Electron Spin Resonance: Volume 9 (Specialist Periodical Reports - Electron Spin Resonance) - Hardcover

Hardcover

ISBN 10: 0851868312 ISBN 13: 9780851868318

Verlag: Royal Society of Chemistry, 1985

Inhaltsangabe

Reflecting the growing volume of published work in this field, researchers will find this book an invaluable source of information on current methods and applications.

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Electron Spin Resonance Volume 9

A Review of the Literature Published between December 1982 and May 1983

By P. B. Ayscough

The Royal Society of Chemistry

Copyright © 1985 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85186-831-8

Contents

Chapter 1 Theoretical Aspects of E.S.R. By A. Hudson, 
Chapter 2 Transition-metal Ions By D. Gatteschi, 
Chapter 3 Inorganic and Organometallic Radicals By M. C. R. Symons, 
Chapter 4 Organic Radicals in Solids By T. J. Kemp, 
Chapter 5 Organic Radicals in Solution By B. J. Tabner, 
Chapter 6 Applications of E.S.R. in Polymer Chemistry By D. J. T. Hill, J. H. O'Donnell, and P. J. Pomery, 
Chapter 7 Spin Labels: Biological Systems By Ching-San Lai, 
Chapter 8 Metalloproteins By N. J. Blackbum, 
Chapter 9 Applications of E.S.R. in Medicine By N. J. F. Dodd, 

CHAPTER 1

Theoretical Aspects of E.S.R.

BY A. HUDSON

l Introduction

The theory of E.S.R. is well understood and this chapter will be largely concerned with applications rather than new developments. There has been a significant increase in the number of papers dealing with spin-polarized (CIDEP) spectra. Moreover, such experiments are now being performed on chemically interesting systems. There was a time when CIDEP seemed to be largely confined to the photolysis of duroquinone.

The development of experimental techniques for the production of radical cations from saturated compounds has provided experimental data on a large number of interesting new radicals. The interpretation of the spectral parameters in terms of molecular geometries has led to a number of controversies and both ab initio and semi-empirical molecular orbital calculations have been used in an attempt to resolve some of the problems involved.

2 Numerical Methods and Spectral Analysis.

A new journal devoted to the use of computers in spectroscopy contains descriptions of microprocessor based data accumulation and reduction systems. Other recently described data systems include one based on an Apple II Plus (48 K) and another based on s-100 bus components.

Morton and Preston have given a detailed account of the use of a computer-assisted two-circle goniometer to assemble the hyperfine-interaction and g2 tensors in the crystallographic axis system. An explanation is given for each of the seven crystal classes. A general method has been described for extraction of the g-tensor from single crystal data. A simplified method has been developed for dealing with spectra showing strong quadrupale interactions and applied to the case of Ir2+ in MgO. Stevenson has published an extensive theoretical discussion of triplet state spectra.

The computer simulation of powder spectra is virtually essential if several hyperfine interactions are involved, A good recent example involves the spectrum of NF3+ trapped in a rigid matrix at 25 K. The program described is accurate to second order in all hyperfine terms and copes with up to four nuclei. Rieger has written a program which performs a least squares analysis of powder patterns with noncoincident principal axes of the g and hyperfine tensors. Graphs have been published which may be used for the interpretation of powder spectra from axial sites with effective spins ranging from S=1 to S=5/2. The resonance fields are presented as functions of the zero-field splitting. A powder pattern analysis has also been described for cubic sites of FE3+ in MgO12. In disordered solids it is necessary to include the distribution of spin Hamiltonian parameters. Bala and Kliava have simulated spectra for d1 ions including Mo5+ in a phosphate glass.

