA). Cycloalkylmethyl radicals with ten-membered, or larger, rings had QA conformers with a(HB) values in the range 38.3-40.4 G and QE conformers with a(H,) values between 27.7 and 32.0 G at 140 K.20F.MacCorquodale and J. C. Walton Table 1. Conformer ratios for nine- to fifteen-membered rings 1 .o 1.3 (139) 1.3 1.6 (184) 0.75 0.8 (172) 1 .o 0.7 (21 1) 0.6b 0.5 (195) 9 P331 10 [2323] 11 WI 12 [3333] 13 P461 15 1 .O" 1.1 (231) [ 3 3 33 31 3237 a Statistical ratio for the indicated conformation. Statistical ratio for the [13333] conformation is 1.25. " Same ratio for other quinquangular conformers. The main cyclodecane conformer has a structure designated quadrangular [2323] in Dale's notation.21 There are thus three possible sites for the CH; 'spin probe', i.e. (13a)-(13c), neglecting conformations with this group on the 'inner edges', which will be severely crowded.Rotation of the CH; group will be most strongly impeded at the corner (13a), because of 1,3-interaction with the syn-H-atom on C(3), so we assign structure (13a) to the QA radical. Conformers (136) and (13c) are expected to give e.s.r. spectra of the QE type, and the spectrum does show some signs of two different QE radicals.20 There are eight corner and six outer-edge sites, so the statistical ratio of conformers [13a]/([l3b] + [13c]) would be 1.33, which is in reasonable agreement with the measured [QA]/[QE] ratio (see table 1). The most important conformations of cycloundecane22 and cycl~dodecane~~~ 24 are probably triangular, [335] (14) and quadrangular [3333] (15), respectively. Both these conformations should give rise to just two types of radical and, as table 1 shows, the3238 E.S.R.of Cycles and Bicycles statistical ratio of corner to outer-edge conformations is in reasonable agreement with the experimental [QA]/[QE] ratio. Five low-enthalpy conformations have been found for cyclotridecane by molecular- mechanics 22i 25 An X-ray diffraction structure of a 13-membered ring containing nitrogen showed that the main conformer had the quinquangular [ 133331 However, the e.s.r. data are in better agreement with the triangular [346] structure (table 1). Furthermore, the corner sites next to the one-bond edge in the quinquangular conformer would be expected to give rise to distinctly different spectra, but none were observed. Cyclopentadecane exists as a mixture of about five quinquangular forms21* 22 all having corner/outer-edge statistical ratios of 1 .O.This is reasonably close to the experimental [QA]/[QE] ratio (table 1). The one-bond, corner sites were expected to furnish novel corner radicals, and the e.s.r. spectrum did indeed show unusually broad lines for the QE radical. Dale26 has shown that interconversion of large-ring conformers takes place by a process of ‘corner migration’. The activation energies for this process in nine- to fifteen- membered rings were determined from the exchange broadening in the e.s.r. spectra. The e.s.r. activation energies were of similar magnitude to the free energies of activation (AGI) estimated from the I3C n.m.r. coalescence temperatures (where available). Our measurements also agreed quite well with available barriers calculated by force-field methods.20 The e.s.r.approach to conformational analysis via the CH; ‘spin probe’ gives a number of valuable insights and should prove useful for examining rings containing heteroatoms. Cyclobutylmethyl and Related Rearrangements Cycloalkylmethyl radicals with three- and four-membered rings rearrange rapidly by p- scission to give alkenyl radicals. Thus, the cyclopropylmethyl radical rearranges extremely rapidly and has been widely used as a fast radical The cyclo- Q-7r - 0 - 0 Scheme 1.F. MacCorquodale and J. C. Walton 3239 Table 2. Kinetics of @-scission in cyclobutylmethyl and related radicals radical k/s-l (25 “C) kre, Elkcal mol-I log (Als-l) (16) 4.7 x 103 I .o 12.2 12.6 (17) 2.8 x 104 6.0 11.5 12.9 (19) 3.9 x 1 0 4 8.4 12.5 13.8 cyclopropylme t hyl a 1.3 x lo8 2.8 x 104 5.9 12.5 a Data from ref.