Deuterium Labeling Studies and Quantum Effects of Radicals in Solids

Abstract

Recent progress in deuterium (D or 2H) labeling studies of radicals in the solid state is reviewed. Emphasis is placed on quantum effects at low temperature. The high-resolution EPR (ESR) spectra of selectively D-labeled methyl radicals were radiolytically generated together with a hydrogen atom methyl radical pair in an Ar matrix at cryogenic temperatures. The methyl radical spectra are discussed in terms of nuclear spin-rotation couplings using a three-dimensional free quantum-rotor model. A hydrogen atom hydrogen molecule (H···H2) pair formation in Ar is discussed in terms of D-isotope effects on quantum mechanical tunneling reaction. The H2 molecule as a “quantum solid” for high-resolution EPR spectroscopy is presented. D-effects on zero-point vibrational energy (ZPVE) are presented in combination with Jahn-Teller (J-T) distortion of some chemically important radical cations of methane, tetramethylsilane, cyclohexane and related radical cations whose mother molecules have high symmetrical structures such as T d and D 3d .D-isotope effects on methyl group conformations are exemplified using selectively deuterated dimethylether and monofluoromethane radical cations; the experimental results were also interpreted in terms of ZPVE incorporated with the mass difference of the two hydrogen isotopes. Furthermore, a D-labeling study on bimolecular homolytic substitution (SH 2) reactions by methyl radical in solid methylsilane (CH3 SiH3) is presented.