A detailed study of the powder spectrum of an 15N enriched nitroxide has led to the conclusion that such probes are significantly better than 14N nitroxides in polycrystalline or amorphous systems since the hyperfine features are well resolved at X-band. An alternative solution is to work at Q-band and simulations have been presented of first and second derivative spectra for a variety of spin labels in frozen solutions. A key factor in the successful simulation of field swept spectra is the Aasa and Vanngard l/g factor. Pilbrow has considered this point in some detail in a discussion of lineshapes for frequency-swept and field-swept spectra. Phillips and Herring have investigated the use of dispersion absorption plots for detecting lineshape distortions attributable to either the spectrometer or the sample. An interactive method allows for the elimination of baseline drift. A fast deconvolution procedure has been described for inhomogeneous resonance lines. The deconvolution of hyperfine splittings is necessary in the investigation of paramagnetic spin distributions by E.S.R. imaging.

There have been further developments in the simulation of isotropic solution spectra using fast Fourier transform methods. A general iterative least-squares procedure has been described for fitting complex dynamic lineshapes in the fast motional region. Two papers have been concerned with the problem of superhyperfine structure when simulating the E.S.R. spectra of nitroxide spin probes in solution. This is relevant to the measurement of spin exchange rates and the use of spin probes for monitoring the concentration of dissolved oxygen in biological samples. Bales has experimentally verified a previously published procedure for extracting spin exchange rates from inhomogeneously broadened lines .

A procedure based on correlation methods has been applied to the analysis of weak spectra. It is particularly useful for locating satellites due to 13C or 29SI in natural abundance. A product function produced by correlation of the digitized experimental spectrum with a test spectrum is used as a criterion of the goodness of fit. The test spectrum is initially a single line but becomes increasingly complex as the analysis procedes and additional coupling constants are located.

3 Spin-relaxation and Line-broadening Effects

An interesting review article by Kurreck and his coworkers includes a good account of the various relaxation processes involved in ENDOR and its extension to triple resonance experiments.

There have been few developments in the theory of spin relaxation and lineshapes. Baram has discussed nonsecular lineshapes in the slow motion region. Exact solutions have been obtained for the modified Bloch equations with n-site chemical exchange and a new fonnalism has been applied to the two-site problem in the intermediate exchange region. Kooser and Resing have investigated the effects of two-dimensional reorientation in partially ordered systems. A generalised treatment for slow diffusional reorientation of axially syrranetric hyperfine centres has been. applied to the spectra of nitroxide probes in amorphous polymers. Formulae have been derived for averaged tensor components when a dynamic process occurs between two symmetry related sites in a single crystal. Relaxation processe have been considered in trimeric clusters including both isotropic and anisotropic exchange interactions. Zaspel has studied the effect of antisymmetric exchange on E.S.R. linewidths and has shown that the angular dependence can have cl period of 360° instead of 180°. The lineshapes of alkali metal biphenyl salts have provided evidence for spin diffusion in these quasi-two-dimensi.onal magnetic systems.

Spin exchange in solution has been investigated for nitroxides in nematic liquid crystals and in hydrocarbon solvents. In the latter case particular emphasis was given to the intermediate exchange region. A useful insight into the effects of spin exchange and rotational diffusion has been obtained by comparing the E.S.R. linewidths for the different magnetic isotopes in a molybdenum complex. A wide range of rotational correlation times has been found for vanadyls isolated from oil shale. Linewidth variations in the solution spectra of dithiazolyl and related radicals have been analysed to calculate hydrodynamic radii. A pronounced temperature dependence in the E.S.R. spectrum of the pentafluorocyclopentadienyl radical originates from averaging of the 19F anisotropic hyperfine tensor.

