Electron Spin Resonance: Volume 13B (Specialist Periodical Reports - Electron Spin Resonance, 13B) - Hardcover

Hardcover

ISBN 10: 0851869114 ISBN 13: 9780851869117

Publisher: Royal Society of Chemistry, 1993

Synopsis

Specialist Periodical Reports provide systematic and detailed review coverage of progress in the major areas of chemical research. Written by experts in their specialist fields the series creates a unique service for the active research chemist, supplying regular critical in-depth accounts of progress in particular areas of chemistry. For over 80 years the Royal Society of Chemistry and its predecessor, the Chemical Society, have been publishing reports charting developments in chemistry, which originally took the form of Annual Reports. However, by 1967 the whole spectrum of chemistry could no longer be contained within one volume and the series Specialist Periodical Reports was born. The Annual Reports themselves still existed but were divided into two, and subsequently three, volumes covering Inorganic, Organic and Physical Chemistry. For more general coverage of the highlights in chemistry they remain a 'must'. Since that time the SPR series has altered according to the fluctuating degree of activity in various fields of chemistry. Some titles have remained unchanged, while others have altered their emphasis along with their titles; some have been combined under a new name whereas others have had to be discontinued. The current list of Specialist Periodical Reports can be seen on the inside flap of this volume.

About this title

Excerpt. © Reprinted by permission. All rights reserved.

Electron Spin Resonance Volume 13B

A Review of Recent Literature to mid-1992

By M.C.R. Symons

The Royal Society of Chemistry

Copyright © 1993 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85186-911-7

Contents

CHAPTER 1 Transition Metal Ions By Alessandro Bencini and Claudia Zanchini, 1, 
CHAPTER 2 Metalloproteins By G.R. Hanson, 86, 
CHAPTER 3 EPR and ENDOR in the Lanthanides By J.M. Baker, 131, 
CHAPTER 4 Simulation and Analysis of ESR Powder Patterns By Philip H. Rieger, 178, 
CHAPTER 5 Inorganic and Organometallic Radicals By Martyn C.R. Symons, 200, 

CHAPTER 1

Transition Metal Ions

BY ALESSANDRO BE NCI NI AND CLAUDIA ZANCHINI

1 Introduction

E. s. r. spectroscopy has been applied in a number of research fields mainly as a diagnostic tool for determining the spin and/or the oxidation state of the transition metal ion. A large number of papers therefore appeared, dealing with straightforward applications of e.s.r., while in other cases e.s.r. was applied to systems which were found to be too complicated to permit a complete spectroscopic characterization. An example of these latter systems are the transition-ion containing clusters found in living organisms (usually formed with iron and manganese) and their synthetic analogues, whose synthesis and crystallographic characterization have received much attention in the last few years. In order to rationalize this harvest of data we follow (with minor modifications) the classification scheme adopted in issue 12B. In part 1 we will report review articles of general interest or books; in part 2 attention will be given to "areas of interest" like studies on phase transitions, mixed valence systems, superconductors, etc.; in part 3 and subsequent sections the systematic of e.s.r. is presented classifying the systems according to the spin of the transition metal ion. Only articles written in English will be generally reviewed and we will try to put emphasis on papers reporting on rather uncommon spin systems or papers in which a characterization as detailed as possible has been performed, especially when data coming from more than one spectroscopic and physico-chemical technique were interpreted together. In order to avoid as much as possible the overlap among different areas of interest we have separated complexes in different sections. Oligonuclear complexes containing mixed valences (generally delocalized) have been grouped in the Mixed Valence Systems section, while those exhibiting long range exchange interactions have been reported in the Extended Systems section. Magnetic exchange interactions will be represented with the spin hamiltonian H = Jij Si • Sj. A negative value of Jij indicates a ferromagnetic interaction. Readers chiefly interested in the nature of the transition ion or in high nuclearity systems must therefore check more than one section..

The book from J.R. Pilbrow entitled Transition Ion Electron Paramagnetic Resonance gives an up to date account of the applications of the e.s.r. spectroscopy to transition metal ions. The book covers the fundamental aspects of e.s.r. of transition metal ions and also presents the principles of both CW and pulsed techniques and zero-field e.s.r. One chapter is also devoted to the principles and techniques of simulation of e.s.r. spectra.