(27). butylmethyl radical (16) rearranges at a rate which is convenient for study by both e.s.r. spectroscopy28 and tri-n-butyltin hydride red~ction.~’ This system was chosen for a study of the effect of increasing resonance stabilisation in the rearranged radical. Thus, the kinetics of the rearrangements of cyclobutylmethyl (16), which gives a primary radical, 3-methylenecyclobutylmethyl(17), which gives an ally1 radical (18) and cyclobut- 2-enylmethyl (19), which gives a pentadienyl radical (20), (scheme 1) were studied by both the kinetic e.s.r. method and the tin hydride reduction method. Excellent correspondence between the two methods was obtained in each case and the rate constants and Arrhenius parameters are listed in table 2.The kinetic e.s.r. data were converted to absolute rearrangement rate constants by use of Fischer’s parameters for the combination of t-butyl radical^,^" corrected for solvent viscosity differences. The tin hydride results were converted to absolute rate constants by using the laser flash photolysis result for the rate of H-abstraction from Bu,SnH by primary alkyl radical^.^' The excellent agreement between the two types of measurement is good evidence of the reliability of these two reference rate constants. The rearrangements given in table 2 provide some of the most trustworthy free-radical clocks. The remarkable result, which the data in table 2 reveal, is that product stabilisation of over 12 kcal mol-l in the case of 2-allylallyl (18) and over 20 kcal mol-1 in the case of pentadienyl (20) has comparatively little effect on the rate and activation energy of ring fission.These experimental results suggest that scission of the CP--C, bond is not synchronised with development of resonance stabilisation in the product radical. Similar non-perfect synchronisation of the development of resonance stabilisation, with proton loss, has been observed in carbanion-forming reactions.32 The three radical re- arrangements of scheme 1 were studied using the semiempirical MNDO method,33 and the computed progress of the rearrangement was plotted in the form of a More O’Ferrall-Jencks diagram.34. 35 These diagrams for radicals (17) and (19) showed the type of curvature typical of non-perfect synchronisation, i.e.the MNDO calculations indicate that these rearrangements are extreme cases in which ring fission precedes development of resonance stabilisation. Thus, even large resonance stabilisation energies in the product radicals cause only a very small lowering of the rearrangement activation energy. Spiroalkyl and Bicycloalkyl Radicals Hydrogen abstraction from spiro[2. nlalkanes produces spiro[2. nIalk-2-yl radicals. Only the two lowest members of the series can be observed by e.s.r. spectroscopy because rearrangement by /3-scission is too rapid for the The spiro[2.2]pent-2-yl radical is a o-radical, like cyclopropyl, and it does not rearrange even at temperatures as high as 380 K. Spiro[2.3]hex-2-y1 radicals (21) can be observed at ca.150 K, but rearrange to cyclobutenylethyl radicals (22) at higher temperatures. The rate of ring fission is ca. an order of magnitude slower than in cyclopropylmethyl radicals : probably because of the creation of the strained cyclobutene ring in (22). The Ha h.f.s. in radical (21) is3240 E.S.R. of Cycles and Bicycles unusually low at 19.6 G,36 suggesting that spin density is delocalised into the Walsh orbitals of the adjacent cyclopropane ring. The orientation of the p orbital containing the unpaired electron is favourable for this type of delocalisation. Thus, the hydrogens at C(2) in spiro[2.3]hexane should be activated towards hydrogen abstraction. Experiments in which the concentration of the spiro[2. nlalk-2-yl