We shall not deal in this chapter with the numerous applications of spin probes in biological systems. However, their use is not restricted to investigating the dynamical properties of biomolecules. Both neutral and positively charged nitroxides have been employed in a study of micelles formed from sulphate surfactants. Large and nearly cylindrical probes are highly ordered within the channels of thiourea-cyclohexane inclusion compounds, whereas smaller and nearly spherical probes reorient 1sotrop1cally45 An investigation of spin-labeled rodlike poly(benzylglutamate) suggests that the mac:romolecule behaves hydrodynamically as a very porous cylinder with an impenetrable core. Meirovitch reports on stretching-induced molecular mobility and the partitioning of spin probes among different sites in semicrystalline low-density polyethylene films. Another contribution deals with the intrinsic flexibility gradient found in hydrocarbon chains in lipid bilayers. Studies of liquid crystals include an investigation of molecular dynamics at the nematic to smectic A transition and an analysis, using parallel-edge lines, of the orientational distribution of a spin probe in rigid MBBA. The E.S.R. spectra of nitroxides in some magnetically aligned liquid crystals formed from surfactants have been interpreted in terms of cylindrical micelle and disc-shaped micelle structures. A combination of ELDOR, saturation and E.s.R. linewidth measurements has been used to study deviations from the Brownian motion model. Additional relaxation terms have been formulated in terms of a slowly relaxing local structure mechanism. The motion of midchain peroxy radicals in poly{tetrafluoroethylene) is consistent with a model involving helical twisting of the polymer axis.

Two simple methods of determining the microwave field strength in E.S.R. should be useful in saturation transfer experiments. Recent applications of the latter technique include a study of multiple motions of the spectrin-actin complex and an investigation of maleimide spin-labeled cowpea chlorotic mottle virus. [15N] nitroxides have been employed to determine the effects of non-coincident magnetic and diffusion tensor axes when there is anisotropic rotational diffusion. Fajer and Marsh have analysed the sensitivity of X-band ST spectra to anisotropic rotation. The influence of experimental parameters has been considered by Delmelle; the effects of overmodulation are difficult to include in simulation& because of very long computation times and Robinson has suggested various appoximations to overcome this problem. The results of varying the modulation frequency have also been the subject of recent attention. Improvements in cavity design have led to renewed interest in the dispersion mode method of recording ST E.S.R. spectra. A technique has been described for dealing with multicomponent spectra and applied to membrane systems. Saturation transfer has also been studied in pulse experiments using an electron spin-echo spectrometer. The motion of nitroxide radicals in dibutyl phthalate is best accounted for by a large-angle jump model. Very slow motion can also be detected by a method involving double modulation of the E.S.R. spectrum. Another possibility is to use an ELDOR technique based upon spin echoes and rapid stepping of the magnetic field.

Baram has developed a theory of 1pin echoes in the slow motion regime68 Pulse techniques feature increasingly in the literature and this tend will continue as the necessary equipment becomes commercially available. The diffusion of spins in the radical cation salt (fluoranthenyl)2 +AsF6 has been measured by observing electron spin echo decays in a magnetic field gradient. Irradiation of methanol absorbed on zeolites generates hydroxymethyl radicals. Electron spin echo spectra of these radicals have then been used to elucidate the geometrical arrangement of surrounding methanol molecules. A treatment of nuclear quadrupole effects on electron spin echo modulation by Shubin and Dikanov has been extended to three pulse sequences by Kevan and coworkers who point out a number of limitations in the expressions obtained by perturbation theory. Quadrupole splitt:Lng parameters have been estimated for 14N nuclei in a nitroxide biradical.

The determination of kinetic parameters from linewidth variations is long standing and well established. Stevenson and his coworkers have investigated the hydrogen bonding of ethanol to the radical anion of p-cyanonitrobenzene in hexamethylphosphoramide and report for the first time activation parameters controlling hydrogen bond formation to an anionic species. Further work has been reported on intermolecular exchange of sodium ions in dinitrobenzene ion pairs. A detailed study of restricted rotation of the acetyl group in m- and p-nitroacetophenone anion radicals has been used to establish an optimum procedure for obtaining thermodynamic parameters from dynamic E.S.R. spectra Ion pair formation has a significant influence on the hindered rotation of the phenyl rings in nitrobenzophenone radical anions. The dynamic behaviour of the 5-hydro-6-methyl-6-yl-uracil radical has been studied in single crystals, polycrystalline powders and aqueous glasses. The detected variations can be accounted for by different molecular packings in the respective matrices. Barriers to rotation determined by E.S.R. have been used to estimate stabilisation energies in aminoallyl, aminopropynyl, and aminocyanomethyl radicals. Unusually large barriers to methyl group rotation have been found in cation radicals formed from methyl and ethyl esters. Barriers to rotation can also be estimated from the terature dependence of β-hyperfine coupling constants. A recent unusual example involves muonic-subatituted ethyl radicals.

