AN AB-INITIO MO STUDY ON HYDROGEN ABSTRACTION FROM METHANOL BY METHYL RADICAL

The H atom abstraction from methanol by methyl radical has been studied theoretically based on ab-initio MO calculations of the reaction system at the HF/3-2lG and MP4SDQ/6-31G**//MP2/6-31G levels. The rate constant was estimated for two reaction channels, CH3OH + CH3 --> CH2OH + CH4 and CH3OH + CH3 --> CH3O + CH4, through the intrinsic reaction coordinate by the RRKM theory including the tunneling effect and through the short-cut paths on the two-dimensional potential energy surface. The Arrhenius plot of the rate constant for both the reaction channels starts to deviate from a straight line at about 100 K and approaches a limiting value, showing the importance of the quantum tunneling effect at low temperature. The H abstraction from the hydroxyl group is theoretically predicted to dominate over that from the methyl group at low temperature, in contradiction with previous results of ESR experiments on the methyl radical in solid methanol. This contradiction is possibly explained by taking into account the hydrogen bonding in condensed phases which effectively blocks the reaction site for the H abstraction from the hydroxyl group.