Several review articles dealing with various applications of e.s.r. spectroscopy and related techniques appeared. Particularly interesting is the article by Trautwein et al. in which Mossbauer, e.s.r., and magnetic susceptibility studies on iron-containing proteins and their synthetic analogues are reviewed. Applications of e.s.r. to study the structure of impurities in insulators and in antiferromagnetic solids and to investigate the surface structure of catalysts. An extensive review article outlines the nature of the information which can be obtained from the subset of spin-spin interactions that yield additional lines in the e.s.r. spectra. Ferromagnetic e.s.r. is considered in ref. and in ref. NMR and e.s.r. in two-dimensional magnets has been presented. Optically detected magnetic resonance has been discussed in ref. 10 and applied to clarify the structure of intrinsic and impurity centres in ionic solids.

The principles and applications of pulse e.s.r. and pulse electron-nuclear multiple resonance have been reviewed. A recently developed technique based on pulsed e.s.r. methods for dynamic polarisation of nuclear spins in semiconductors is reviewed.

Modern techniques in electron paramagnetic resonance, such as two-dimensional and Fourier-transform e.s.r., far infrared e.s.r, imaging, spin-echo, and high field e.s.r. are presented in three review articles.

2 General

A number of new e.s.r. spectrometers have been described and the apparatuses are under patent licence. Some instruments, however, are rather peculiar and will be mentioned here. A CW e.s.r spectrometer working at the magnetic earth-field with a frequency of 1.845 MHz with optimized sensitivity is described. Only very narrow lines can of course be detected (maximum sweep 125 µT). A modular low frequency spectrometer with sensitivity of 8x1021 spins T-1 has been constructed using commercially available components. Coupling spins to other degrees of freedom in order to obtain dynamical polarization and/or greater detection sensitivity constitutes rather new approaches to magnetic resonance. Several of these techniques have been described. A reduction of the overall receiver noise figure of 24.6 dB has been obtained for a Varian E-110 spectrometer working at Q-band frequency by the use of a low-noise Gunn diode oscillator. Simple modification of a Varian E-line series of spectrometers is suggested in order to perform fast direct detection of the e.s.r. signaf. An e.s.r. signal from a sample of irradiated quartz which can be easily saturated was used to monitor the performance of an automatic frequency control (AFC) circuit, measuring the different saturation behaviour of the absorption and dispersion signals. Modulation of the magnetic field to detect the e.s.r. signal is still under development, while electron paramagnetic rotary resonance is being studied as an alternative method.

A software package containing routines for data acquisition, processing and graphic representation of e.s.r. (and other) spectra has been described. The data acquisition system allows one to perform kinetic studies as well and includes spectra accumulation.

Pulse e.s.r. technique is receiving much attention and we mention in particular the following advances from Arthur Schweiger and co-workers: suppression of artefacts and undesired echoes by selection of coherence-transfer pathways; detection of small hyperfine and quadrupole interactions; generation of electron-spin-echo-envelope modulations (ESEEM) in paramagnetic systems that do not contain nonsecular hyperfine interactions; resolution of an e.s.r. spectrum in a second dimension.

Electron-spin transient nutation spectroscopy has been studied by Astashkin and Schweiger in order to find a way of simplifying complicated solid state e.s.r. spectra. The technique is based on the physical fact that the nutation frequency of a spin system precessing around a magnetic field is given by the product of the transition matrix element and the strength of the magnetic field causing the nutation. In solid state spectra these matrix elements may vary from line to line and the nutation frequency is therefore different from line to line. In a transient nutation experiment the microwave magnetic field is suddenly switched on at t=O and the nutation intensity is measured at any fixed time as a function of the static magnetic field flux density. Examples of applications to the resolution of single crystal spectra of copper(ll) and manganese(ll) samples have been furnished.

E.s.r. imaging using the microwave scanning technique has been applied by Furusawa and lkeyato to detect the distribution of nitrogen and nickel impurities in diamond crystals, and the same authors also describe a method of producing high-quality linear field gradients,35 which constitutes the more common method of e.s.r. imaging. High fields (5 T) have been used in imaging with spatial resolution up to 1 µm in one dimension.36 The perspectives in 3D e.s.r. imaging using CW methods and electron spin-echo (ESE) detection have been analysed.

