Electron Spin Resonance Volume 12B

A Review of Recent Literature to mid-1990

By M.C.R. Symons

The Royal Society of Chemistry

Copyright © 1991 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-891-2

Contents

CHAPTER 1 Transition Metal Ions By A. Bencini and C. Zanchini, 1,
CHAPTER 2 Laser Maqnetic Resonance Spectroscopy By D.K. Russell, 64,
CHAPTER 3 Electron Spin Resonance of Transition Metal Ions in Zeolites By L. Kevan, 99,
CHAPTER 4 Metalloproteins By G.R. Hanson, 129,
CHAPTER 5 EPR Imaging By S.S. Eaton and G.R. Eaton, 176,
CHAPTER 6 Inorganic and organometallic Radicals By M.C.R. Symons, 191,

CHAPTER 1

Transition Metal Ions

BY ALESSANDRO BENCINI AND CLAUDIA ZANCHINI

1 Introduction

After some years the task of preparing the Specialist Periodical Reports on Transition Metal Ions returns back to Firenze. It is with great pleasure that we are now starting to prepare this issue.

The basic format of the chapter will be left unchanged, but in order to cover some recent applications of the e.s.r. spectroscopy, we add some new paragraphs. In the Extended Systems section we will review application of e.s.r. spectroscopy to compounds which contain transition metal ions in three and lower dimensional magnetic lattices. In the Semiconductors section we will resume the main information that e.s.r. spectroscopy can give in semiconducting solids containing transition metal ions either as main constituents or as impurities. In another section, called Superconductors, we will try to resume the main applications of e.s.r. in a research field which is largely attracting the attention of many researchers after the discovery of high Tc superconductors formed by copper oxides.

Due to the extremely large number of papers reporting the application of e.s.r. in many fields of physics and chemistry we cannot be exhaustive, but we will mention here the most relevant applications. Particular emphasis will be given to single crystal measurements and to works in which the physical properties of the compounds are investigated with the largest number of techniques. Papers written in languages different from English, in general, will not be referred, since we, like many other researchers, have a lot of difficulties in moving through the babel of human languages.

Review articles covering particular aspects of the applications of the e.s.r. spectroscopy will be reported in each section.

Without any doubt some overlap will occur with other chapters of this book, but this already occurred in the past and did not cause any damage to the reader. This time, perhaps, the reader will be much more involved in understanding our English, which, we hope, will not be too italian-like.

Two books dealing with e.s.r. spectroscopy in exchange coupled systems appeared. The first oneby Yablokov, Voronkova and Mosina, in russian, is entitled “Paramagnetic Resonance of Exchange Clusters”; the second one by Bencini and Gatteschi , entitled “Electron Paramagnetic Resonance of Exchange Coupled Systems”, is intended to collect in one place as much information as possible on the use of e.s.r. spectroscopy in the analysis of systems in which two or more spins are magnetically coupled.

2 General

The conventional technique of detection of an e.s.r signal makes use of the modulation of the magnetic field. To achieve the optimum signal one must then use a modulation amplitude comparable to the linewidth, which leads, however, to lineshape distortion or to a decrease in the signal to noise ratio especially for broad resonances. Alternative methods to field modulation in e.s.r. spectroscopy have been reviewed by Hyde, Sczaniecki and Froncisz. In particular the use of Loop Gap Resonators and the Electron Paramagnetic Rotary Resonance is emphasized. This latter technique uses the resonance that occurs when the difference of two incident microwave frequencies matches the precession frequency of the rotating frame, in the Bloch formalism.

In the field of pulsed e.s.r. a detection technique based on the measurement of the longitudinal magnetization, Mz, is described (LOD-PESR). This technique can be used to measure the rapid changes of Mz during short microwave pulses of duration tp ≤ T1, T2, and flip angles β ≤ π. Using this technique, data acquisition of the echo envelope modulation can be initiated after much shorter time than in a conventional electron spin echo experiment.

