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| ISBN-13: | 9780851868813 |
|---|---|
| Publisher: | RSC |
| Publication date: | 01/01/1990 |
| Series: | Specialist Periodical Reports - Electron Spin Resonance , #14 |
| Edition description: | Edition. ed. |
| Pages: | 208 |
| Product dimensions: | 5.45(w) x 8.50(h) x (d) |
Read an Excerpt
Electron Spin Resonance Volume 12A
A Review of Recent Literature to mid-1989
By M. C. R. Symons
The Royal Society of Chemistry
Copyright © 1990 The Royal Society of ChemistryAll rights reserved.
ISBN: 978-0-85186-881-3
CHAPTER 1
Organic Radicals in Solution
BY B. J. TABNER
1 Introduction
In my report for Volume 12A, which covers the period June 1987 to May 1989, I have retained the same general layout as used previously.
It is pleasing to note that the number of papers published dealing with neutral and charged organic radicals in solution has increased slightly compared with two years ago. Together these papers continue to include a very wide range of applications and interest in the technique is obviously maintained. E.s.r. has now celebrated its 40th birthday and commercial spectrometers have been readily available for over 25 years. Nevertheless new, and sometimes quite ingenious, applications are regularly reported. Modern spectrometers are undoubtedly more sensitive than those of 25 years ago but, perhaps more importantly, there have been other significant developments in instrumentation. A recent review claims that "this is an exciting time in EPR; several research labs are currently developing new experimental techniques and new interpretations that will revolutionise the method's possible applications. Prominent among these developments are new micro-wave sources and resonators that promise much greater signal-to-noise ratios, time-domain spectroscopies that will open a new dimension of spin interactions and dynamics, Fourier transform EPR that will revolutionise spin-label and other organic radical studies, EPR imaging that will yield a spectrum at any point in the sample, in vivo spectroscopy that will make whole biological organisms accessible to EPR, and computation capabilities that will simplify complex simulations we did not dare dream about just a few years ago."
One of the instrumental developments which has been available for several years is ENDOR spectroscopy and no doubt many readers will find a recent book, 1 ENDOR Spectroscopy of Radicals in Solution', invaluable. Also invaluable is a new volume in the Landolt-Bornstein series dealing with the Magnetic Properties of Free Radicals. The most recent Volume covering 'Nonconjugated Carbon Radicals' is particularly relevant to the subject area of my report. Also now available is the collection of papers presented at the XVth Annual International Conference on ESR in Organic and Bio-organic Systems, held at Cardiff in 1988.
A significant development over the last 10 years has been the study of radical cations formed by ionising radiation in frozen halocarbon solutions. A review covering these developments is, therefore, timely. Other recent reviews cover the way in which the ·CH2 moiety can be used as a 'spin probe' in the study of conformational problems, and the methods available for determining and analysing radical self-reactions and additions. Also reviewed is the analysis of dynamic e.s.r. and ENDOR spectroscopy of organic radicals in solution. It is worth noting that the ENDOR technique can be sensitive to dynamic processes in the 'rate' range where e. s. r. spectra are unaffected. Also described is hyperfine-selective ENDOR which is based on a pulsed ENDOR scheme. The main advantage appears to be its capability to measure the ENDOR subspectrum originating exclusively from nuclei with a predicted splitting constant.
Although Fourier transform NMR has become widespread the same is not true in e.s.r. However, a detailed description of a two-dimensional Fourier transform e. s. r. spectrometer has now been presented which includes applications of the method.
Unfortunately in a conventional spectrometer it is not possible to sweep the magnetic field rapidly enough to study transient radicals produced in flash photolysis experiments. However, Mclachlan and Stevens describe how this problem can be overcome by producing radicals repetitively in a series of flashes as in flash photolysis e.s.r.
Bond et al. have reviewed the various simultaneous e.s.r.-electrochemical techniques. They cover both stationary and flow-through cells. The same authors also describe a novel bubble electrode which appears to operate reproducibly and allows longterm experiments in the absence of both oxygen and light. A new design of flat cell, which incorporates a long capillary folded within a conventional flat cell, is convenient and can be oriented either parallel or perpendicular to the magnetic field.