radical, relative to that of other secondary radicals derived from the ring, was measured by e.s.r.spectroscopy indicated significant activation. A similar study of spiro[3.3]heptane (23), which would not be expected to show any activation at C(2), gave essentially equal concentrations of the two possible H-abstraction products (after statistical correction). In order to quantify the extent of this cyclopropylmethyl (cpm) stabilisation the rotation barrier about the CB-C; bond in cyclopropylmethyl radicals was examined. Exchange broadening due to restricted rotation was observed at low temperature^,^' and from the matching of experimental and computed spectra a barrier of 2.8 kcal mol-1 was obtained. This implies a small but chemically significant stabilisation in cpm radicals of ca. 2.5 kcal mol-l.Bicyclo[n . m . llalkanes can undergo H-abstraction either at the bridgehead or at the methylene groups in the bridges. The simplest member of this series, bicyclo- 1.1. llpentane (24) preferentially undergoes bridgehead abstraction to give a radical with a very large y - h . f . ~ . ~ ~ This indicates significant cross-ring orbital overlap, which may explain the unusual bridgehead reactivity. The bridgehead radical from the next member of the series, bicyclo[2.1. llhexane (25), has been observed previo~sly~~ and the large y- h.f.s. from the remaining bridgehead hydrogen suggested that H-abstraction from the bridgehead of (25) might also be favoured. We found that the only radical detectable by e.s.r. in this case was bicyclo[2.l.l]hex-2-yl derived by abstraction of one of the bridge (C,) hydrogens.However, the less selective bis(trimethylsily1)aminyl radicals did give significant bridgehead attack; see scheme 2. For bicyclo[2.2. llheptane (norbornane) (26) the bridgehead radical shows only a small h.f.s. from the remaining bridgehead hydrogen4' and, as would be expected on this basis, bridgehead hydrogen abstraction was negligible. We thank NATO for a travel grant, without which much of this work could not have been undertaken.F. MacCorquodale and J. C. Walton 324 I 69.9 100 50 22.5 0 0 2 9 2.25 + 27.7 20. 0 BU'O. 50 CI 1.3 0.7 H 0.7 H 3.9 >270' 100 Bu'O* 100 Br 71 ( Me3Si )zN' 0 3 100 Br* 97 (Me,S i IzN Scheme 2. Bicyclo[n. m . llalkyl radicals. References 1 F. A. L. Anet and R. Anet, in Dynamic Nuclear Magnetic Resonance Spectroscopy, ed.L. M. Jackman and F. A. Cotton (Academic Press, New York, 1975), chap. 14, p. 543. 2 J. Dale, Top. Stereochem., 1976, 9, 199. 3 U. Berkert and N. L. Allinger, Molecular Mechanics, ACS Monograph 177 (American Chemical Society, Washington, D.C., 1982), p. 89. 4 R. Briere, H. Lemaire and A. Rassat, Bull. SOC. Chim. Fr., 1965, 3273; R. E. Rolfe, K. D. Sales and J. H. P. Utley, J. Chem. SOC., Perkin Trans. 2, 1973, 1171; R. Chiarelli and A. Rassat, Tetrahedron, 1973, 29, 3639. 5 G. A. Russell, in Radical Ions, ed. E. T. Kaiser and L. Kevan (Wiley-Interscience, New York, 1968), chap. 3, p. 87. 6 M. L. Kemball, J. C. Walton and K. U. Ingold, J. Chem. SOC., Perkin Trans. 2, 1982, 1017. 7 K. U. Ingold and J. C. Walton, J. Am. Chem. SOC., 1985, 107, 6315. 8 K.U. Ingold and J. C. Walton, J. Chem. SOC., Perkin Trans. 2, 1986, 1337. 9 E. L. Eliel, N. L. Allinger, S. J. Angyal and G. A. Morrison, Conformational Analysis (Interscience, New York, 1967), chap. 2, p. 36. 10 G. A. Russell, G. R. Underwood and D. C. Lini, J. Am. Chem. SOC., 1967, 89, 6636. 11 J. A. Hirsch, Top. Stereochem., 1967, 1, 199. 12 H. Booth and J. R. Everett, J. Chem. Soc., Chem. Commun., 1976, 278. 13 H. Booth and J. R. Everett, J. Chem. SOC., Perkin Trans. 2, 1980, 255. 14 J. C. Walton, J. Chem. SOC., Perkin Trans. 2, 1986, 1641. 15 F. A. L. Anet and M. Z. Haq, J. Am. Chem. Soc., 1965, 87, 3147. 16 F. R. Jensen and C. H. Bushweller, J. Am. Chem. SOC., 1969, 91, 5774.3242 E.S.R. of Cycles and Bicycles 17 F. MacCorquodale and J.C. Walton, J. Chem. Soc., Chem. Commun., 1987, 1456. 