Electron Spin Resonance Volume 14

A Review of Recent Literature to 1993

By N.M. Atherton, M.J. Davies, B.C. Gilbert

The Royal Society of Chemistry

Copyright © 1994 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-921-6

Contents

Chapter 1 Organic Radical Ions By A.G. Davies and G. Gescheidt,
Chapter 2 Time Resolved ESR Studies of Free Radicals By K.A. McLauchlan and M. T. Yeung,
Chapter 3 High-field FSR By Ya. S. Lebedev,
Chapter 4 Transition Metals in Inorganic Systems By F.E. Mabbs and D. Collison,
Chapter 5 Radiation Damage in DNA By Michael D. Sevilla and David Becker,
Chapter 6 Spin Labelling in Biological Systems By Derek Marsh,
Chapter 7 EPR Studies of Photosynthesis By K. Möbius,
Chapter 8 Biological Spin Trapping By Janice A. DeGray and Ronald P. Mason,
Author Index, 303,

CHAPTER 1

Organic Radical Ions

BY A. G. DAVIES AND G. GESCHEIDT

1 Introduction

This report covers the two years from January 1992 to December 1993 inclusive, with some references from 1991 which were not included in Volume 13A. The references were obtained from our personal reading, CAS Online and Current Contents searches.

There is an increasing recognition of the importance of electron-transfer reactions in chemistry and biochemistry, and the interest in the initial radical ions in such reactions and in their subsequent transformations is leading to an increasing need for understanding the ESR spectra. The methods for the generation of radical ions are well established, but increasing use is now being made of photochemically assisted reactions.

2 Bibliography

The basic principles and practice of the observation of the ESR spectra of organic radical anions and cations in fluid solution have been reviewed, and a more thorough account of the structure and reactivity of organic radical cations (115 pages, 427 references) covers methods of generation, methods of observation, reactions, and unusual structures.

A book on Radical Ionic Systems: Properties in Condensed Phases, edited by Lund and Shiotani, includes reviews on electronic structure, spectroscopy, and photochemistry of organic radical cations, ESR studies on radical cations of saturated hydrocarbons, and of cycloalkanes and saturated heterocycles, of deuterium labelling studies of cation radicals, of radical cations of aliphatic ethers, of studies of radical cations by time-resolved magnetic resonance, and of ion pairs in liquids, together with other chapters on the study of radical ions by techniques other than ESR.

3 Radical Cations

3.1 Experimental methods

There is little new to report on new techniques for generating radical cations in fluid solution, but it is worth emphasising again that the species which is observed is not necessarily the same as the initial substrate. The ESR technique pinpoints the solute with the lowest ionisation energy, which can give a long lived radical cation, and even very small amounts of impurities or of transformation products which may result from the often strongly acid conditions, can be potentially misleading. This is particularly so when radical cations are generated with a Lewis acid such as aluminium chloride in dichloromethane, and Friedel Crafts reactions can occur with the solvent.

Thus (Me5C6)6Sn2, (Me4HC6)6Sn2, Mes6M2 (Mes = mesityl, M = Ge or Sn), and Mes4Ge with aluminium chloride in dichloromethane, and (Me5C6)2CH2, (Me5C6)(Me4HC6)CH2, and (Me4HC6)zCH2 + CH2O in trifluoroacetic acid (TFAH) all show the same spectrum of the 1,2,3,4,5,6,7,8-octamethylanthracene radical cation 1•+, the structure of which has been confirmed by X-ray crystallography; the species obtained from hexa- and pentamethyl benzene had previously been misidentified as the 1-methylene-2,3,4,4,5,6-hexa-methylcyclohexa-2,5-diene radical cation 2•+. Similarly a series of 1-aryl3-methyl-3-p-anisyl triazenes all show the same spectrum of the dimethoxy-N,N-dimethyldihydrophenazine radical cation.

The part that laboratory lighting may play in inducing photoxidation in TFAH solvent has often been overlooked. These reactions are usually regarded as involving electron transfer from the substrate to its conjugate acid (equation 1), but Eberson has proposed that the key to the effectiveness of TFAH is that electron transfer occurs from photoexcited ArH to TFAH dimer (equation 2), back electron transfer being avoided by synchronous proton transfer within the dimer.

[FORMULA NOT REPRODUCIBLE IN ASCII] (1)

[FORMULA NOT REPRODUCIBLE IN ASCII] (2)

The current techniques for generating radical cations in fluid solution are limited to substrates with ionization energies below about 9.5eV and which can survive the acid conditions that are usually involved. Most simple classes of compounds which can be handled by these techniques have now been investigated to some degree, and studies are focusing on compounds with special structural features such as steric strain, bridged rings, or heterocyclic groups. On the other hand, a lot of attention is being paid to the generation of radical cations in frozen freons by γ-irradiation, which makes it possible to extend studies to substrates which may be sensitive to acids, and particularly those with higher ionisation energies (ca. 12 eV), including saturated hydrocarbons.