Other experiments using pulsed techniques can be found in references 5,6.

The double modulation electron spin resonance spectroscopy (DOMESR) has been analyzed both theoretically and experimentally.

Apparatus for high pressure measurements and for low temperature Q-band measurements have been described.

A review on the future developments of the e.s.r. instrumentation has appeared.

The measurement of e.s.r. spectra of ions having large zero field splitting of the ground state is attracting the attention of researchers. For this purpose submillimeter wave e. s. r. in high magnetic fields and ODMR techniques should be applied. On this subject we cite the measurements on CsFeC13 at 4.2 K using FIR laser at 103.6 cm-1 and 84.2 cm-l in pulsed magnetic fields up to 17 T, and the measurement of the spin hamiltonian parameters of the triplet luminescent state of a Ba3(VO4)2 crystal. Spectrometers working with frequencies up to 16 cm-l and with magnetic fields up to 20 T have been described in references 13,14.

An apparatus which can be derived from a conventional e.s.r. spectrometer has been used to monitoring the superconducting transition in high Tc oxides. The experiment consists in the measure of the imbalance of the bridge when the sample is loaded in the cavity arm of a four arm microwave bridge X-band e.s.r. spectrometer.

The application of e. s. r. spectroscopy in microscopy, dating and dosimetry is reviewed in reference 16. In particular the use of e. s. r. microscopy and imaging are evidenced. The construction of a portable e.s.r. spectrometer to be used for microscopy, dating and dosimetry is described.

In the field of the analysis of defect states in semiconductors some applications of photo-e.s.r. techniques and the effects of non-resonant absorption due to free carriers has been reviewed. The two techniques have been found useful in the identification of the nature of the defects in semiconductors and in the determination of the free carrier concentration.

Pilbrow reviewed the use and possibilities of the electron paramagnetic resonance spectroscopy of transition metal ions.

Theory. The calculation of the spin hamiltonian parameters from molecular orbital functions obtained from some SCF procedure is certainly the best way to check the quality of the function describing the ground state of the molecule. Among the theoretical models available up to date for computing the electronic structure of open shell transition metal complexes the MS-Xa model still receives much attention. Ruiz-Lopez and Natoli have computed optical excitations, g values, and XANES in vanadyl porphyrins. Here the principal g values are computed using the standard perturbation solution for a D4h d1 system and taking the covalency parameters from the ground state Xa-wavefunction. Barriuso, Aramburu and Moreno computed the electronic structure of a Ni+ impurity into LiF. The calculations give a reasonable correlation between the Ni-F bond distance and optical transitions and hyperfine values.

Other molecular orbital methods have been applied to compute the g tensor in ferricytochrome c and azidomyoglobin and the g tensor and the anisotropic part of the hyperfine tensor in vanadyl complexes.

The use of approximate d orbitals of double zeta quality allows one to compute the Racah parameters and to be used in Ligand or Crystal Field calculations. Although some criticism appeared on the choice of the basis function the method has been applied with some success, for example, to iron(II).

Ligand Field calculations have been also applied to the e.s.r. spin hamiltonian parameters in a number of complexes.

Some troubles seem to arise among Chinese researchers on the applicability of e.s.r. techniques to the determination of crystal structure parameters in particular from zero field splitting parameters and from hyperfine and superhyperfine coupling constants. It is our opinion that, apart from the accuracy with which these parameters are measured, a serious lack in obtaining precise measurements of the crystalline parameters is the theoretical model (s) one has to use to establish the required correlation. Ligand Field models, either including vibronic effects, are always overparameterized, except for cubic site symmetries, and Molecular Orbital calculations, at any level of sophistication, often fail to give accurate descriptions of the radial function of the ground state needed to evaluate hyperfine terms. As a matter of fact, however, e.s.r. spectroscopy is invaluable in individuating impurities or defect sites in solids and the correlation between geometry of the coordination site and e.s.r. parameters must be used to guess, with all its uncertainty, the local site symmetry of the sites, having in the mind that X-ray absorption or EXAFS spectroscopy cannot be used in these cases. Of course much more information should be obtained in all the cases in which endor spectra can also be measured.

