Temperature Dependent Anisotropic Line Splitting And Linewidth In Motionally Averaged Epr Spectra Of Ch3 Radicals In Solid Ar

Abstract

Rotational dynamics is one of the most fascinating features of molecular solids. This report presents EPR studies of methyl radical rotation in solid Ar, a matrix possessing no nuclear magnetic moments that facilitates recording magnetic resonance spectra of high resolution. As a result, we were able to make a thorough investigation of the lineshape temperature behavior of the A- and E-symmetry multiplets related to different rotational states of the radical. Methyl radicals in solid Ar matrix were recorderd at the liquid helium temperature 4.2 K and in the range from 9 through 31 K. Both EPR multiplets exhibited anisotropic features which were explained by a model accounting for fast rotation about the radical C$_{3}$ axis and relatively slow reorientation about the in-plane C$_{2}$ axes. We present the first observation of anisotropic splitting in the powder EPR of methyl radical E-states suggesting that the A- and E- rotational states which differ in their spatial characteristics should exhibit a varying degree of coupling to the surrounding cage in the matrix. The simulation model of the E-state doublet was tentatively based on the transition of radical dynamics from rotational jumps at low temperatures to rotational diffusion at high temperatures with a conditional border 19 K. For the molecule in A-state , temperature assisted tunneling was supposed to occur at temperatures below 16 K while temperature activated rotational diffusion was taking place at higher temperatures.\footnote{Y. D. acknowledges a partial financial support from Russian Foundation for Basic Research under grant 13-02-00373-a.}