A number of research groups are also studying the ESR spectra of radical cations in zeolites, often generated again by radiolysis, but sometimes by exploiting the intrinsic acidity of the zeolite. They provide very convenient microreactors which retain their rigidity and dimensions over a much wider range of temperature than can be covered by rare gas matrices or the freons, and permit studies to be carried out up to and above room temperature. The polarity of the medium can be varied by changing the SiO2/Al2O3 ratio, the reaction volume can be controlled by regulating the size of the cavity, and intra- and inter-molecular reactions can be studied by varying the loading of the substrate in the zeolite. On the theoretical front, Bally has given a critical account of the scope and limitations of the various MO methods that can be used for calculating the spectroscopic properties of radical cations.

3.2 π-Systems

3.2.1 Hydrocarbons

Tetramethylethene ionises spontaneously in an HZSM-5 (SiO2:Al2O3 40.7) or H-mordenite (SiO2:Al2O3 8.9) zeolite, or on γ- irradiation in ZSM-5 or silicalite S-115 to give the radical cation Me2C = CMe2•+ with a(4Me) 1.72 mT. Its mobility within the cavity can be interpreted by quantitative simulation of the spectra. Decay occurs by formation of the π-dimer (Me2C = CMe2)2•+, with a(8Me) 0.83 mT, then deprotonation. 1- and 2-pentene in halocarbon matrices between 77 K and 145 K similarly show spectra typical for localised π-bond ionisation if account is taken of the conformation imposed by the matrix.

The spectrum of the radical cation of benzvalene 3 has been observed by γ-irradiation in a CF3CCl3 matrix. At 115 K it shows hyperfine coupling by the two olefinic protons (0.835 mT), by the inplane adjacent (H) protons (0.158 mT), and the γ-protons (2.70 mT) which lie in a W-orientation with respect to the p-orbitals. On heating or irradiation with visible light, the benzene radical cation is formed.

If a variety of pentadienes, or hexa-1,5-diene are adsorbed onto Hmordenite, the dominant spectrum which is observed is that of the radical cation of 9-octalin; comparison of the behaviour with that in freon matrices suggests that the rearrangements occur through Brønstedacid catalysis before radical cations are formed.

Radical cations of allenes, such as Me2C = C = CMe2•+, a(4Me) 0.83 mT, have been prepared in freon matrices. 18 The radical cations of the butatrienes H2C = C = C = CH2 and Me2C = C = C = CMe2 show a(4H) 0.83 mT and a(4Me) 1.3 mT, respectively, and comparison with the results of semiempirical MO calculations suggests that the skew angles are 25° and 50° respectively.

γ-Irradiation of semibulvalene (4), cubane, or cuneane, or irradiation of the cyclooctatetraene radical cation with red light in frozen freons, gives a spectrum with a(2H) 3.62 mT, a(4H) 0.77 mT which is ascribed to the bishomobenzene radical cation (5•+)2 in which the unpaired electron occupies the b2 SOMO where the coupling by the bridgehead protons is enhanced by the similar sign of the SOMO on the atoms on either side (the Whiffen effect; though this argument has been questioned).

Irradiation of 5•+ with blue light brings about a further transformation. The spectrum now consists of a triplet, a(2H) 1.35 mT, which was initially ascribed to the tetracyclic radical cation 7•+, but this species has been shown to represent only a shallow local minimum on the AM1/UHF hypersurface, and that the species which is observed is the dihydropentalene radical cation 6•+ which is formed by hydrogen migration. The large coupling is ascribed to the protons at the termini of the triene system, and the CH2 coupling is lost within the line width (ca. 0.5 mT) because the p-orbital coefficients on either side are of opposite sign.

The ESR spectrum of the permethylated derivative of 5•+, with a spectrum consistent with that of 5•+, has been observed previously as the decay product of the tetramethylcyclobutadiene radical cation.

γ-Irradiation of a 1% solution of quadricyclane 8 or of norbornadiene 9 in a frozen freon shows only the spectrum of the norbornadiene radical cation 9•+, but more dilute solutions show also the spectrum of the bicyclo[3.2.0]hepta-2,6-diene radical cation 10•+, which can also be obtained by UV irradiation of 9•+.

Similar behaviour is observed in a non-polar Silicalite matrix. Even down to ca. 4K, 8 and 9 give only 9•+; at 77 K, 9•+ decays to give the 7norbornadienyl radical, but at 100 K, 10•+ is also observed as a decay product. 26 In a polar ZSM-5 zeolite however, (SiO2:Al2O3 70), both 8 and 9 show the spectrum of 9•+ at 100 K, but at 200 K, decay occurs by a retro-Diels Alder reaction to give ethene and the cyclopentadiene radical cation 11•+.

