Yurij Dmitriev

Anomalous EPR Intensity Distribution of the Methyl Radical Quartet Adsorbed on the Surface of Porous Materials. Comparison with Solid Gas Matrix Isolation

The two inner lines of the EPR quartet of methyl radicals trapped in cryogenic gas matrices are superpositions of the inner transitions of an A-proton-spin quartet and an E-proton-spin doublet. Their intensity relative to the outer lines provides information on the population of the methyl-rotation quantum states. The above intensity ratio for the CH3 in solids is a challenging problem of the quantum dynamics and statistical thermodynamics. The influence of the quantum-mechanical/inertial rotation on the intensity distribution of the hf components of methyl radical on the surface of porous materials, e.g., silica gel, is investigated by EPR line shape simulations and compared with spectra of the radical isolated in the bulk of solid gas samples. The experimental part of this study includes the first in literature EPR observation of methyl radical in the bulk of N2O solid and provides new essential information on CH3 in CO2 and Ar matrices, thus, covering both strongly hindered and almost free rotation of the radical. We verify the observation of nonrotating methyl radicals in a N2O matrix, discovered earlier in cold CO2, give a thorough account of their EPR characteristics, and explore their formation at the inner surface of porous materials. Combination of a classical spin-Hamiltonian with employment of quantum effects due to nuclear spin-rotation coupling and the radical symmetry were used to interpret the experimental spectral observations. The cause of experimentally found unexpected contribution of the excited degenerate E-doublets to the EPR spectrum down to 4.2 K and A/E transition amplitude ratios sometimes as high as ca. 1:8 at liquid-N2 temperature is sought. The validity of Bose-Einstein quantum (BEq-) statistics of the spin rotation states in addition to the classical Maxwell-Boltzmann (Boltzmann) statistics was also assessed against experimental population A/E-ratio data. The BEq-statistics were not previously applied to similar systems. Furthermore, detailed consideration of the laboratory/free space rotational degeneracy and the planarity of methyl radical was also included in this work.

Dynamical Effects in CW and Pulsed EPR

Typical molecular processes with EPR accessible dynamical parameters are listed and evaluation of their timescales according to the different EPR methods used for this purpose is reported. The detection and description of the dynamics of cyclic and some other small radicals and related nitroxide labels isolated in solids and the connection to the structural parameters are given. Both fast-motion averaging and the method of lineshape modification due to chemical exchange are outlined. Some usually overlooked anomalies concerning the activation parameters of the thermally activated rotary motion and their relation to the microscopic variables of the spin-motion system are discussed. The definition of the thermodynamic limits differentiating diffusional motion from quantum motion and the particular ways of the couplings of these motions to the spin system are exemplified. Several experiments manifesting their difference, such as comparison of EPR spectra for classical and for tunnelling rotors, as well as severely distorted EPR spectra including totally quenched (stopped) methyl-type rotors are reviewed and explained. Spin-lattice relaxation and broadening are discussed for fast and slow motions in solids and the characterization of the different dynamics according to the effects of motion on the ESE (Electron Spin Echo) decay are considered in the framework of pulsed EPR. Finally, some standard biological applications in determining the timescales and the motional pattern of disordered matter at the molecular level are described with emphasis to the relatively recent pulsed EPR techniques and some relevant developments. Also the classical EPR methods for dynamics investigations will be reviewed focusing in the application of fundamental theoretical tools.

Methyl Radical in Clathrate Silica Voids. The Peculiar Physisorption Features of the Guest–Host Molecular Dynamics Interaction

EPR line shape simulations of CH3/SiO2 clathrates and comparison to CH3/N2O and CH3/SiO2 experiments reveal the motional conditions of the CH3 radical up to the unusual regime of its stability, the high-temperature diffusional regime, at 300 K. In the low-temperature region, the CH3 in clathrates is found to rotate around the in-plane axes even at as low temperatures as 3.8 K. However, nonrotating methyls performing only libration about the C2-axes as well as around the C3-axis are also found, proving the existence of special sites in the clathrate voids that begin to accumulate a significant fraction of methyl radicals at temperatures below approximately 7 K. A distinctive feature in the spectrum anisotropy and line width temperature profiles is found nearby 25 K, which is interpreted as the radical physisorption inside the voids that occurs with the sample temperature lowering. The unusual increase of the CH3/SiO2 clathrate EPR spectral width with temperature over approximately 120 K has its origin in repeated angular momentum vector alterations due to frequent collisions with the clathrate void walls between periodical free rotation periods. This relaxation mechanism resembles to spin-rotation interaction known only for small molecular species in nonviscous fluids but unknown earlier for methyl hosted in solids.

Epr of CH3 radicals in SIO2 clathrate

EPR lineshape simulations of CH3/SiO2 clathrates reveal the motional conditions of the CH3 radical up to the unusual regime of its stability, the high temperature diffusional regime. This was obvious by the isotropic magnetic interaction at the highest experimental temperatures over 140 K. Special motional and thermodynamics conditions for methyl radical may however prevail for the CH3/SiO2 clathrates system due to the limited space of the host voids, compared to solid gas isolation. The lowest temperature in the experiment was 4.1 K, while the highest one was 300 K. The EPR parameters of the radical revealed non-monotonic temperature dependence. The extremely wide temperature range of the radical stability may be attributed to the solidity of the clathrate voids and the small diameter of their channels that do not allow molecular collisions between the radical species. At the lowest sample temperatures, a portion of the radicals stopped to rotate thus indicating their attachment to specific matrix sites with large radical-host interaction. The unusual increase of the width of the CH3/SiO2 clathrate spectra with the temperature at high sample temperatures indicates resemblance to the spin-rotation interaction relaxation mechanism known only in the case of small species in non-viscous fluids, and is contrasted to the normal difussional decrease of the width in the CH3 hosted in a series of solid. The effect was explained by adopting extremely frequent radical collisions with the clathrate void walls leading to repeated angular momentum alterations, a kind of “reorientation”.a