Radical Pair Spectra Research Articles

Temperature dependence of the CIDEP spectrum of acetone

The CIDEP spectrum of acetone has been studied by means of time-resolved EPR over a wide range of temperature. It was found that the spectrum varied remarkably depending on temperature indicating that different CIDEP mechanisms are involved at different temperatures. The CIDEP mechanisms operating in each temperature region are determined quantitatively mainly from the simulations of the spectra and their dependence on the diffusion constant of the solvent; from room temperature to −90 °C, the radical pair mechanism involving S–T0 mixing (ST0M)is always dominant, but the triplet mechanism and the radical pair mechanism involving S–T−1 mixing (ST−1M)become important above and below −45 °C, respectively. The relative importance of ST−1M as compared with ST0M determined below −60 °C is in good agreement with the prediction of the theory. We have also found two interesting phenomena that relatively strong E/A peaks appeared in the center portion of the complete E/A spectrum around −45 °C, and that the line shapes of the peaks were seriously distorted below −70 °C. We examined in detail these observations in terms of the contribution of the radical pair spectrum. The temperature dependence of a hyperfine coupling constant for the hydroxyl proton in the acetone ketyl radical is discussed in terms of spin polarization and hyperconjugation. CIDEP spectra of similar aliphatic carbonyls were investigated and the results support the generality of the conclusion obtained in acetone.

ESR and X-ray diffraction studies of diacyl peroxides in urea and aluminosilicate hosts

Electron spin resonance (ESR) spectroscopy was used to study the photodecomposition of long-chain diacyl peroxides trapped in channels within zeolites (silicalite and ferrierite) and urea clathrates. ESR spectra of radical pairs in single crystals of the urea clathrates of diundecanoyl peroxide (UP), lauroyl peroxide (LP) andbis(6-bromohexanoyl) peroxide (6-BrHP) show that the alkyl radicals respond to the CO2 stress field by recoiling along the channel. In each clathrate, the inter-radical distance for the most relaxed pair is approx. 9.5 A, suggesting nearly complete relaxation of stress from the CO2s. The rotational mobility and exceptional kinetic stability of the radicals is attributed to relaxation of stress and the lack of a convenient escape route for the CO2s. X-ray diffraction indicates one-dimensional ordering of guests in 6-BrHP/urea and 3-dimensional ordering of guests in UP/urea. Solid state NMR experiments on LP/urea suggest high guest mobility under ambient conditions. When UP and 6-BrHP were intercalated into silicalite, photolysis yielded isolated radicals, but no radical pairs, even as low as 20 K.

CIDNP-detected ESR of radical pairs in the photolysis of quinones

CIDNP-detected ESR spectra of short-lived radical pairs in the photolysis of quinones ( p-benzoquinone and duroquinone) have been studied. The influence of the saturating rf-field amplitude on the ESR spectra of radical pairs detected by this method has been demonstrated and investigated.

Ion—molecular charge transfer as studied by the method of optically detected ESR of radical pairs

Abstract Effects of ion—molecular charge transfer upon optically detected (OD) ESR spectra of radical pairs have been studied experimentally and theoretically. Theoretical analysis has shown that under certain conditions (often realized in experiments) OD ESR spectra are analogous to standard ESR ones. Broadening and collapse of the hyperfine structure (exchange broadening) have been observed experimentally depending on the concentration of neutral acceptors. Ion—molecular charge transfer rate constants have been calculated for some radical-anions from the experimental data.

Theory of optically detected magnetic resonance spectra of radical pairs

The characteristic features of optically detected magnetic resonance spectra of radical pairs associated with dynamic nature of the signal have been studied theoretically. The dependence of the spectral line width and shape upon the microwave intensity and the radical pair lifetime has been determined. It has been shown that a time delay in the signal detection can narrow the spectral hyperfine components observed to a size less than the natural width and thus make it possible to detect resolved spectra of short-lived radicals.

ChemInform Abstract: RADICAL PAIRS IN THE STUDY OF ELEMENTARY CHEMICAL REACTIONS IN SOLID ORGANIC SUBSTANCES

Methods for the investigation of elementary chemical reactions and molecular motions on the basis of the EPR spectra of radical pairs are examined. The results of the study of the stereochemistry and kinetics of processes involving radical pairs are used to discuss the nature of the characteristic features of the elementary solid-phase reactions: the existence of a broad spectrum of the frequencies of the reactions and molecular motions, the reaction mechanisms at low temperatures, and the influence of intra- and inter-molecular motions on the rates of chemical reactions. The bibliography includes 92 references.

