Mechanism of transmethylation in anisole decompostion over Brønsted acid sites: Density Functional Theory (DFT) study

In this work, the mechanism and intrinsic reaction energy barriers of transmethylation, as the initial stage of the catalytic and non-catalytic anisole decomposition, were investigated by Density Functional Theory (DFT). Molecule analyses indicated that methyl free radical transfer happened in the absence of catalyst, and the catalytic transmethylation over Brønsted acid sites was considered based on the dual electrophilic attack mechanism with protonation and carbocation substitution respectively. Reactions modelling for the formation of methyl-contained compounds in both non-catalytic and catalytic anisole decomposition indicated that the energy barriers were significantly decreased in the presence of catalyst by 60 kcal/mol at the most in the case of o-cresol. The results also revealed that the intrinsic transmethylation orientation preferred the ortho- and para-positions on the acceptor compounds contained oxygen-rich substituents due to its large electronegativity, and the lowest energy barrier was observed in the case of transmethylation towards the para-position of the cresol molecule (54.1 kcal/mol).