Analysis of the Spectra and Computing. Misra has developed a least-squares-fitting procedure to analyze single crystal e.s.r. and endor spectra with the spin hamiltonian

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

allowing for the non coincidence of the g, D, Ai, gi, and Pi tensors. The procedure is based on a second order calculation of the transition energies, AEi, and minimisation of the function

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where the summation runs over all the observed (e.s.r., endor) lines and δi is the effective weight factor for the ith line position, and it is related to the uncertainty in the measure of the static magnetic field and position of the crystal with respect to the field. This procedure is a generalization of a previously reported one and based on first order calculations. As an example of application, the spectra of VO2+ doped K2C2O4•H2O were fitted using the spin hamiltonian parameters obtained from the first order formulae as starting parameter values. Unfortunately no evaluation of the standard deviations of the measured spin hamiltonian parameters was done, thus making difficult any meaningful test.

Rudowicz developed the expressions which relate the g and D tensors for s = 2 ions at sites of various symmetries to the spin-orbit coupling constant, spin-spin coupling constant, and the energy level splitting of the 5v ground multiplet. Applications to Fel-xAxF2 (A = Mn, Zn, Mg) and FeCl2•2H2O have been discussed.

Expressions for the position of forbidden hyperfine lines, ΔM=±1, Δm=±1, where M and m are the electron and nuclear azimuthal spin quantum numbers, have been developed to third-order in perturbation theory.

A high order perturbation formula has been worked out for computing the g values of d8 ions in trigonal symmetry and the spin hamiltonian parameters of d3 ions possessing C2 symmetry.

Polycrystalline powder and solution spectra are still more easy to handle than single crystal ones and a number of papers deal with the development of computer programs to simulate them in order to extract the principal values of the spin hamiltonian parameters.

E.s.r. and endor powder spectra for S = 1/2 spin systems including hyperfine and superhyperfine coupling from nitrogen nuclei have been considered by Greiner and Baumgarten.

A fitting procedure of polycrystalline powder spectra which uses a least-squares method has been developed by She, Chen and Yu and applied to the fitting of copper(II) spectra with and without hyperfine splitting. The parameters used in the calculations are the principal values of the g and A tensors and the linewidths. The methods can use energy levels computed using perturbation solutions of the spin hamiltonian or complete matrix diagonalization. Also in this case, however, no estimate of the standard deviations on the parameters neither of the correlation between them has been reported. In particular it is this latter quantity which gives any physical reality to the best fitting parameters.

Multifrequency e.s.r. spectra of solutions of copper(II) complexes have been studied with the purpose of minimizing the uncertainty in the determination of motional and magnetic parameters. Always for solutions, an analysis of the effect of the temperature on the e. s. r. spectra of hexaaquoions of several transition metal ions has been reported.

Phase Transitions. -The nature of the phase transitions in a series of dipropionates of formulae Ca2M(C2H5COO)6 M = Pb, Ba, Sr have been investigated using single crystal measurements on Mn2+ doped samples. In particular Ca2Pb(C2H5COO)6 undergoes a paraelectric-ferroelectric second order phase transition around Tc1 = 343 K and a first order phase transition around Tc2 180 K. The e.s.r. spectra revealed that the structure is still tetragonal both above and below Tc2. In the ferroelectric phase below Tel the critical behaviour was followed by measuring the temperature dependence of the angular splitting of the maximum field along the z axis of one manganese ion. The high field resonance separates into two maxima below Tel, and the variation with temperature of the high field resonance follows the equation

Θ = Θ0(Tc1-T)β (3)

where Θ0 = 1.148° and β = 0.51(3). Θ being the angle with the c direction. The ordering of the propionate molecules around the manganese ion in the series of propionates is discussed.