Calculation of the geometry of the complex of spatially hindered quinones and phenols from E.S.R. spectra of radical pairs

Geometry of the binary complex of 3,6-di-t-butyl-o-quinone and 2,4,6-tri-t-butyl phenol is investigated. Photolysis of this system produces two types of radical pairs with unequal zero-field splitting tensors. The values of the components, D and E, for each type of radical pair were calculated by diagonalizing the dipole-dipole interaction matrix; the elements of this matrix were calculated by using spin density distributions, either known from the experiment or obtained theoretically. The four parameters (three coordinates of the centre of the benzene ring in the phenoxy radical, and one Euler angle) were found by variational search techniques, through comparing the experimental and calculated values of the zero-field splittings. It is shown that benzene rings of the two radicals are displaced with respect to one another by approximately 1·6 A and rotated relative to one another by 86·5°. This configuration is interpreted in terms of interaction between the t-butyl groups, hydroxyl and carbonyl groups.

Mechanism of hydrogen atom transfer in the photoexcitation of sterically hindered quinones and phenols

1. Analysis of the angular dependence of the ESR spectra of radical pairs formed under irradiation with polarized light showed the momentum vector for electron transfer from 3,6-ditert-butyl-o-benzoquinone to be directed at the angle of 70 ± 5° with respect to the plane of the molecule. 2. In frozen Vaseline solution, a triplet-triplet mechanism of energy transfer from 3,6-ditert-butyl-o-benzoquinone to 2,4,6-tri-tert-butylphenol is ineffective. 3. We have proposed a phototransfer mechanism in which the H atom is detached from the phenol molecule by a quinone molecule which is, itself, in a singlet excited state. Although it is clear that there is no isotope effect in phototransfer, it is equally clear that such an effect will come into play in the reverse process. The magnitude of this effect has been estimated from an analysis of the equation for the photoyield of radical pairs.

Trapped Free Radicals and Electrons in Organic Glasses

Free radicals, hydrogen atoms, and electrons produced in rigid organic glasses at sufficiently low temperatures have lifetimes of minutes to years. They can be studied by their electron spin resonance spectra and, in the case of electrons, by their optical spectra, recombination luminescence, and electrical conductivity. The decay kinetics of these reaction intermediates serve to distinguish those trapped as geminate pairs or in spurs of high concentration from those formed with random distributions. Electron spin resonance studies of relaxation times and of the spectra of radical pairs provide further evidence on geometrical distributions. The decay rates of radicals combining with reactive geminate partners are dependent on the size and shape of the radical, the temperature, and the nature of the matrix. Decay is much slower in deuterated matrices than in protiated matrices. The factors that control the physical trapping of electrons in organic glasses are under intensive investigation. There is evidence that many electrons trapped relatively weakly during irradiations at 4 degrees K deepen their traps by orientation of dipoles when the matrix is warmed; that most electrons are trapped in the field of the geminate positive ion; that in some matrices the traps have a bound excited state to which the electron can be promoted without detrapping; and that trapped electrons can tunnel to solute molecules with a higher electron affinity than the trap depth.

Hyperfine anomaly arising from the nuclear spin-forbidden transition, and absolute sign determination of the zero-field splitting constant

It has been found in the triplet E.S.R. spectra of radical pairs in irradiated potassium deuterium fumarate that the hyperfine structure of the two transitions, M s = 1↔0 and M s = 0↔+1, are entirely different. This anomaly has been interpreted in terms of the forbidden transition arising from the mixing of the nuclear spin states by the anisotropic hyperfine interaction. The theory has been developed for multiplet electron spin systems and includes the nuclear Zeeman interaction which is often neglected. The theoretical predictions are in good agreement with the observed separations and intensities of the anomalous hyperfine lines. In addition, it has been found that since the forbidden lines of the electron spin multiplet system with S ≥ 1 appear strongly only in transitions which include some specific electronic spin states, the anomalous features of the spectra make it possible to determine the absolute sign of the zero-field or hyperfine splitting constant, if the sign for one of them is known. Using...

ESR spectra of radical pairs in irradiated oriented polyethylene

ESR spectra of radical pairs in irradiated oriented polyethylene