Electron release by a tertiary carbon atom

Summary Heterocyclic iminoxyl free radicals display the effect of electron release through a tertiary carbon atom via the sigma bond system of a molecule.     

A model of macroradical recombination in dilute solutions

The dependence of exchange rate constants and diffusion coefficients of stable macroradicals (modified polydimethylsiloxane ($\text{P}\text{= 15, 29, 97}$) and polyethylenoxide ($\text{P}\text{= 25, 47, 140}$) on the length of polymer chain in various solvents is investigated. The results are compared with the rate constants for diffusive bimolecular recombination of macroradicals of various lengths. The dependence of rate constants of exchange and combination for macroradicals of different lengths obey the same law.The rate of combination for macroradical with P = 15-10⁵ is shown to depend mainly on the mobility of an active centre, limited by diffusion of the whole macromolecule. The diffusion rate constant of combination can be described by the Smoluchowski equation in which the radius of an active centre is used as the effective radius of interaction.A model of bimolecular termination of macroradicals, a model of “freely penetrating coils” (FPC), is proposed. It implies that k_ter ～ P^{?0.9 ± 0.1} (for “good” solvents). The reactivity of an active centre in recombination is independent of the length of a macroradical chain for P > 15.The exchange interaction in a radical pair broadens the ESR lines of radicals and recombination with equal effectiveness.     

Numerical method of quantum capture probability determination for molecular collisions at ultralow temperatures

The numerical method suggested by Truhlar and Kuppermann (J. Am. Chem. Soc., 1971, vol. 93, p. 1840) to determine tunneling probabilities is adapted for quantum capture calculations in barrierless molecular processes by means of absorbing boundary conditions imposed in the range of strong interactions. It is shown that the phase uncertainty of the singular scattering problem, which arises during the extrapolation of the long-range interaction potential to short distances, is revealed as the oscillatory dependence of the transmission coefficient on the point at which the boundary conditions were imposed. The mean transmission coefficient computation makes it possible to decrease the uncertainty of the results. The method is evaluated to calculate the KRb + KRb reaction rates and K₂ + K vibrational relaxation at ultralow temperatures using model dispersion and adiabatic channel potentials derived from ab initio calculations. The results are in good agreement with the data of analytic models based on the solution of the singular scattering problem close to Bethe-Wigner energy threshold and, within the capture approximation accuracy, with the data of a rigorous quantum scattering theory.     

Magnetic isotope effect

The electron spin selectivity of radical reactions is accompanied by nuclear spin selectivity, i.e., sorting of the isotopic nuclei relative to their magnetic moment. This property of spin-selective reactions produces separation of magnetic and nonmagnetic isotopes known as the magnetic isotope effect. The chemical physics of this phenomenon is examined along with the conditions and magnitude of its manifestation as well as the prospects for its use in theoretical and experimental chemistry.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 3, pp. 144–152, May–June, 1995.     

Quantitative theory of chemically induced dynamic nuclear polarisation in high magnetic fields

Further development of the diffusional model of the radical pair is suggested. A function is proposed for the exponential distribution of the diffusional trajectories of the radicals in a radical pair. By this function, the differences in the populations of nuclear spin levels in the molecules are calculated for the S, T and U precursor pairs. The equations which express the relationship between the theoretically calculated population differences and the experimentally determined enhancement coefficients are derived.