Photo-e.s.r. has been applied to characterize deep levels in cadmium telluride.37 Laser-flash excitation following by ESE-e.s.r. has been applied to characterize excited states in chromate(VI) doped dipotassium sulphate.

Zhong and Pilbrow have given a simple interpretation of the saturation-transfer e.s.r. spectroscopy based on the effects of spectral diffusion induced by molecular motion on adiabatic rapid-passage responses.

Very low temperatures, as low as 0.4 K, have been reached with a X-band e.s.r. spectrometer in which the sample enclosed in a small-helix resonator inside a 3He cryostat is reached by the microwave radiation through a semirigid coaxial cable and a small loop. Detection of a minimum of 1.5x105 as spins T-1 was reported.

Quantitative detection of the number of spin and preparation of the standards for intensity measurements have been described.

Theory. Numerical calculations of the zero-field splitting (ZFS) parameters for high spin d5 ions in cubic and tetragonal symmetries have been performed by Wan- Lu and Rudowicz. The calculations of the second (b02) and fourth degree (b04,b44) terms of the zero-field splitting spin hamiltonian, written using Stevens operators, have been performed in the crystal field framework including spin-orbit coupling as a perturbation on the crystal field and electrostatic interactions. All the 252 states of the d5 configuration have been included in the calculations and the perturbation procedure was extended to fifth order. The parameter b02 was found to be mainly determined by second and third order perturbation terms, i.e. interactions among the 6S ground state and the excited quartet states, while the fourth degree ZFS parameters were affected by both quartet and doublet levels. Relationships between the zero-field splitting parameters and the crystal field, electrostatic, and spin-orbit coupling parameters have been examined. Agreement with the experimental data is obtained for manganese(ll) and iron(lll) in fluoroperovskites. The energies of the electronic transitions and the zero-field splitting of high spin iron(III) (S = 5/2) in cubic fields, which is parameterized by a= b04 = 2b44/5, have been computed including all the 252 states of the d5 configuration and e.s.r. data for iron(III) in yttrium gallium garnet have been reproduced. A paper dealing with the calculation of the e.s.r spectra of high spin iron(II) (d6, S = 2) in C2v symmetry based on the crystal field theory appeared. The formalism developed seems to consider only 5A1 as the ground state and presents the calculation of Zeeman splitting between adjacent states. The computed zero-field splitting, D = b02 = 8.8035 cm-1 proves to be axial, but the actual value seems to depend on the nature of the states chosen for the calculation. Electronic transitions and e.s.r. spectrum of nickel(II) (d8, S=1) in lithium niobate(V) have been calculated using the crystal field formalism assuming a C3v symmetry.

Xα-SW molecular orbital calculations of the isotropic and dipolar contribution to the hyperfine tensor have been performed for copper(II) (d9, S = 1/2) in a [CuCl4(NH3)2]2- centre to clarify the observed dependence of the superhyperfine splitting on the metal-ligand distances. Calculations of the spin hamiltonian parameters on iridium(II) (d7, S 1/2),49 copper(II) complexes, metal phthalocyanines, and metal-nitroxide complexes using various MO methods have been reported.

G. Shen, C. Xu, and G. Bai53 analyzed the e.s.r. spectrum of the binuclear complex [Ph4AsCuCl3]2 since, they stated, "few theoretical analyses to interpret the results" (for binuclear copper(ll) systems) "have been made". Apparently, however, the authors are not aware of a bulk of scientific literature: useful references can be 54 and 55. Their theoretical treatment is based on a rather peculiar hamiltonian which uses a crystal field hamiltonian which includes the Dirac-Van Vleck-Heisenberg isotropic exchange spin-hamiltonian and the conclusions showed a relationship between the zero-field-splitting of the triplet state and the isotropic exchange coupling constant, contrary to well established literature results.