Considerable attention continues to be paid to the computational analysis and interpretation of complex hyperfine patterns. Both a maximum entropy method and a novel symmetry development transformation approach have been described. The former is suitable for noisy spectra. Several research groups describe methods of obtaining simulations which give the best possible fit to experimental data. Heikki describes a second-order simulation suitable for the Apple II Plus whereas Beckwith and Brumby deal with the old problem of determining radical concentrations. Finally, a simple and convenient subtraction technique, not limited by noise, has been developed.
2 Carbon-centred Radjcals
I have slightly modified the first part of my report so as to include a small separate section on a-electronic radicals. As in previous reports interests cover the kinetics and mechanisms of radical reactions as well as radical decompositions, additions, isomerisations, and other related topics.
2.1 Nonconjugated Carbon-centred Radicals
2.1.1 Alkyl Radicals. -A common radical reaction is the addition of a small radical across a carbon-carbon double bond. Ozawa et al. have studied the addition of the sulphite radical anion to a range of olefinic compounds to give the corresponding adduct. These reactions have a rate comparable with that of ·OH addition, but the e.s.r. spectra of the resulting radicals indicate that addition is influenced by steric factors to a greater extent. The addition of the hydroxyl radical to alkenenitriles generally favours the least substituted carbon atom but spectra resulting from the alternative addition are sometimes also observed.
The mechanism of rearrangement and isomerisation reactions has attracted considerable attention for many years. This interest is reflected in a number of reports. For example, Asmus and Gilbert et al. have studied the oxidation of 2,3-dimethyl-butane-2-ol using time-resolved pulse radiolysis, complimented by steady-state e.s.r. to identify the radicals involved. Reaction with hydroxyl radicals gives ·CH2CHMeCMe2OH, Me2CHC(OH)(Me)CH2 and Me2CCMe2OH [a(6H) 2.30 and 0.05 mT]. At pH ca. 1 the latter radical is replaced by ·CH2C(Me)=CMe2 [a.(3H) 1.575, 1.245, and 0.30 and a(H) 1.31 and 1.28 mT]. The formation of this latter radical, by acid catalysed loss of hydroxide, provides evidence Electron Spin Resonance for the formation of a radical cation intermediate. The rearrangement of 3-ethoxycarbonylbut-1-enyl to 1-ethoxycarbonyl-but-1-enyl, at 233 K, has also been demonstrated. The former radical has been generated from three different halides at 200 K [a(H) 2.315, 2.20, 2.15, 0.163, 0.113, and 0.05 mT (note that the two β-protons are slightly inequivalent)]. At 160 K the CH2=CHCH(CO2Et)CH2CH2 radical [a(2Hα) 2.235, a(Hβ) 2.36, and a(2H) 0.05 mT] is observed which, upon raising the temperature, rearranges to give the former radical.
A muon spin rotation (µSR) study of the isomerisation of α-carbonyl-, α-carboxyl-, and α-carbamide-substituted alkyl radicals indicates that the activation energy for the isomerisation is not significantly influenced by methyl substitution at the radical centre or by replacement of the ester group by a dimethylamido group. These µSR results indicate a considerable barrier to rotation about the partial double bond between the alkyl radical centre and the substituent and confirm earlier e.s.r. measurements.
Spectral assignments can be complicated by the presence of a chiral centre and such centres can have long-range effects on the magnetic equivalence of β-methylene protons. For example, in the CH2(OH)CH(OH)CHCH2OH radical all six observed hyperfine couplings are non-equivalent implying that the β-carbon chiral centre causes inequivalence of the nonadjacent β-H(CH2OH) protons as well as the adjacent γ-H(CH2OH) protons.
Radicals derived from amines are important in radiation chemistry and in biochemistry and can be readily generated by photolysis of a solution of the amine and di-t-butyl peroxide. The e.s.r. spectrum of the α-aminoethyl radical, CH3CHNH2, has a(Hα) 1.47, .a(N) 0.44, and a(3Hβ) 2.07 mT and two non-equivalent amino protons [a(H) 0.25 and 0.545 mT]. The barrier to rotation was found to be 31.9 kJ mo1-1. The spin-trapping technique (employing n-.t-butyl-a-phenylnitrone) has also been used to study the radicals formed upon photolysis of mixtures of CH4, NH3, and H2O. The trapped radicals [a(N) 1.48 and a(H) 0.36 mT] are assigned to the ·CH2NH2 adduct. The photolysis of di-t-butyl peroxide forms a convenient route to hydrogen atom abstraction from CH2(Ome)2. Both the ·CH(OMe)2 radical [a(Hα) 1.19 and a(H[gama]) 0.0795 mT) and the ·CH2OCH2OMe radical [a(Hα) 1.784 and a(Hγ) 0.078 mT] are formed. The relative proportions of the two radicals appears to be influenced by their stabilities.