The interaction between the paramagnetic probe and the lattice has been studied, within the Crystal Field theory, for chromium(III) doped RbCdF3 below the phase transition.

The phase transition in the (Cd,Mn)S, a semimagnetic semiconductor, was also studied. In this and in the (Zn,Mn)S system a transition from a paramagnetic state into a spin-glass-like phase, determined by the antiferromagnetic interaction between the manganese(II) ions, occurs at a Neel temperature which depends on the manganese(II) concentration, TN(xMn). The temperature dependence of the linewidth of the observed signal was found to follow the law

B(T) = A[T-TN(xMn)]-1 (4)

for small differences, with l/A = 0.60(4) K-1T-1. An equation showing the variation of TN with xMn is also given.

Studies of the phase transitions in K2(Cr,Cd)F4, KSc(MoO4)2, (Zn, Mn)TiF6•6H2O, (NH4)2Cd2(SO4)3, and Ba(Ti,Fe)O3 were also reported.

Jahn-Teller Effect. The theory of the Jahn-Teller interactions in metallocenes has been developed to include orthorhombic Ligand Field components and applied to the analysis of cobaltocene, chromocene and ferricenium cation.

The e.s.r. spectra of copper(II) doped Cd(NH4)(SO4)2•6H2O have been studied both on single crystals and on polycrystalline powder down to 4.2 K. The principal values of the g and A tensors were found to be temperature dependent while the principal directions of the two tensors remained parallel within experimental error. The temperature dependence of the spectra was attributed to a pseudo-Jahn-Teller distortion experienced by the Cu(H2O)62+ chromophore and rationalized using a model with three adiabatic potential valleys. The relative ordering of these three valleys arising from the Jahn-Teller distortion was found to be similar to that observed in the Tutton salt but with the higher energy distortion at significantly smaller energy.

Resonance Raman spectroscopy studies performed on radical cations of nickel(II), copper(II), and iron(III) chloride complexes of octaethyl-and tetraphenyl-porphyrins have been interpreted by a pseudo-Jahn-Teller mixing of the a1u and a2u molecular orbitals (D4h labelling) via the A2g vibrational mode. These results are in agreement with previously reported e.s.r. spectra of a cobalt(II) nitrosyl complex in which the observed hyperfine coupling with both the cobalt nucleus and protons of the saturated ring were rationalized through a a1u/a2u admixture.

(Continues…)Excerpted from Electron Spin Resonance Volume 12B by M.C.R. Symons. Copyright © 1991 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Electron Spin Resonance Volume 11A

A Review of Recent Literature to mid-1987

By M. C. R. Symons

The Royal Society of Chemistry

Copyright © 1988 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-861-5

Contents

CHAPTER 1 Organic Radicals in Solution By B.J. Tabner,
1 Introduction, 1,
2 Carbon-centered Radicals, 3,
3 Nitrogen-centered Radicals, 15,
4 Oxygen-centered Radicals, 17,
5 Nitroxy Radicals, 19,
6 Sulphur-centered Radicals, 22,
7 Radical Cations, 23,
8 Radical Anions, 34,
9 CIDEP, 43,
CHAPTER 2 Theoretical Aspects of E.S.R. By A. Hudson,
1 Introduction, 55,
2 Numerical Methods and Spectral Analysis, 55,
3 Spin Relaxation and Line Broadening Effects, 58,
4 CIDEP and Related Phenomena, 63,
5 Pulsed E.S.R. Spectroscopy, 65,
6 Applications of Quantum Chemistry, 67,
CHAPTER 3 Spin Labels: Biological Membranes By Ching-San Lai,
1 Introduction, 77,
2 Proteins, 77,
3 Nucleic Acids, 83,
4 Properties of Model and Biological Membranes, 83,
5 Lipid -Protein Interaction, 86,
6 Cellular Membrane Dynamics, 88,
7 Modification of Membrane Functions, 91,
8 Miscellaneous, 91,
9 Synthesis, 98,
CHAPTER 4 Free Radical Studies in Biology and Medicine By N.J.F. Dodd,
1 Introduction, 109,
2 Tissues, 109,
3 Radiation Effects in Biological Molecules, 113,
4 Radical Reaction of Drugs and Toxic Chemicals, 119,
5 Enzymes, 128,
6 Oxygen Radicals, 130,
7 Other Systems, 134,
CHAPTER 5 E.S.R. of the Conformation of 5-and 6-Hembered Cyclic Nitroxide (Aminoxyl) Radicals By A. Rockenbauer, M. Gyor, H.O. Hankovszky and K. Hideg,
1 Introduction, 145,
2 Computer Simulation of Spectra, 146,
3 E.S.R. Spectroscopic Data for Nitroxide Radicals, 147,
4 Pyramidal or Out-of-Plane Distortion of the C(C)NO Group, 153,
5 Ring Conformation, 166,
6 Conclusion, 179,

