High-resolution ESR study of the H...CH3, H...CHD2, D...CH2D, and D...CD3 radical pairs in solid argon

Rotation of methyl radicals in a solid krypton matrix

Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH(3)) in a solid krypton matrix at 17-31 K temperature range. The radicals were produced by dissociating methane by plasma bursts generated by a focused 193 nm excimer laser radiation during the krypton gas condensation on the substrate. The ESR spectrum exhibits only isotropic features at the temperature range examined, and the intensity ratio between the symmetric (A) and antisymmetric (E) spin state lines exhibits weaker temperature dependence than in a solid argon matrix. However, the general appearance of the methyl radical spectrum depends strongly on temperature due to the pronounced temperature dependency of the E state linewidths. The rotational energy level populations are analyzed based on the static crystal field model, pseudorotating cage model, and quantum chemical calculations for an axially symmetric, planar rotor. Crystal field strength parameter values of -140 cm(-1) in Ar and -240 cm(-1) in Kr match most closely the experimentally observed rotational energy level shifts from the gas phase value. In the alternative model, considering the lattice atom movement in a pseudorotating cage, the effective lowering of the rotational constants B and C to 80%-90% leads to similar effects.

Hyperfine coupling in methyl radical isotopomers

The hyperfine coupling constants (hfcs) of two methyl radical isotopomers, CH2Mu and CD2Mu, have been measured over a wide range of temperature in ketene and ketene-d2, from which the radicals were generated. The magnitudes of the hfcs of these muoniated methyl radical isotopomers are larger than those of CH3 and CD3 due to larger zero-point energy in the out-of-plane bending mode. In contrast to CH3 and CD3, where the coupling constants become smaller with increasing temperature, the negative hfcs of the muoniated radicals were found to increase in magnitude (become more negative) with temperature, passing through a maximum near the boiling point of ketene. This behavior is attributed to a solvent-induced change in the force constant of the out-of-plane bending mode. The opposite temperature effect known for CH3 and CD3 is explained by excitation of the low frequency out-of-plane bending mode. This effect is much smaller in the muoniated radicals, where the vibrational frequency is significantly higher due to the light mass of muonium; consequently, the solvent effect dominates at low temperatures.