G. G. Lazarev

Stable radical pairs under the photolysis of 2,6-di-t-butyl quinone diazide in the single crystals of sterically hindered phenols: New type of double-spin probes

Abstract Intermediate radical products, which are formed when monocrystals of sterically hindered phenols containing 10 −3 M 2,6-di- t -butyl quinone diazide are photolysed, have been studied employing EPR. We have identified the formation of carbenes and several types of radical pairs. It is found that, with a rise in temperature, a red-shift is observed for the long-wave boundary involving the formation of radical pairs. The high stability of radical pairs made it possible to measure the thermal coefficients for the linear expansion of the monocrystals. Two types of asymmetric relaxation have been identified in the EPR spectra of radical pairs.

Hydrogen and electron transfer in the photoreduction reaction of quinones

Abstract The primary products of the photoreduction reaction of 3,6-di t -butyl- o -quinone in single crystals of 3,6-di- t -butyl-pyrocatechol are studied by EPR spectroscopy. Ion radical and neutral radical pairs are identified distinctly. In the case of the same o -quinone in single crystals of 2-6-di t -butyl-4-methylphenol the primary product is the radical pair composed of the two hydroxyphenoxyl radicals of phenol. This indicates the possibility of transfer of two hydrogen atoms in a single elementary photochemical event.

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.

The mechanism of formation of stable radical pairs by photolysis of 2,6-di-tert-butylbenzoquinone diazide in crystals of 2,6-di-tert-butyl-4-methylphenol

Abstract The nature of radical pairs obtained by irradiation of single crystals of sterically hindered O-deuterated phenols containing 10 −3 M 2,6-di-tert-butylquinone diazide has been studied by electron-polarization-resonance (EPR) spectroscopy. A possible channel for the hydrogen-atom transfer was derived on the basis of calculated D and E values from the EPR spectra of the observed radical pairs. The thermal coefficients for the linear expansion of the crystal of deuterated phenol and diazide were measured.

Radical paris during photolysis of 1H-2,4,6,8-tetrakis(t-butyl)phenoxazin-1-one in single crystals of 3,5-di-t-butylpyrocatechol

Abstract The formation of radical pairs upon photolysis of single crystals of 3,5-di- t -butylpyrocatechol doped with 10 −2 M 1H-2,4,6,8-tetrakis( t -butyl)phenoxazin-1-one was studied by EPR spectroscopy in the temperature range between 20 and 77 K. A temperature-dependent, reversible transition of one type of radical pair into another type was demonstrated. The relaxation in the EPR spectra of the radical pairs was studied and found to be asymmetric for some of the radical pairs detected.

EPR spectroscopic and conductometric studies of polyaniline after combined action of high pressure and shearing deformation

Abstract Relaxation processes of polyaniline after the combined action of high pressure and shearing deformation were studied by EPR spectroscopy and conductivity measurements. It was found from the temperature dependence of conductivity at different angles of rotational displacement that the conjugation length was reduced to about one tenth with an increase of angle of rotation.

Mechanism of the formation of weak complexes of the spatially hindered quinones and phenols

1. Study of absorption spectra and molecular models has shown that 3,6-di-tert-butyl-o-benzoquinone and 3,6-di-tert-butylpyrocatechol can form either charge transfer complexes or complexes with two hydrogen bonds (HBC). 2. The ESR spectra suggest the presence of four types of radical pairs in irradiated frozen solutions of quinone and pyrocatechol. A mechanism, involving H-atom phototransfer from the pyrocatechol to the quinone in the binary complex, has been proposed to account for the formation of all radical pairs, and the geometry of the complex established. 3. Differences in the fine structure constants for the deuterated and nondeuterated compounds have been explained in terms of the equilibrium established between the hydrogen bonded complexes and the sandwich complexes. The geometry of the sandwich complex varies, depending on whether the complex has been formed from the HBC or from the individual quinone and pyrocatechol molecules.

Formation of 3,6-di-tert-butyl-2-hydroxyphenoxyl radicals upon the inhibition of polymerization of methyl methacrylate by the system containing 3,6-di-tert-butylpryocatechol and 3,6-di-tert-butyl-1,2-benzoquinone

Conclusions3,6-Di-tert-butyl-2-hydroxyphenoxyl radicals (QH.) were detected by EPR spectroscopy in a mixture of 3,6-di-tert-butylpyrocatechol (QH2) and 3,6-di-tert-butyl-1,2-benzoquinone (Q) in the polymerization of methyl methacrylate. The rate constant for the reaction QH2 + Q⇄ 2QH. and the rate contants for the forward and back reactions were determined at 60°C.

Mechanism of formation of radical pairs during the photolysis of 2,6-di-tert-butylquinone diazide in single crystals of 2,6-di-tert-butyl-4-methylphenol

In the preceding investigations [I, 2], radical pairs (RP) arising during low-temperature photolysis of 2,4-di-tert-butyi-quinone diazide in single crystals of 2,6-di-tertbutyl-4-methyl-phenol (ionol) were studied by the EPR method, whereby the formation of two types of radical pairs was observed with dipole-dipole splitting constants D I = 13.2 and D 2 = 3.9 mT, respectively. The RP l are stable up to room temperature, which made it possible to use these radical pairs as a two-spin probe for measuring the thermal expansion coefficient of the ionol crystal.

Relaxation of the magnetic structures in high temperature superconducting ceramics Tl2Ca2Ba2Cu3Ox. Investigation of the microwave response

A new method to study relaxation of magnetic structures is suggested, based on the observation of the relaxation of microwave response. A new explanation of these processes is proposed, using the theory of “polychromatic” kinetics in the solid state.