Electron spin transfer between naphthalene π-systems, which was first studied in 1961 by Atherton and Weissman, is still the subject of interesting research. A recent study involved a careful analysis, using E.S.R. and ENDOR, of intramolecular transfer between naphthalene rings separated by a variable number of spirobonded cyclobutane rings. With 3 or 5 of the latter, the intramolecular spin transfer was slow on the hyperfine time scale, but fast exchange was observed for (1) in media of high solvating power. The E.S.R. and ENDOR spectra actually consisted of a superposition of fast and slow exchange spectra and these have been assigned to the syn- and anti-conformations of (l) since the distance between the naphthalene units is significantly shorter in the former.

4 CIDEP

The problem of developing a consistent mathematical theory for a spin 1/2 system undergoing first-order spin-selective reactions, has been considered by Pottinger and Lendi, who derive generalised Bloch equations employing the theory of quantum Markovian master equations. The results of a theory of CIDEP and Heisenberg spin exchange have also been presented in terms of Bloch-type equations. An unusual pattern of oscillations in the time-resolved CIDEP signal has been observed in a study of photoelectrons in Rb/THF solutions. Baer and Paul have observed stationary nutations in the spectra of benzyl radicals produced by photolysis of methyl benzyl ketone with modulated UV light. This experiment yields both the relaxation times and the CIDEP of the spin system. Another novel experiment involves using the extra sensitivity offered by chemically induced polarization effects to detect the ENDOR of short lived radicals. If radicals are generated by a laser pulse in the absence of a microwave field which is later applied in a continuous wave fashion, the evolution and observation of the magnetization can be separated in a two-dimensional experiment, which yields the spin-lattice relaxation time and the polarization ratio. Basu and McLauchlan have shown how a non-uniform concentration of free radicals produced by a light pulse can affect the subsequent kinetic behaviour of the system. It has been pointed out that overlapping spectra can often be separated in a time-resolved experiment by utilizing the different temporal variations of the two signals. A theory based on the Bloch equations has been used to account for the influence of electron transfer reactions on CIDEP spectra. The Bloch equation approach has also been used to analyse a time-integration method which removes spurious sidebands in the E.S.R. spectra of spin-polarized radicals produced by laser flash photolysis. The same paper also contains a useful discussion of numerical resolution-enhancement methods.

The time integration method has been used to study the spectra of a-aminoalkyl radicals produced by flash photolysis of benzene-1,2:3,4-tetracarboxylic dianhydride in the presence of tertiary amines. The oxford group have also reported on the photophysics and photochemistry of diazanaphthalenes and methylpyrazines. Interesting results have been found for radicals derived from aliphatic ketones. It has been shown that the zero-field splitting constant in the photoexcited triplet state of such molecules is normally positive. The 700 G doublet splitting of phosphonyl radicals shows a marked E-A polarization when generated by photolysis of a dialkylphosphite in di-t-butyl peroxide. This is the first example of CIDEP for a radical in which the unpaired electron is located on phosphorus. Wan and his coworkers have also reported on CIDEP studies involving chromone and chromanone, ascorbic acid, and some sulphur cation radicals. A method of discriminating between F-pair polarization and the triplet mechanism has been applied to the durosemiquinone radical in a variety of solvents.

(Continues...)

Excerpted from Electron Spin Resonance Volume 9 by P. B. Ayscough. Copyright © 1985 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Details zur Ausgabe

• Verlag: Royal Society of Chemistry
• Erscheinungsdatum: 1985
• ISBN 10: 0851868312
• ISBN 13: 9780851868318
• Einband: Tapa dura
• Sprache: Englisch
• Anzahl der Seiten: 400
• Herausgeber: Ayscough P B
• Kontakt zum Hersteller: Nicht verfügbar