18 N. L. Allinger and J. T. Sprague, J . Am. Chem. SOC., 1977, 94, 5734. 19 D. N. J. White and M. J. Bovill, J . Chem. Soc., Perkin Trans. 2, 1977, 1610. 20 K. U. Ingold and J. C. Walton, J . Am. Chem. Soc., 1987, 109, 6937. 21 J. Dale, Acta Chem. Scand., 1973, 27, 11 15. 22 F. A. L. Anet and T. N. Rawdah, J . Am. Chem. SOC., 1978, 100, 7810. 23 J. D. Dunitz and H. M. M. Sheaver, Helv. Chim. Acta, 1960, 43, 18; J. D. Dunitz, Perspect. Struct. 24 F. A. L. Anet and T. N. Rawdah, J. Am. Chem. Soc., 1978, 100, 7166. 25 B. H. Rubin, M. Williamson, M. Takeshita, F. M. Menger, F. A. L. Anet, B. Bacon and N. L. 26 J. Dale, Acta Chem. Scand., 1973, 27, 1130. 27 D. Griller and K. U. Ingold, Acc. Chem. Res., 1980, 13, 317. 28 K. U. Ingold, B. Maillard and J. C. Walton, J . Chem. Soc., Perkin Trans. 2, 1981, 970. 29 A. L. J. Beckwith and G. Moad, J . Chem. Soc., Perkin Trams. 2, 1980, 1083. 30 H. Schuh and H. Fischer, Znt. J. Chem. Kinet., 1976, 8, 341. 31 C. Chatgilialoglu, K. U. Ingold and J. C. Scaiano, J. Am. Chem. Soc., 1981, 103, 7739. 32 C. F. Bernasconi, Acc. Chem. Res., 1987, 20, 301. 33 J. J. P. Stewart, QCPE No. 455, 1983. 34 R. A. More O’Ferrall, J . Chem. SOC. B, 1970, 274. 35 D. A. Jencks and W. P. Jencks, J, Am. Chem. SOC., 1977, 99, 7948. 36 C. Roberts and J. C. Walton, J . Chem. SOC., Perkin Trans. 2, 1985, 841. 37 J. C. Walton, Magn. Reson. Chem., 1987, 25, 998. 38 B. Maillard and J. C. Walton, J. Chem. Soc., Chem. Commun., 1983, 900. 39 T. Kawamura and T. Yonezawa, J. Chem. Soc., Chem. Commun., 1976, 948. 40 T. Kawamura, M. Matsunaga and T. Yonezawa, J. Am. Chem. Soc., 1975, 97, 3234. Chem., 1968, 2, 1. Allinger, J. Am. Chem. SOC., 1984, 106, 2088. Paper 8/01525B; Received 18th April, 1988

ISSN:0300-9599    

DOI:10.1039/F19888403233

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(10), 3243-3248 Is Gram-negative Shock a Free-radical-mediated Condition? Simon K. Jackson and J. Marshall Stark Department of Medical Microbiology, University of Wales College of Medicine, Heath Park, Cardig CF4 4XN, Wales Christopher C. Rowlands and Jeffrey C. Evans* Department of Chemistry, University College Cardif, P.O. Box 78, Cardig CFI IXL, Wales The involvement of free radicals in endotoxic shock (a severe condition occurring in many Gram-negative septicaemias) has been investigated in a mouse model. Administration of endotoxin produced an increase in the e.s.r.-detectable signals due to ascorbyl radicals. In vivo spin-trapping experiments with the spin trap phenyl t-butyl nitrone resulted in the formation of poorly resolved adduct spectra.The results suggest that free radicals are produced during the endotoxic shock crisis and may play a significant role in the pathophysiology of this condition. Bacterial lipopolysaccharides (endotoxins) are an integral component of the outer cell membrane of Gram-negative micro-organisms. Liberation of endotoxin is considered to be the main cause of the high mortality associated with bacteraemia caused by Gram- negative organisms, despite the introduction of new antibiotics and improved patient care.l Injection of nanogram or microgram amounts of lipopolysaccharide into susceptible experimental animals can produce the well known sequence of events in septicaemic (endotoxic) shock. These are associated with multiple haematologic events, disseminated intravascular coagulation, intestinal bleeding and death.Despite a wealth of experimental observations, little is known of the precise sequence of biochemical events initiating this frequently fatal condition. Recent evidence suggests that Gram-negative (endotoxic) shock can be regarded as an oxidative crisis event.3* We have used a mouse model of endotoxic shock to determine if free radicals are produced in this crisis. Experimental Mice Adult female TO strain mice were used. Mice