CHAPTER 1

Organic Radicals in Solution

BY B. J. TABNER

1 Introduction

My report for Volume 11 A covers the literature published between June 1985 and May 1987. This is double the period covered in my previous report.

It has become apparent during the preparation of this report that the study of organic radicals in solution by e.s.r. spectroscopy continues to retain a considerable interest. This is not only due to the continual development of new areas of study but also reflects recent advances in instrument technology (for e.g., the development of the spin echo technique). Consequently e.s.r. is now being applied to study new problems, particularly those involving transient species. In addition to these new areas, e.s.r. spectroscopy will always play an important role in the investigation of radicals present as reaction intermediates and in the study of the kinetics of their reactions. There is also a wealth of structural information that can be obtained from a study of e.s.r. spectra. With several new techniques available for the analysis of complex hyperfine patterns, particularly ENDOR spectroscopy, it is now possible to examine systems of increasing size and complexity. Thus, with the unpaired electron able to act as a probe, information can be obtained on the structure of larger and more complex systems. The hyperfine data obtained in such investigations not only reveals information on the unpaired electron distribution throughout the π-system but is also available to obtain information on such features as substituent effects and conformational preferences.

Readers interested in ENDOR spectroscopy and its methodology will find a useful review in Volume 10 B of these reports. This latter review covers several recent developments as well as instrumental aspects and advanced techniques. An important nucleus in magnetic resonance is 13C as it provides a means of obtaining information on those positions in a molecule to which no proton is attached. Its hyperfine splitting constant is often quite sensitive to structural changes. Normally 13C-enriched samples are required but deuteriation provides a means of studying 13C ENDOR when the nucleus is present in natural abundance.

It is pleasing to see that the text by Wertz and Bolton, originally published in 1972, has now been reprinted, and also that collections of papers presented at several conferences have now been published. Since my last report the plenary lectures and papers presented at the 22nd AMPERE Congress held at Zurich, and at the Faraday Discussion on ‘Radicals in Condensed Phases’ have both appeared. In a new innovation the papers presented at the 18th International Conference on ESR in Organic and Bio-organic Systems, held at Leeds, have been published in a special issue of the Faraday Transactions.

Several reviews have appeared which readers interested in organic radicals will find valuable. Symons and Cox have compared the power of µSR and ESR in probing structures of radicals. Due to the greater zero-point energy of muonium (relative to hydrogen) large hyperfine isotope effects are observed in µSR. There appears to be the possibility of preparing some novel species, such as muonated radical cations and radical anions. Another technique that has developed greatly over the last few years is that of ESR imaging. In two reviews Ohno describes the technique and some of its applications. This technique has applications to the study of diffusion of organic radicals in polymers and to the study of the distribution of radicals after the mixing of two flowing reagents.

The application of e.s.r. spectroscopy to the study of small strained radicals has been reviewed by Ingold and Walton. The use of the technique to study the rearrangement reactions of such radicals, and to provide information on their configuration, is described.

