V.A. Morozov

Theory of spin-peierls transitions in chains of exchange clusters.

A theoretical approach to the analysis of magnetostructural phase transitions of chain polymeric heterospin complexes is suggested. The approach is based on a model of the spin-Peierls transition in chains of exchange clusters. The chain elasticity parameter is found to be a main factor determining the order of phase transition.

Integral encounter theories of multistage reactions. IV. Account of internal quantum states of reactants

The matrix-form kinetic formalism developed earlier for multistage chemical reactions is generalized to the case of reactants with internal quantum quasiresonant states. Such internal quantum states may arise due to spins of the unpaired electrons of radicals or nuclear spins. The formalism gives an infinite hierarchy for matrix correlation patterns that can be truncated in two different ways. The simplest one reproduces the conventional integral encounter theory (IET). The second way results in the generalized modified encounter theory (MET). In the spinless problems such as intermolecular energy transfer MET is known to improve the results obtained with other methods. Thus we believe that the generalized MET taking account of all binary contributions will have wider applicability range than the conventional IET.

Application of the integral encounter theory to the description of degenerate electron exchange reactions

Consistent kinetic description of degenerate electron exchange (ion-molecular charge transfer) reaction in the liquid phase based on the encounter theory has been performed. This has made it possible to allow for re-encounters of reactants and the attendant additional dephasing of electron spin during the encounter. The applicability range of the approximation of noncorrelated spectral migration that is commonly used to describe spectroscopic manifestation of degenerate electron exchange in strong magnetic fields has been refined. It is shown that for radicals with large hyperfine interaction constants the discrepancy between the results obtained, and the approximation of noncorrelated frequency migration may be considerable.

Theory of multiquantum optically detected ESR spectra of radical pairs. I. General theory. Resonances in parallel radio-frequency field

Abstract A general theory of multiquantum optically detected ESR spectra of radical pairs has been developed neglecting the processes of internal spin relaxation. The theory has been used to calculate multiquantum resonances in radiofrequency field which is parallel to the Zeeman field. The position and form of spectral components at arbitrary values of nuclear partner spins have been studied assuming a Poisson life-time-distribution of radical pairs. A physical interpretation of the results obtained has been proposed. The position of the spectral lines of a resolved spectrum for a complex nuclear structure of radical pair partners has been established.

A Copper–Nitroxide Adduct Exhibiting Separate Single Crystal-to-Single Crystal Polymerization–Depolymerization and Spin Crossover Transitions

A complex cascade of solid-state processes initiated by variation of temperature was found for the heterospin complex [Cu(hfac)2L(Me/Et)] formed in the reaction of copper(II) hexafluoroacetylacetonate [Cu(hfac)2] with stable nitronyl nitroxide 2-(1-methyl-3-ethyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L(Me/Et)). The cooling of the compound below 260 K initiated a solid-state chemical reaction, which led to a depolymerization of chains and formation of a pair heterospin complex [Cu(hfac)2L(Me/Et)2][[Cu(hfac)2]3L(Me/Et)2]. Further decrease in temperature below 144 K led to a spin transition accompanied by a drastic decrease in the effective magnetic moment from 2.52 to 2.24 μB. When the compound was heated, the order of effects was reversed: at first, the magnetic moment abruptly increased, and then the molecular fragments of the pair cluster united into polymer chains. Two hysteresis loops correspond to this cascade of temperature-induced structural transformations on the experimental dependence μeff(T): one at high (T↑ = 283 K and T↓ = 260 K) and the other at low (T↑ = 161 K, T↓ = 144 K) temperature. The spin transitions were also recorded for the [[Cu(hfac)2]3L(Bu/Et)2] and [[Cu(hfac)2]5L(Bu/Et)4] molecular complexes, which are models of the trinuclear fragment of the {[Cu(hfac)2]3L(Me/Et)2} pair cluster.