The reports covered so far illustrate only some of the common radical reactions. Gilbert et al. have examined the role of electronic and steric factors in determining the ease of abstraction, fragmentation, and ring-opening reactions in order to establish if intermediate radicals formed via 1, 5-shifts are involved in these reactions. For example, the •CHMeOEt radical reacts with butynedioic acid (HO2CC[equivalent to]CCO2H) to give 'CH(Me)OCH(Me)C(CO2H)=CH(CO2H) [a(3H) 1.25 and a(H) 1.33 and 1.25 mT]. These radicals undergo fragmentation and ring-opening and can trap on a second alkyne molecule. These results show that oxygen-conjugated radicals add particularly rapidly to butynedioic acid and that in most cases the resulting vinyl radical undergoes a rapid 1,5-hydrogen transfer.
The e.s.r. spectra observed following reaction of 'OH with N-acetylamino acids, in a flow system, indicate the formation of MeCONHCHCO2H from N-acetylglycine and MeCONHC(Me)CO2H from N-acetylalanine. In the former case the MeCONHCH2 radical, formed by decarboxylation, is also observed. When a flow-system is not employed the spin-trapping technique is invaluable for systems which produce a very low steady-state radical concentration. This latter technique has been employed to study alkyl, and other radicals, formed during the photolysis of N-hydroxypyridine-2-thione esters and during the decomposition of diacyl peroxides.
Kinetic parameters for radical reactions have always attracted interest, particularly where they apply to 'clock' reactions. Ingold et al. have now reported a secondary alkyl radical reaction, the cyclisation of 1-methy1-5-hexenyl, which is useful in this respect. Several other useful clock reactions reported include the related 3-oxahex-5-enyl, 2-oxahex-5-enyl, and 2,2-dimethylbut-3-enoyloxymethyl radicals all of which undergo cyclisation.
There has been considerable interest in cycloalkylmethyl radicals where the ·CH2 moiety is attached to alkyl rings of various sizes. The smallest member of the series, the cyclo-propylmethyl radical, has been the subject of two studies. Walton has prepared this radical by bromine atom abstraction from cyclopropylmethyl bromide. At 122 K the e.s.r. spectrum can be interpreted in terms of a(2Hα) 2.09, a(Hβ) 0.250 and a(2Hγ) 0.296 and 0.200 mT. At lower temperatures the central multiplet in the spectrum broadens indicating restricted rotation about Cβ-Cα (Ea = 11.5 ± 0.8 kJ mol-1). Newcomb et al. have established that in the temperature range 236 -323 K the ring opens to give the 3-butenyl radical, CH2=CHCH2CH2. This reaction is useful as a radical clock rearrangement. The spectra of cyclohexylmethyl radicals indicate the presence of two conformations, one in which the CH2 moiety is equatorial and the other in which it is axial. The line broadening observed in the axial conformer is attributed to restricted rotation of the ·CH2 moiety (ca. 25 kJ mol-1). This relatively high barrier results from steric interactions with the syn axial hydrogens at positions 3 and 5. The e.s.r. spectra of cycloalkylmethyl radicals with 9-to 15-membered rings all indicate the presence of two conformers. It is apparent that the ·CH2 moiety can provide a very useful "spin probe" since it reveals information on the preferred conformation with respect to the Cβ-Hβ bond and also, in some cases, on the dynamics of ring interconvertions.
2-(Cycloalkenyl)ethyl radicals with C4 C7 rings have also been reported. The e.s.r. spectra of all of these radicals are characterised by a basic triplet of triplets [a(Hα)ca. 2.16 - 2.25 mT]. The negative temperature coefficients for the _a(Hβ) splittings reveal a preferred conformation about the Cα-Cβ bond in which the bond makes an angle of 90° with the plane of the ring. Restricted rotation about the Cα-Cβ bond is observed with the barrier to rotation' decreasing as the size of the ring increases.
