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

摘要

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, CH3OD, CHD2OD, CD3OH, and CD3OD indicates a structure that is locally similar to the (disordered) bgr;hyphen;phase crystal structure of methanol, with the methyl radical replacing a methanol molecule and occupying a position close to its methyl position. The resulting methylndash;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 bgr;hyphen;phase crystal.