The growing interest in identifying radicals very shortly after their formation and following their subsequent reactions is the subject of a review, by McLauchlan, of flash photolysis e.s.r. One of the main continuous-wave techniques for obtaining e. s. r. spectra of transient radicals produced by flash-photolysis involves time-integration. A variant of this method (MISTI), involving switching on the microwave field after radical creation, provides a method for enhancing signals. A further development in the study of radicals very shortly after their creation, and their subsequent recombination reactions, involves the study of the 1: Organic Radicals in Solution effects of magnetic and microwave fields. The problems encountered as a result of non-uniform concentration distributions following photochemical radical generation have also been discussed.

The post-experimental treatment of spectroscopic data continues to be aimed at resolution enhancement (so that information can be obtained on long-range small hyperfine couplings) employing the Fourier transform method, and at the extraction of hyperfine splitting constants in complex or poorly resolved spectra with the aid ofmicrocomputers.

2 Carbon-centred Radicals

As on previous occasions I have divided this, the first part of my report, into two sections. In the first section I shall cover ‘simple’ alkyl radicals (including cyclic alkyl radicals) and in the second section I shall cover delocalized radicals. Once again the investigation of alkyl radicals by e.s.r. spectroscopy covers a wide range of interests. The study of the structure and conformation of these radicals continues, as does that of their formation by addition and abstraction reactions. There are several reports of rearrangement reactions, and the determination of rate constants for some important reactions has continued to attract attention.

2.1 Alkyl Radicals.-Ingold and Walton have exploited the e.s.r. technique to study the conformation of cyclohexylmethyl radicals. The spectra of the cyclohexylmethyl and (4-alkylcyclohexyl)methyl radicals (at 140 K) have a(β-H) ca. 4.2 – 4.3 mT when the CH2 group adopts the axial conformation, and a(13-H) ca. 3.0 – 3.1 mT when it adopts the equatorial conformation. Thus the two conformations can be readily distinguished from the magnitude of their a(β-H) splitting constants. Both conformers are separately observed up to 400 K indicating that the rate of their interconversion is slow on the e.s.r. timescale. However, it has been possible to determine the rate constant for ring inversion for the cis-4-methy1cyclohexylmethyl radical. Both the quasi-axial (1) and the quasi-equatorial (2) conformers can be observed in the cyclohex-2-enylmethyl radical. The magnitude of a(β-H), and its variation with temperature, shows that these radicals adopt an eclipsed conformation about the Cα-Cβ bond. Ring-opening occurs in some methyl-substituted cyclobutylmethyl radicals. For example, the trans-2-methylcyclobutylmethyl radical has a(2α-H) 2.21, a(β-H) 0.80, a(2γ-H) 0.15, and a(3δ-H) 0.073 mT when prepared at 140 K. However, at temperatures above 240 K ring-opening occurs and the hex-5-en-2-yl radical, with a([apha]-H) 2.20, a(2γ-H) 0.07, and a(5β-H) 2.51 mT, is observed. At 90 K rotation of the methyl group is effectively frozen in the trans-2-methylcyclobutylmethyl radical with one of the δ-hydrogens of the methyl group occupying an all trans-conformation with respect to the radical centre.

Other conformational studies include those of the 2, 2-dimethy lbuty1 radical and some acyclic alkyl radicals. The 2,2-dimethylbutyl radical has a(2α-H) 2.21 and a(8γ-H) 0.09 mT at 250 K. Below 120 K the rotation of both the methyl groups and of the ethyl group are at the slow exchange limit [a(5γ-H) 0.07 and a(γ-H) 0.60, 0.21, and 0.14 mT at 85 K]. The observation of long-range hyperfine couplings are a common feature when radicals have a fixed geometry. Consequently spectral interpretation indicates that many acyclic alkyl radicals, such as 2-methylalkyl and 2,2,3,3-tetramethylbutyl radicals, exist in a preferred conformation about the Cα-Cβ and Cβ-Cγ bonds. Information concerning the conformation about Cγ-Cδ bonds, however, is often limited by the resolution of the appropriate hyperfine splittings.

