Interpretation of \u3b2-hydrogen hyperfine splittings in the electron spin resonance of the 3-methyl-3-phenylbut-1-yl radical

At low temperatures the PhCMe2CH2CH2 radical preferentially adopts a conformation in which the radical center and the phenyl substituent are gauche to each other when viewed along the C\u3b2-C\u3b3 bond. The \u3b2-hydrogens are magnetically nonequivalent and the magnitudes of their hyperfine splitting represent an average over rotation about the C\u3b1-C\u3b2 bond. On the basis of the equation \u3b1H\u3b2= 54\u3008cos2 \u3b8\u3009 G, we have derived a simple, two-component rotational potential which supports the suggestion that there is a weak, attractive interaction between the singly occupied C\u3b1 2pz orbital and the \u3c0-cloud of the phenyl substituent. The conformations of some related radicals are also discussed.Peer reviewed: YesNRC publication: Ye

Hydrogen abstraction by methyl radicals in glasses

An experimental and theoretical study is reported of the abstraction of hydrogen atoms by methyl radicals from organic glasses, in particular methanol and several of its deuterated analogues. Rate constants are obtained for hydrogen and deuterium transfer as a function of temperature for a distribution of trapping sites of the radical in the glass. The site dependence is investigated by an analysis of the non-exponential decay of the methyl radical concentration based on the use of Laplace transforms. With the help of a recently developed theoretical model for hydrogen tunnelling, a relationship between tunnelling rate and tunnelling distance is established on the basis of these observations. The results yield a detailed picture of the structure of the trapping sites: roughly spherical cages in which the tumbling radical is surrounded on average by eleven rotating methyl groups, with the closest of which it reacts. The model yields a quantitative description of the rate of this reaction, expressed in terms of spectroscopic, thermodynamic and quantum-chemical input parameters.Peer reviewed: YesNRC publication: Ye

Temperature and site dependence of the rate of hydrogen and deuterium abstraction by methyl radicals in methanol glasses

Rate constants are reported for hydrogen and deuterium abstraction by methyl radicals in CH3OH and CD3OD glasses in the ranges 5-89 and 77-97 K, respectively. At each temperature, they show a distribution due to a variation of radical trapping sites. The rate constants of this distribution are analyzed theoretically to yield a quantitative relation between tunneling rate and equilibrium tunneling distance.Peer reviewed: YesNRC publication: Ye

Competition between hydrogen and deuterium abstraction by methyl radicals in isotopomerically mixed methanol glasses

Rate parameters are reported for hydrogen and deuterium abstraction of methyl radicals embedded in glassy mixtures of CH3OH and CD 3OD. The mole fraction of CH3OH in these isotopomeric mixtures is 0, 0.05, 0.075, 0.10, 0.15, or 1. The nonexponential time dependence of the radical concentration is analyzed in terms of distributions of first-order rate constants. For the isotopomerically pure matrices, lognormal distributions describe the decay satisfactorily. The large difference between characteristic H and D transfer rate constants indicates tunneling. In the mixtures, there is competition between H and D abstraction processes which depends on the local structure about a radical, so that the corresponding rate parameters contain information about this structure. On the basis of earlier work [J. Chem. Phys. 86, 6622 (1987)], the analysis begins with the assumption that the structure about a radical resembles one of the crystalline phases of methanol. The entire set of decay curves is described by a (disordered) \u3b2-phase structure in which the radical replaces a methanol molecule and is located near the position associated with a methyl group. However, this static picture is inadequate because the radical can diffuse through the glass on the time scale of the kinetic measurements. Diffusion allows the radical to encounter more CH3OH molecules than would be expected for the static structure on a statistical basis - the effective mole fraction of CH 3OH in the mixtures is higher than the analytical concentration. For the xH=0.05 mixture, we estimate that on the average the radical encounters approximately 26 methanol molecules before abstraction occurs. This corresponds to diffusion over roughly 1100 pm through the lattice. \ua9 1993 American Institute of Physics.Peer reviewed: YesNRC publication: Ye

Electron spin resonance (ESR) investigation of the structure of methyl radical trapping sites in methanol glass

Measurements are reported of ESR spectra of methyl radicals trapped in methanol glasses. In these spectra, forbidden lines appear as satellites of the lines of the methyl quartet as a result of dipolar coupling of the unpaired electron with protons of neighboring methanol molecules. The relative intensity of the satellites is used to study the structure of the sites where the radicals are trapped. Comparison of intensities observed in CH3OH, CH 3OD, CHD2OD, CD3OH, and CD3OD indicates a structure that is locally similar to the (disordered) \u3b2-phase crystal structure of methanol, with the methyl radical replacing a methanol molecule and occupying a position close to its methyl position. The resulting methyl-methyl distances are compared with those deduced from the observed rate constants of the hydrogen abstraction reaction taking place at the trapping sites. If volume changes due to cooling and phase transitions are taken into account, the distances obtained in the two experiments are found to be compatible. This confirms earlier conclusions that methanol glass has many structural features in common with the \u3b2-phase crystal.Peer reviewed: YesNRC publication: Ye