were made sensitive to endotoxin by intravenous injection of Bacille-Calmette-Guerin (BCG) (Glaxo, ca. 1 O7 live organisms per mouse) 14-1 8 days before endotoxin administration. Endotoxin Lipopolysaccharide from Escherichia coli (E. coli) 0 1 1 1 : B4 (phenol extract, Sigma) was used.Endotoxin was injected intraperitoneally (i.p.) in 0.25 cm3 physiological saline (10 pg per mouse). In Vivo Spin Trapping The spin trap phenyl t-butyl nitrone (PBN) was dissolved in 10% dimethyl sulphoxide (DMSO) (1 50 mmol dm-3) and injected i.p. (0.25 cm3) either 15 min before or at the 32433244 Is Gram-negative Shock Free-radical Mediated? A 6 H 0.5 mT Fig. 1. E.s.r. spectra obtained from mouse spleen homogenates before (A) and 2 (B) and 6 (C) h after administration of endotoxin in vim. H 0.5mT Fig. 2. E.s.r. spectrum of 1 mmol dm-3 aqueous solution of ascorbic acid oxidized in air. same time as the endotoxin. 1, 2 and 4 h after endotoxin treatment, the animals were sacrificed and the livers and spleens removed. The tissues were homogenised in chloroform-methanol (2: 1) to extract lipids as per the procedure of F01ch.~ E.S.R.E.s.r. spectra were recorded on a Varian El09 spectrometer operating at 9.5 GHz with 100 kHz field modulation. Results Fig. 1 shows the development of the e.s.r. signal seen in mouse spleen homogenates before administration of endotoxin (A), and 2 h (B) and 6 h (C) after administration ofS. K . Jackson et al. 3245 A t0S;;;ii B Fig. 3. E.s.r. spectrum of mouse spleen homogenates from animals given endotoxin either with (A) or without (B) polymyxin-B. Fig. 4. E.s.r. spectrum obtained from liver lipid extract from mouse receiving endotoxin and the spin trap PBN. Fig. 5. E.s.r. spectrum of PBN- and endotoxin-treated mice showing a triplet signal due to rearrangement of the spin trap in viuo.I07 F A R I3246 Is Gram-negative Shock Free-radical Mediated? endotoxin. The doublet spectrum, aH = 0.178 mT, is characteristic of the ascorbyl free radical ;6 oxidation of an aqueous solution of ascorbic acid (1 mm dm-3) in air produced an e.s.r. signal with the same splitting constant (fig. 2). When BCG-primed mice were injected with the endotoxin binding agent polymyxin- B 10 min after an injection of endotoxin, the resulting spleen homogenate ascorbyl signal was decreased (fig. 3B) compared to animals that had not received polymyxin (A). Administration of ascorbic acid (50 mg) to BCG-primed mice did not produce a significantly increased ascorbyl radical e.s.r. signal in tissue homogenates. Fig. 4 shows the e.s.r. spectrum obtained from the lipid extract from livers of BCG-primed mice injected with endotoxin and PBN.This spectrum shows a triplet (a, = 1.50 mT) with a small doublet splitting, which could not be properly resolved. Often, the signal obtained with the spin trap was only a triplet (fig. 5), a , = 1.50 mT, with no /?-hydrogen coupling. Discussion The results demonstrate that endotoxin induces the formation of ascorbyl radicals and adducts of the spin trap PBN in mice. The ascorbyl radical has been detected in various tumour tissues7 and may be associated with tissue damage. In our experiments, endotoxin administration consist- ently resulted in an increased e.s.r. signal due to ascorbyl radicals. Tissues from mice not given endotoxin sometimes produced a very small doublet due to ascorbyl radical.The ascorbyl radical concentration was greatest ca. 6 h after endotoxin administration in vivo, the time at which the endotoxic crisis in BCG-primed mice is most severe. This suggests a definite link between the production of ascorbyl radicals and the intensity of the shock state. Ascorbyl radicals (A'-) decay to ascorbate (AH-) and dehydroascorbic acid (A) : 2A- + H + e A H - +A. Dehydroascorbic acid is converted back to ascorbic acid by reductase, using glutathione as a reducing agent.8 It is postulated that ascorbic