The Me3CCH2C(OMe)COOMe radical is stable enough to be observed by e.s.r., possibly because of capto-dative stabilization. The radical becomes locked in a twisted conformation upon complexation with SnCL4 with two non-equivalent β-protons (0.746 and 1.030 mT). Aminoalkyl radicals, (H2NCHR), containing an acceptor substituent (R) should also be capto-dative stabilized. These radicals have two non-equivalent hyperfine splittings from the amino hydrogens. A two jump model has been used to estimate the barrier to rotation about the C-N bond in these radicals.

Addition and abstraction reactions are both widely used in the preparation of radicals. Addition to a C:C bond invariably occurs at the least substituted carbon atom (to give a tertiary or a secondary radical). However, ‘head’ addition to methylenecyclo-alkanes becomes thermodynamically more favoured as the size of the ring decreases. Indeed addition of MeSiO’ to methylenecyclo-propane appears to be predominantly ‘head’ (3) giving (4) as the radical observed [a(2α-H) 2.23, a(2β-H) 2.63, and a(δ-H) 0.034 mT]. Addition of Cl2-,SO4-, and ·OH also occurs readily to the C=C bond in alkenols and alkenoates. However, the addition of SO4: or Cl2: (or ·OH at pH <1) to Me2C:CH(CH2)2CH(Me)OH leads to the observation of radical (5), formed via a cyclization reaction. Aliphatic alcohols, such as 3-buten-2-ol, also yield radicals by ·OH addition to the C=C bond. However, three different radicals are formed by hydrogen atom abstraction from 1 ,3-butanediol [CH(OH)CH2CH(OH)CH3, CH2(OH)CHCH(OH)CH3, and CH2(OH)CH2C(OH)CH3]. Vinyl and propenyl ethers can also give radicals by either addition or abstraction. For example, the t-butoxyl radical reacts with MeCH20CH:CH2 to give MeCH2OCH=CH2 [a(H) 1.532 and 0.014, A(3H) 2.248, and a(2H) 0.123 mT] and MeCH2OCHCH2OCMe3 [A(H) 1.628 and a(2H) 0.738 and 0.240 mT]. The photochemical hydrogen atom abstraction from alcohols by benzophenone has also been reported.

There is increasing evidence for hydrogen shifts in certain radicals. For example, the addition of MeCHOEt (generated from Et2O) to an alkyne, such as butynedioic acid, gives a spectrum with a(H) 2.55, 3.23, and 3.22 and a(2H) 0.04 mT. It is proposed that this spectrum arises from the cyclic radical (6) and that the first formed vinyl radical undergoes a rapid 1,5-shift followed by ~.d..Q cyclization to the observed radical. There is evidence for a 1,6-shift, again followed by cyclization, upon addition of CH20CMe3. A 1, 5-shift has also been observed in the rearrangement of RCH2(CH2)CCl2 to RCH(CH2)3CHCl2 (R = Me, Et, or MeO).

There is an interesting report by Symons et al, that γ-irradiation of MeCl in CD3CN at 77 K gives a methyl radical-chloride ion adduct. The spectrum of the adduct clearly exhibits coupling to 35Cl and 37Cl nuclei and it is proposed that the adduct is formed by electron capture. The methyl radical is irreversibly formed on annealing. Metal assisted radical reactions, such as polymerization with Lewis acids, are of some interest and a report has been made that the spin population in Me3CCH2C(SEt)CN varies upon coordination with SnCl4.

Davies et al. have studied the radicals formed upon photolysis of carboxylic acids either neat or in CCl4. When R = H, Me, Et, or Pr only RCHCO2H radicals are observed. However with pivalic acid only the Me3c radical is observed. Two models are proposed to explain the regiospecific formation of RCHCO2H.