2.1.2 Cyclic Alkyl Radicals.-Three-membered ring radicals studied include 1-substituted cyclopropyl radicals48 and the oxiranyl radical. At 203 K the 1-methylcyclopropyl radical has a(3H) 1.95 and a(4H) 2.1 mT. At lower temperatures line-broadening occurs which is complicated by slow rotation of the methyl group. However, at 92 K the a(4H) splitting becomes a(2H) 2.485 and 1.655 mT. The former value is assigned to the two ring protons, the latter to the two anti ring protons (Ea [??] 13 kJ mol-1). Deuterium substitution has been employed to study the inversion of the oxiranyl radical. In the [2-2H]oxiran-2-yl radical the inversion is frozen at low temperatures and the two β-protons are distinguishable [a(Hβ) 0.523 and 0.474 mT]. In the oxiranyl radical itself the two β-protons remain equivalent (0.506 mT) although some line broadening is observed. A curved Arrhenius plot is obtained for the latter radical from which it is concluded that inversion proceeds via quantum-mechanical tunnelling. The oxiranyl radical has also been observed during the photolysis of CH3CN in oxirane/cyclopropane.
The e.s.r. spectra of 3-methylenecyclobutyl (1) and cyclo-pent-3-enyl (2) have a(Hα) 2.20, a(4Hβ) 3.76, and a(2Hδ) 0.035 mT 0 and a(Hα) 2.12, a(4Hβ) 3.69, and a(2Hγ) 0.046 mT respectively. Both of these radicals have only small couplings of the protons attached to the C=C bond indicating very little unpaired electron density reaches this bond.
Reaction of cyclopentanone with hydroxyl radicals gives two radicals, (3) and (4). Radical (3) predominates and has a(H1) 2.123, a(2H2) 3.493, a(2H4) 0.090, and a(2H5) 3.758 mT. This spectrum displays higher-order effects arising from the interaction of two pairs of nearly equivalent β-CH2 protons.
The methylcyclohexyl radical (5) has been prepared by γ-irradiation of the thiourea canal complex. Spectra have been recorded from 77 to 294 K and interpreted in terms of a pyramidal structure with a CCC angle of about 117°. At low temperatures, two planar conformations can be identified which are related via an umbrella inversion.
The addition of the sulphate radical anion to a variety of substituted uracils, to give a range of C-5 and C-6 hydroxyl adducts, has been studied as has the addition of the CH2OH radical to some α-enones. The spin-trapping technique has been employed to study the radicals produced from 2-aryl-1,3-dithianes during photolysis with benzophenone.
(Continues...)
Excerpted from Electron Spin Resonance Volume 12A by M. C. R. Symons. Copyright © 1990 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
Contents
CHAPTER 1 Organic Radicals in Solution By B.J. Tabner, 1,1 Introduction, 1,
2 Carbon-centred Radicals, 3,
3 Nitrogen-centred Radicals, 13,
4 Oxygen-centred Radicals, 15,
5 Nitroxyl Radicals, 17,
6 Sulphur-centred Radicals, 22,
7 Radical Cations, 23,
8 Radical Anions, 37,
9 CIDEP, 46,
CHAPTER 2 Triplets and Biradicals By A. Hudson, 60,
1 Introduction, 60,
2 Ground and Thermally Excited Triplets, and States of Higher Multiplicity, 60,
3 Photoexcited Triplet States, 66,
CHAPTER 3 Spin Labels: Biological Systems By Ching-San Lai, 74,
1 Introduction, 74,
2 Proteins, 74,
3 Nucleic Acids, 80,
4 Properties of Model and Biological Membranes, 80,
5 Lipid-Protein Interaction, 84,
6 Cellular Membrane Dynamics, 88,
7 Modification of Membrane Functions, 90,
8 Methods, 93,
9 Nitroxide Reduction, 94,
10 Synthesis, 95,
CHAPTER 4 Applications of E.S.R. in Polymer Chemistry By D.J.T. Hill, J.H. O'Donnell, and P.J. Pomery,
104,
1 Introduction, 104,
2 Polymer Degradation, 104,
3 Polymerization, 112,
4 Polymer Structure, 120,
5 Conducting Polymers, 124,
CHAPTER 5 Free Radical Studies in Biology and Medicine By N.J.F. Dodd, 136,
1 Introduction, 136,
2 Tissues, 136,
3 Radiation Effects in Biological Molecules, 143,
4 Radical Reactions of Drugs and Toxic Chemicals, 150,
5 Enzymes, 162,
6 Oxygen Radicals, 164,
7 Other Systems, 167,