Yu.N. Molin

Optical detection of ESR absorption of short-lived ion-radical pairs produced in solution by ionizing radiation

Abstract Optically detected ESR signals from short-lived (naphthalene) − -(naphthalene) + radical pairs have been observed in ≈0.01 M naphthalene solution in squalane irradiated by fast positrons. The signal is a single line with a g -value of 2.00 and a line width of 14 G at half height. The signal to noise ratio amounts to 10 for an average radical pair concentration in the sample of 20.

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.

The induction of quantum beats by hyperfine interactions in radical-ion pair recombination

Abstract Quantum beats were detected in the recombination of spin-correlated radical-ion pairs. These beats are induced by singlet-triplet transitions in a pair due to hyperfine interactions. Singlet radical-ion pairs were generated by radiolysis of hydrocarbons involving acceptors of electrons and holes. The singlet pair recombination was detected by the luminescence of the product (singlet-excited molecules). The beat frequencies measured for (tetramethylethylene) + /(paraterphenyl- d 14 )-and (durene) + /(paraterphenyl- d 14 ) − pairs correspond to the hfi constants in the cation radicals. The character of the beats differ in high and zero magnetic fields and their shape depends upon the solvent.

The induction of quantum beats by the Δg mechanism in radical ion pair recombination

Abstract Quantum beats induced by the difference in g factors between radicals in a pair have been detected in recombination fluorescence kinetics in dilute solutions of diphenyl sulfide and paraterphenyl in cis-decalin following irradiation by fast electrons. Good agreement between theory and experiment is obtained.

Ion-molecular charge transfer reactions of hexafluorobenzene and cis-decalin in nonpolar solutions studied by linewidth broadening in MARY spectra

Abstract The effect of an external magnetic field on the radiofluorescence arising from the recombination of spin-correlated radical ion pairs (MARY spectrum) has been studied for dilute nonpolar solutions of hexafluorobenzene and cis-decalin. MARY spectra of these systems show additional local fluorescence intensity maxima at zero field and in the field equal to triple the hfi constant. The breaking down of spin coherence in the course of the ion-molecular charge transfer reaction leads to broadening of the maxima. Relations between the line broadening and the rate of charge transfer reaction have been derived. Rate constants of charge transfer reactions for hexafluorobenzene radical anion in squalene and cis-decalin radical cation in hexane have been obtained experimentally.

Optical detection of the ESR spectrum of hexafluorobenzene anion-radicals in squalane at room temperature

Abstract The technique of optical detection of ESR spectra from recombining radical pairs has been employed to observe the hyperfine structure of C 6 F 6 − ion-radicals formed in squalane under ionizing irradiation. Anthracene introduced into the solution enhances the ESR intensity. The spectra are well described by second-order perturbation theory with a hyperfine interaction constant 133.6 G and g -value 2.0014.

Theory of optically detected magnetic resonance spectra of radical pairs

Abstract 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.

Studies of various mechanisms of nuclear polarisation due to a resonant high-frequency field in radical reactions

Abstract Two different mechanisms of creation of nuclear polarisation in radical reaction products are studied by means of the high-frequency field influence on intermediate radical pairs (stimulated nuclear polarisation, SNP) and on intermediate short-lived radicals (dynamic nuclear polarisation, DNP). Criteria are formulated for distinguishing the contributions of DNP and SNP effects, and experimental demonstration of this possibility is presented for photoinduced reactions of quinones.

On optically detected ESR spectra from radical cations of liquid hydrocarbon solvents

Abstract The optically detected electron spin resonance (OD ESR) method has been employed to study the origin of radical-cation ESR signals in some saturated hydrocarbons with small amounts of 2.5-diphenyloxazol or p-terphenyl under radiolysis. In cyclohexane, the ESR, signal with resolved hyperfine structure was ascribed to c-C6H10+/PPO− radical-ion pairs produced from primary c-C6H12+/PPO− ones by monomolecular decay, of cyclohexane radical cations to cyclohexene radical cations. Cis- and trans-decalin under radiolysis accumulate 9,10-octalin which captures solvent holes and form 9,10-octalin radical cations giving a resolved OD ESR spectrum. 9,10-octalin is present in non-irradiated commercial decalin as an impurity. The OD ESR technique has been shown to be very sensitive to some impurities in hydrocarbon solvents.

Od ESR spectra of cis-decalin radical cations in liquid and glassy solutions: Evidence of radical cation stabilization in glassy cis-decalin

Abstract The resolved optically detected electron spin resonance (OD ESR) spectrum for cis-decalin radical cations has been taken in dilute solutions of cis -decalin and naphthalene in liquid 3-methylpentane at 140 K. A similar spectrum has been observed in the glassy solution of 10 −3 M naphthalene in cis-decalin. This spectrum is associated with the signals from stabilized radical cations of the solvent, cis-decalin.