A common radical reaction is fragmentation. A nice example of this type of reaction has been observed by Gilbert et al. when studying the photolytic decomposition of peroxydisul phate in the presence of carboxylic acids and related compounds. With ethanoic acid both CH3 and CH2CO2H are observed with their relative proportions varying with the ethanoic acid concentration. It appears that attack at the carboxy group is followed by decarboxylation. With propanoic acid, however, the primary reaction appears to be abstraction of a β-hydrogen atom, al though attack at the carboxy group also appears to occur. An example of a de hydration reaction is also reported. The initial α, β-dihydroxyalkyl radical formed from ethane-1,2-diols undergoes an acid catalysed dehydration reaction to give the corresponding carbonyl-conjugated CR2R3C(O)R1 radical. A kinetic study of these reactions indicates the importance of the electronic effects of substituents.

The thermal rearrangement of 2,2-bis(ethylthio)-3,3-dimethyl-pent-4-enal into 2,2-bis(ethylthio)-5-methylhex-4-enal is an example of a [1,3] carbon-carbon shift in an acyclic system. The observation of the (EtS)2CCHO radical during the course of this reaction supports a radical pathway for this isomerization.

Alkanes react with CC1 4 by a radical chain mechanism and three groups have studied various aspects of this process. Griller et al. have photolytically generated alkyl radicals (cyclopentyl and t-butyl) in CCl4· The spectra of the alkyl and the CCl3 radical are both observed. The rate constant for reaction of the alkyl radicals is greater in solution than in the gas phase possibly due to the solvation of a polar transition state. Similar reactions have been studied at 353 K using the spin-trapping technique and the rate constants for the reaction of Me2CCN and Me2CPh with CBr4 and CBrCl3 determined. The reaction between 1-hexene and CCl4 has also been studied using the spin-trapping technique and the presence of both CCl3 and Cl3CCH2CHCMe3 radicals confirmed.

The knowledge of absolute rate constants and activation parameters is necessary for a proper understanding of chemical reactivity. Several different research groups have reported results relevant to this topic. Roberts et al. have used laser flash photolysis in conjunction with e.s.r. spectroscopy to determine the activation energy for the decarboxylation of the t-butoxycarbonyl radical (45 – 50 kJ mol-1) and for the hydrogen atom abstraction by ButO. from cyclopentane (14 – 25 kJ mol-1). Ruegge and Fischer have also studied the former reaction and report an activation energy of 49 kJ mol-1. The self-and cross-termination reactions of the Me3COCO and Me3C radicals are both diffusion controlled (Ea ca. 8 kJ mol-1). Griller et al. have studied the reaction between simple alkyl radicals and 1,4-cyclohexadiene and find rate constants of ca. 104 – 105 mol-1 l s-1 at 300 K and activation energies of ca 21-30 kJ mol-1. Employing a spectrometer operating at 2 MHz magnetic field modulation Bolton et al. have determined the rate constant for the trapping of Me2COH by 5,5-dimethyl-1-pyrroline 1-oxide (1.1 ± 0.1 x 108 mol-1 l s-1).

The simplest member of the series of fully conjugated cyclic radicals of the general formulae C2n+1H2n+1 is cyclopropen-2-yl. The e. s. r. spectrum of the radical formed following y-irradiation has now been observed at 20 K. The spectrum consists of a large 1:1 doublet indicating that the “ethylenic” structure (7) is preferred over the “allylic” structure (8). The structure of the cyclopropyl radical has aroused interest. In this radical the 13C hyperfine couplings provide information about the degree of nonplanarity. In cyclopropyl and 1-methyl-cyclopropyl the a(α-13C) values of 9.59 and 9,8 mT respectively confirm these radicals have a ‘bent’ structure. In contrast, however, values of a(13C) 4.09 and a(4β-H) 2.70 mT in 1-(trimethylsilyl)cyclopropyl indicate that this radical is planar, or nearly so.

(Continues…)Excerpted from Electron Spin Resonance Volume 11A by M. C. R. Symons. Copyright © 1988 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.