Ya. S. Lebedev

Pulsed EPR in 2-mm band

A pulsed 2-mm band EPR spectrometer is described. The perspectives of 2-mm band EPR are comprehensively discussed.

Electron paramagnetic resonance of coals. New approaches to an old problem with multifrequency electron paramagnetic resonance and spin echo

The origin of spin paramagnetism of coals is still under question as well as its correlation with chemical and technological properties of different coals. Recent applications of high-frequency EPR (150 GHz) together with electron spin echo at 10 GHz revealed new details about the EPR signals of conjugated aromatic hydrocarbon radicals in coal. The shape of the high-frequency EPR spectra clearly demonstrates both g anisotropy and ‘strain broadening’ caused by different local g shifts for non-equivalent radical centres. T1 and T2 values, as measured by two- and three-pulse spin echo, depend on the degree of excitation of inhomogeneous spectra.The model of strong spin exchange between centres with different conjugation areas is suggested in order to explain observed effects.

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.

Diffusion of free radicals as studied by tomography

Abstract A linear magnetic field gradient superimposed on the main static field was used to study one-dimensional diffusion of perchlorotriphenylmethyl radicals in decalin and of oxygen molecules in polytetrafluoroethylene. A simple method is developed for deducing diffusion coefficients from both time-dependent and single-scan experiments provided that the natural linewidth is small relative to the field change over the diffusion distance.

The structure of phenoxyl radicals as studied by 2 mm band ESR

The g-tensor components of a number of phenoxyl radicals including Coppingers radicals have been measured by ESR spectroscopy in the 2 mm band (140 GHz). Assuming that the g-tensor anisotropy arises mainly from spin-orbit interaction on the oxygen atom, the geometrical structure of Coppingers radical was determined. Some additional assumptions for nitrogen contributions to the g-tensor allowed the estimation of the geometrical structure of N-Coppingers radical as well. The validity of such assumptions was confirmed by quantum chemical calculations using the unrestricted Hartree-Fock method.

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.

Magnetic properties of metal–quinone high-spin complexes prepared by solid-state mechano-activation and by chemical synthesis in solution. Comparative study with very high-field and low-temperature EPR and ENDOR

EPR spectroscopy at high frequencies and low temperatures and ENDOR spectroscopy at X- and W-bands have been applied to investigate two- and three-spin complexes of the type MQ2˙ and MQ3˙ generated by mechanochemical treatment (by grinding or by the action of elastic waves) of solid mixtures of quinones and metals (M = Al, Ga, In, Cd, Sn, Zn). In addition, complexes obtained in liquid-phase reactions of metal amalgams with quinones were investigated. The magnetic-resonance parameters and the observed thermal spin polarization provide evidence for the identity of the magnetic complexes produced in solutions and by solid-state mechanochemical treatment. In particular, the presence of ground-state triplet and quartet species has been demonstrated in both cases. In addition, the exchange and the dipole–dipole interaction parameters were evaluated, their comparison providing information about the electronic intramolecular interaction.

ESE-detected EPR of triplet radical pairs produced by mechano-chemical treatment of 2,6-di-t-butylquinone diazide mixed with 3,5-di-t-butylpyrocathechon

Abstract Electron spin echo experiments at high microwave frequency (95 GHz) have been performed to study mechanically induced radical pairs. It is shown that the sign of the spin—spin interaction of the radical pairs produced by mechanical treatment of a crystalline-powdered mixture of 2,6-di-t-butylquinone diazide and 3,3-di-t-butylpyrocathechon is positive. The asymmetry of the relaxation rates between the triplet sublevels is explained by the presence of a nearby singlet state.

Investigation of Spin Labels and Probes by Millimeter Band EPR

The enhanced effectivity of spin-label and probe techniques is discussed. The 2-mm wavelength band is suggested for detecting EPR in high fields. The general principles of construction of 2-mm band EPR spectrometers are described and the physical background of resolution improvement in high fields is discussed. Experimental results from the determination of a full set of magnetic resonance parameters of nitroxyl radicals and biradicals in various media are reported. It is shown that these data allow adequate selection of spin labels and probes for biological species. The experimental evidence for molecular motions of spin labels in synthetic and biological objects is reported. The 2-mm band EPR permits one to obtain unique information on the anisotropy of label rotation and select molecular reorientation models. The technique also extends the range of the correlation times under investigation. Orientational ordering is studied for some samples. Thus, although the experimental setups required for 2-mm band studies at high magnetic fields (up to 50 kG) is quite complicated, the technique has important advantages as it is very informative and extends the range of applications of the EPR method for investigating paramagnetic models.

Very high field and multifrequency ESR study of a coal sample

Abstract The ESR spectra of a coal sample have been obtained at very high fields (8.837 T, 12.805 %, 15.715 T). The linewidth values, as compared to those obtained at lower field, allow one to assign numerical values to the g factor dispersion and to the spin-exchange frequency.