acid/dehydroascorbic acid ratios are sensitive indications of cell physiology, including pathogenic ~ t a t e s . ~ Since glutathione is the reducing agent for dehydroascorbic acid reductase, a shift in the ascorbic acid/dehydroascorbic acid ratio or an increase in the production of ascorbyl radicals might be related to the oxidation state of the cell.Polymyxin-B is an antibiotic produced by Bacillus polymyxa that binds preferentially with the lipid component of lipopolysaccharide, presumably, therefore, preventing the interaction of lipopolysaccharide with its target cells. Its use in humans, however, is limited owing to its toxicity. In our mouse model, polymyxin-B reduced the e.s.r. signal due to ascorbyl radicals and it is thus shown that endotoxin is directly affecting the production of this radical. The in vivo spin-trapping studies showed that administration of endotoxin to susceptible mice resulted in the formation of spin adducts (fig. 4). The spectra of the PBN spin adducts, extracted in the lipids from the liver, were poorly resolved owing to line- broadening.It is known that endotoxin can promote lipid peroxidation'' and stimulates the production of antioxidant enzymes. It is therefore possible that lipid radicals formed during the peroxidation of membrane lipids were being trapped. PBN administration protected rats from the lethal effects of endotoxin," lending weight to the idea that endotoxin toxicity is due, at least in part, to the production of free radicals. Recently, we have demonstrated that endotoxin can stimulate the production of superoxide radicals from activated macrophates in vitro.13S. K. Jackson et al. 3247 The triplet spectrum obtained with PBN (fig. 5 ) with no b-H splitting is clearly due to a breakdown product of the spin trap, which might be rearranging and trapping t-butyl radicals to form di-t-butyl nitroxide.This species was not detected in samples from animals not receiving endotoxin, however, and so the endotoxin administration might be altering the metabolism of this compound in vivo. Taken together, the results suggest that endotoxin is promoting tissue damage via free- radical mechanisms. The initial free radical result is probably release of superoxide from activated macrophages and neutrophiles, known to accumulate during the endotoxin crisis.14 The ascorbyl radical might be a sensitive indicator of this tissue damage. References 1 B. E. Kreger, D. E. Craven, P. C. Carling and W. R. McCabe, Am. J. Med., 1980, 68, 332. 2 D. C. Morrison and J. L. Ryan, A h . Immunol., 1979, 28, 293.3 K. A. Knox, G. Smith, P. L. Yap and H. A. Leaver, Biochem. SOC. Trans., 1987, 15, 544. 4 E. S. Lee, A. G. Greenburg, P. W. Maffuid, E. D. Melcher and T. S. Velky, J. Surgical Res., 1987, 5 J. Folch, M. Lees and G. H. Sloane-Stanley, J. Biol. Chem., 1957, 226, 497. 6 I. Yarnazaki, H. S. Mason and L. Piette, J. Biol. Chem., 1960, 235, 2444. 7 N. J. F. Dodd and H. M. Swartz, Br. J. Cancer, 1984, 49, 65. 8 Y. Yamamoto, M. Sat0 and S. Ikeda, Bull. Jpn SOC. Sci. Fish., 1977, 43, 59. 9 J. A. Edgar, Nature (London), 1970, 227, 24. 42, 1. 10 T. Yoshikawa, M. Murakami, Y. Furukawa, H. Kato, S. Takemura and M. Kondo, Thromb. 1 1 J. M. Stark, S. K. Jackson and J. Parton, in Free Radicals in Lipid Chemistry and Human Pathology, ed. 12 G. P. Novelli, P. Angiolini and R. Tani, in Free Radicals in Liuer Injury, ed. T. F. Slater (IRL Press 13 S. K. Jackson, J. M. Stark, C. C. Rowlands and J. C. Evans, Free Rad. Biol. Med., submitted. 14 M. Suematsu, S. Miura, M. Suzuki, H. Nagata, T. Morishita, C. Oshio and M. Tsuchiya, J. Clin. Lab. Haemost., 1983, 49, 214. C. Rice-Evans and T. Dormandy (Richelieu Press, London, 1988), chap. 9. Oxford, 1985), pp. 225-228. Immunol., 1988, 25, 41. Paper 8/01562G; Received 20th April, 1988 107-2

ISSN:0300-9599    

DOI:10.1039/F19888403243

出版商:RSC    

年代:1988

数据来源: RSC