The manifestation of degenerate electron exchange in stimulated nuclear polarization at high magnetic fields

Abstract A detailed theoretical and experimental investigation of the degenerate electron exchange (DEE) effect on the stimulated nuclear polarization (SNP) intensity and spectra at high magnetic fields has been made. The theoretical description of DEE processes is performed in two different ways: (i) The numerical solution of equations of noncorrelated frequency migration theory and (ii) the analytical solution for simple two-site model for the dependence of the SNP intensity on the rate of DEE at low microwave (mw) field amplitudes ( B 1 ⪡ a , a is a constant of hyperfine interaction (HFI) of radical-ions) and low DEE rates ( aτ e ⪢ 1, τ e is the characteristic time of DEE). It is shown that the increase of DEE rate leads to the decrease of splitting in SNP spectra, and also to a substantial decrease of the SNP signal intensity due to an overlapping of hyper-fine structure lines with opposite signs. A numerical evaluation of DEE rate constants can be done by comparison of experimental and calculated spectrum shape in the case of fast DEE rates and from the dependence of SNP intensity in resonance on the concentration of diamagnetic molecules.

RADICAL POLARIZATION IN DOUBLE SWITCHING OF EXTERNAL MAGNETIC FIELD

Abstract Theoretical treatment of radical spin evolution under the action of double switching of external magnetic field is proposed. Account is taken of evolution of the radical spin state during laser pulse which generates paramagnetic particles. It is shown that the most effective beats in the nuclear magnetization of diamagnetic products of recombination occur upon the jump into zero magnetic field after laser pulse. The phase of observed beats bears information about the type of the initial radical polarization. The frequency of the beats is determined by radical hyperfine structure.

Theory of multiquantum optically detected ESR spectra of radical pairs. II. Resonances in perpendicular radio-frequency field

Abstract The previously developed general theory of multiquantum optically detected ESR spectra of radical pairs has been used to analyse multiquantum resonances in the radiofrequency field that is perpendicular to the Zeeman field. The cases of circular polarization (resonance and nonresonance rotation directions) have been considered. The position and form of spectral components at arbitrary values of nuclear spins have been studied assuming the Poisson lifetime distribution of radical pairs. A physical interpretation of the results obtained has been proposed. The resonances of the partners with a complex nuclear structure have been discussed. The nonresonance corrections in the spectra studied have been analysed. A simple method for their estimation has been proposed.

Ferromagnetic Coupling in the Heterospin Bis-Catecholato–Manganese(IV) Complex with Pyridine Substituted by Nitronyl-nitroxide

A new bis(3,6-di-tert-butyl-catecholato)manganese complex with two 4-NIT-Py ligands was synthesized and characterized [4-NIT-Py = pyridine substituted at position 4 with nitronyl-nitroxide radical, 2-(pyridin-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl]. X-ray diffraction indicated an octahedral environment of the manganese atom with a trans arrangement of ligands. Bonds lengths in the inner coordination core of the metal and in the chelate cycles that are representative of the charge distribution between the metal and ligands displayed a Mn(IV)(Cat2-)2 charge distribution. Variable-temperature magnetic susceptibility measurements detected intramolecular ferromagnetic coupling between the Mn(IV) S = 3/2 spins and spins of nitronyl-nitroxyls and intermolecular ferromagnetic interactions of spins of adjacent nitronyl-nitroxide fragments in a chain of molecules at low temperatures. The last phenomenon is revealed by short contacts between nitronyl-nitroxide radicals of adjacent complex molecules.

Exchange coupling transformations in Cu (II) heterospin complexes of “breathing crystals” under structural phase transitions

Family of “breathing crystals” is the polymer-chain complexes of Cu(hfac)2 with nitroxides. The polymer chains consist of one-, two- or three-spin clusters. The “breathing crystals” experience simultaneous magnetic and Jahn-Teller type structural phase transitions with change of total cluster spin and drastic change of bond lengths (ca. 10-12%). For the first time the intra-cluster magnetic couplings in ”breathing crystals” have been calculated both by band structure methods GGA + U and hybrid DFT (B3LYP and PBE0) for the isolated exchange clusters. The temperature dependence of the magnetic coupling constant was calculated for two polymer-chain compounds of the “breathing crystal” family - C21H19CuF12N4O6 with the chains containing two-spin clusters and C22H21CuF12N4O6 with the chains of alternating three-spin clusters and one-spin sites. It was found that adding a Hubbard-like parameter not only to the copper 3d electrons but also to the oxygen 2p electrons (GGA + Ud + Up approach) results in an improve...