V.M. Grigoryants

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.

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.

Phase shift of quantum oscillations in the recombination luminescence of spin-correlated radical ion pairs

Abstract The phas shift of quantum oscillations observed in the recombination fluorescence of the (diphenylsulfide- d 10 ) + /( p -terphenyl- d 14 ) − radical ion pair in isooctane has been detected experimentally. The shift is caused by the delay in (diphenylsulfide- d 10 ) + formation in the reaction of solvent holes with a diphenylsulfide molecule. Comparison with a theoretical model has given for the rate constant of isooctane hole capture by diphenylsulfide molecules the value (3.5±1) × 10 10 M −1 s −1 which exceeds the diffusion-controlled one. The fraction of singlet-correlated pairs in the track has been estimated from the oscillation amplitude to be approximately 35%.

Determination of a fraction of spin-correlated radical ion pairs in irradiated alkanes by quantum oscillation technique

Abstract Amplitudes of quantum oscillations observed in the recombination fluorescence of the (diphenylsulfide-d10)+/(p-terphenyll-d14)− radical ion pairs in alkanene solutions have been used to estimate a fraction of spin-correlated singlet pairs Θ in the radiation track. Θ values of 0.36 – 0.54 for the solvent series studied were found. Values for a fraction of singlet recombinations fs (0.52–0.66), calculated from Θ values, are in satisfactory agreement with those available from literature. It has been observed that Θ and fs values decrease with increasing solvent viscosity (from n-hexane to cis-decalin).

The hfi parameters and structure of radical-anions of substituted tetra- and pentafluorobenzenes. Theory and experiment (OD ESR)

Abstract Optically detected electron spin resonance (OD ESR) of radical pairs has been employed to take ESR spectra of short-lived radical-anions of substituted fluorobenzenes in liquid non-polar solutions at nearly room temperatures. The quantum-chemical hfi analysis has taken into account possible manifestations of pseudo-Jahn—Teller effects. As shown by comparison with experiment,C6F5OCH− 3, p-H-C6F4OCH− 3 and p-(CH3)2-C6F− 4 are pseudo-π-radicals with non-planar distortions. A peculiar feature of these radicals is that their structure corresponds to the Jahn—Teller stabilization of excited Π* states. The effect results from a stronger interaction of the Σ* state with Π* than with the ground Π state, because the SOMO space localizations coincide in the Σ* and Π* states.