Inertial rotation and matrix interaction effects on the EPR spectra of methyl radicals isolated in ‘inert’ cryogenic matrices

Abstract

The CW-EPR lineshapes of methyl and small methyl-like radicals trapped in noble gas matrices at liquid He temperatures are substantially different from the expected classical EPR spectra. At low temperatures they show small or negligible anisotropy in studies using different experimental techniques and have a temperature dependence that differs from systems whose motional dynamics is diffusion controlled. At liquid He temperatures, before the Boltzmann statistics take over in the classical high temperature realm, the spectral intensities are dominated by quantum statistics. These properties, which were obtained experimentally at temperatures about 5 K and lower, and up to about 20 K, can be attributed to quantum effects of inertial rotary motion and its coupling to the nuclear spin of the radical. Methyl-like radicals have nuclear-exchange symmetry and contain the lightest possible isotopes, protons, and deuterons. In the ideal case of absent radical-matrix interaction, the methyl rotation about the central heavier carbon atom guaranties minimal moments of inertia. However, the theoretical interpretation of the above effects and other related quantum effects, as well as recognition of the important physics which lead to them, is not a simple matter. The literature accumulated on the subject over the years is successful but contains several unresolved questions. Recently obtained spectra of methyl radicals in Kr, N(2) and CO matrices, which are less inert than the smaller noble gas Ar, were shown to exhibit greater, but certainly slight, overall anisotropic spectral features while in earlier experimental studies the anisotropy was practically absent. Even gases of smaller radii such as Ne and H(2) at liquid He temperatures show interesting differences as hosts of methyl radicals compared to Ar. Investigation of other possible causes of this difference, not excluding the experimentally controlled ones related to the sample preparation and the MW power saturation of the CW-EPR measurement, were conducted in this work.

Counterion influence on dynamic spin properties in a V(iv) complex

“…7). Indeed, at such temperatures, methyl group rotation is not frozen and can proceed via tunnelling 63–65. Such a possibility is supported by the relative magnitudes of the low-temperature drop in T m for 1–4 , being weaker for the larger R 3 NH + ions.…”

Low-temperature tunneling of CH3 quantum rotor in van der Waals solids

“…The case of Kr matrix is of particular interest. Indeed, the amplitude sequence of the EPR spectrum of CH 3 in solid Kr matrix is a mirror image of the CH 3 spectrum in practically all the other solid gas matrices, Ne, Ar, N 2 , CO, N 2 O, CO 2 [13,18,19]. This "mirror" effect originates from the opposite sign of g-tensor anisotropy in Kr compared to Ar matrix [13,18], modifying the linear, cross relaxation term of Eq.…”

“…In Fig. 1(a), the reciprocal square roots of the hf amplitudes, 1/ A are proportional to the line width [12] that may be approximated by the following quadratic in m F formula [19]:…”

Properties of methyl radical trapped in amorphous SiO2 and in natural SiO2-clathrate Melanophlogite

“…Methyl radical plays a relevant role in many fields of research, as in biochemistry [1,2], in solid state and chemical physics [3][4][5][6][7][8][9][10] and in geology [11]. It is usually recognized in EPR spectra by its characteristic multiplet of four hyperfine components with relative intensities 1:3:3:1 (for T>40 K) and about 2.29 mT splitting between adjacent lines.…”

“…Methyl radicals were also observed and characterized in a variety of clathrate and zeolite materials [11,12], where they are generated by radiation induced dissociation of a fraction of the CH 4 molecules which naturally reside into the cages/channels of these materials. Finally, many efforts have been also devoted to characterize methyl radicals isolated in inert matrices at very low temperatures (T= 4 K), as in these special conditions they exhibit the typical features of a planar quantum rotor [3][4][5].…”