Hydrogen atom abstraction by methyl radicals in methanol glasses at 15–100 K: evidence for a limiting rate constant below 40 K by quantum-mechanical tunneling

Abstract

Rate constants for hydrogen atom abstraction by methyl radicals in methanol glasses have been measured from 100 to 15 K. The Arrhenius plot is nonlinear and the reaction rate constant appears to reach a limiting value below 40 K. The results are discussed in terms of simple models for quantum-mechanical tunneling in the solid state at low temperatures. Assuming that the methyl group rotation in methanol brings about a merging of the energy level distribution at the potential barrier, the observation of temperature-independent rate constants below 40 K may be attributable to a freezing out of this rotation such that tunneling occurs only from the zero-point vibrational level.