The way in which different freon matrices can influence the stability and chemical behaviour of solute radical cations is illustrated by the radical cations bicycloheptadiene 12•+, bicycloheptene 13•+, and bicyclohexene 14•+. If these are generated from 12, 13, and 14 respectively in CF2ClCFCl2, they decay mainly by deprotonation to give the corresponding allylic radicals, but if they are generated in CFCl3, decay is mainly by ring opening to give the cycloheptatriene (15•+), cycloheptadiene (16•+) and cyclohexadiene (17•+) radical cations respectively; then, in CF2ClCFCl2, 17•+ deprotonates to give the cyclohexadienyl radical.

Again, if 15•+ is generated in CCl3CF3 at 70 K, the ring is non-planar and the methylene protons are nonequivalent, with a(1H) 5.70 and 4.67 mT respectively; at higher temperature, ring inversion can be observed with ΔE 1.7 kcal mol-1, then above 90 K, a proton is lost to give the cycloheptatrienyl radical. On the other hand, in a CCl3F matrix, 15•+ is more nearly planar, and the difference between the hyperfine coupline constants of the axial and equatorial protons is lost within the line width of ca. 0.5 mT. The radical cation 16•+ (in CCl3F3) is more rigid than is 15•+, and shows non-equivalent β-protons with a(Hax) 2.85 mT, and a(Heq) 1.0 mT, up to 120 K when the spectrum is lost.

The methylene protons in the cycloheptatriene radical cation 15•+, like those in the cyclohexadienyl radical, show a large hyperfine coupling (5.15 mT and 4.71 mT respectively) because of the operation of the Whiffen effect. Similarly the cyclohexa-1,4-diene radical cation shows a large hyperfine coupling to the methylene groups (7.54 mT); ab initio calculations show that the 2B1u orbital lies above the 2B3g orbital as a result of through-bond interaction, and again the AO coefficients on either side of the methylene groups will have the same sign.

The spectrum of the radical cation of endo 5-vinylnorbornene (18) in CF2 ClCFCl2 shows that the electron vacancy is largely localised in the double bond in the ring; at 110 K, 18•+ undergoes a Cope rearrangement, at about 330 K lower than does the parent 18, to give the radical cation 19•+ of the bicyclo[4.1.0]heptadiene 19. Similarly in the 4-vinylcyclohexene radical cation 20•+, the electron is lost from the double bond in the ring; rearrangement now leads to the bicyclo[3.2. l]octane radical cation 21•+.

The C8 analogue 22 of quadricyclane on γ-irradiation in a freon shows an ESR spectrum consisting of a quintet, a(4H) 0.68 mT, which is ascribed to the bicyclo[2.2.2]octadiene radical cation 23•+. Exposure of this to visible light gives reversibly a species with a new spectrum with a(4H) 0.57 mT, a(2H) 0.27 mT, which is identified as 22•+.

In pagodadiene•+, in which the two syn-periplanar parallel C = C bonds are separated by 2.62 Å, the radical cation shows cyclic electron delocalisation. Similarly, in 1,16-dodecahedrane, in which the double bonds are separated by 3.52 Å, the radical cation has a delocalised structure on the ESR time scale. An interesting anomaly occurs in the spectra of the ring-strained benzocyclo-butenes and -pentenes, which have been recorded in fluid solution. Competitive generation of the spectra from a mixture of, for example, hexamethylbenzene and 1,4-dimethylbenzodicyclobutene 24, indicates that a pair of ortho methyl groups repel electrons more strongly than does a fused cyclobutene ring, but the spectrum of 24•+ shows that the unpaired electron occupies the MO, which would normally be taken to imply that two methyl groups repel electrons less strongly than a fused cyclobutene ring. It has been suggested that this can be understood in terms of the FinneganStreitwieser model of the Mills-Nixon effect (25). Angle strain within the fused ring puts more p-character into the bonds within the strained ring, and thus more s-character into the adjacent bonds within the benzene ring, and the consequential partial charges which are induced destabilise the antisymmetric MO and stabilise the symmetric MO.

The diphenylcarbene radical cation labelled with 13C has been generated in a CF2BrCF2Br matrix by y-irradiation of diphenyldiazo-methane, and shows aC(iso) 9.83 ± 0.05 mT. This is much larger than would be expected for a diphenylmethyl π-radical, but is in line with what would be expected for a σ-radical, and it is concluded that Ph2C•+ has the σ•/π+ configuration 26•+

The methano-bridged [10]annulenes 27 show different properties in the radical anion and cation. In the anion, the methylene groups bridge ring positions through which passes a vertical node in the SOMO and a(CH2) is small (0.058 mT). In the radical cation, the LCAO has substantial coefficients of the same sign at the bridged positions in the ring, which, by the Whiffen effect, impart to the outer pair of the methano protons a large coupling (0.872 mT) which dominates the spectrum.

(Continues…)Excerpted from Electron Spin Resonance Volume 14 by N.M. Atherton, M.J. Davies, B.C. Gilbert. Copyright © 1994 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.