Analysis of the Spectra and Computing. -A fundamental aspect in the calculation of e.s.r. spectra is the correct formulation of the lineshape function. In particular, a few questions have arisen in recent years about the equivalence or nonequivalence of the field and frequency modulation schemes and about the inclusion of the 1/g factor in the simulation of S = 1/2 spin systems. Zhong and Pilbrow worked out an expression of the e.s.r. absorption in single crystals and polycrystalline powders, which we briefly reported here. The basic equation, for single crystal absorption, is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where as usual η is the sample filling factor, Q0 is the unloaded quality factor of the sample resonant cavity (or loop-gap resonator), C is an instrumental constant, α and β are the states (eigenvalues of the spin hamiltonian) between which the transition, caused by the interaction of the microwave magnetic field with the spin system (µx), occurs. Fω is the lineshape function explicitly expressed as a function of the flux density of the static magnetic field, B, of the microwave frequency, ω, and of the phenomenological linewidth, δ, in frequency units. Z is the partition function of the Boltzmann distribution. The fourth term in (1) represent; in fact, the difference in thermal population between the two levels in the high temperature approximation, hv « kT. The resonance centre of the absorption, ω0, is defined by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

ω=ω(θ,φ) being the angular variable. Equation (1) explicitly includes a Boltzmann factor as a frequency dependent quantity. It is a common practice, when dealing with field sweep spectra, to work with a lineshape function expressed using field variables, FB, instead of Fω. A simple relationship connects Fω and FB:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

Equation (3) is rigorously valid for a Dirac-δ distribution of states and its use in e.s.r. was proposed by Aasa and Vanngard and accepted for symmetric lineshapes with finite widths. In general the lineshape function F has the following property:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

This allows one to write down the connection between the frequency and field modulations as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

Neglecting the field dependence of the transition probability and of the linewidth, σω, in (1) a simple form of the e.s.r. spectrum is obtained:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

where G represent all the terms in equation (1) other than Fω. Using (5) and (4) one gets

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

or, using (3), the more familiar expression

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)

as already found by van Veen. For spin systems in which the resonance condition can be expressed as hω =gµ8BB, equation (8) takes the simpler form

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (9)

The expression for the e.s.r. signal of polycrystalline powders can be easily found by integrating (7) or (8) over the angular variables Ω, or equivalently

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (10)

A general treatment of the lineshape functions to be used in e.s.r. spectral simulations and the effect of the asymmetry of the lineshape functions can be also found in ref. 2.

The computer simulation of powder spectra is important in a large number of situations in which crystalline solids cannot be obtained, and this is one reason why a part of the e.s.r. literature is still dealing with developing new algorithms for an efficient simulation of polycrystalline powder spectra. It must be stressed, however, that not all the computer codes agree on the exact form of the resonance absorption function and expressions different from (10) or not including the [??]ωo/[??]B part are still used. Differences can thus be found in the literature on the actual simulations. The actual form of the lineshape function used, in general, does not severely affect resonances which are symmetric around the transition frequency, for which equation (3) applies, but its effects become observable when asymmetric resonances are considered (g strain and A strain effects). A list of computer programs developed to analyze and fit e.s.r. spectra can be found in Appendix T of ref. 2. Gribnau, van Tits and Reijerse reported a procedure to speed up the calculation of e.s.r. spectra in their program MAGRES. The program is based on matrix diagonalization and can in principle handle any spin system; the suggested procedure consists of accumulating a large number of single crystal spectra, but only 2-10% of these spectra are computed by exact diagonalization. All the other points necessary for a meaningful spatial integration have been computed via a fitting procedure. The authors claim that the computer time is reduced by a factor of 10 without apparent deterioration of the quality of the spectra. Simultaneous calculation of e.s.r. and ENDOR powder spectra has been obtained by Kreiter and Huttermann with the program package MSPEN/MSGRA. The program is based on direct diagonalization of the spin hamiltonian. A FORTRAN computer program, FILMCAL, has been announced to be developed to simulate e.s.r. spectra of transition metal ions in thin layer of phthalocyanine films.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 1993
• ISBN 10: 0851869114
• ISBN 13: 9780851869117
• Binding: Hardcover
• Language: English
• Edition number: 1
• Number of pages: 278
• Editor: Symons M C R