Theoretical O–CH3 bond dissociation enthalpies of selected aromatic and non-aromatic molecules

Abstract

Although methyl transfer reactions are important in both chemical and biological systems, there is a need for thermodynamic parameters related to methyl affinity and O–CH3 bond dissociation enthalpies (BDEs) relevant to a full understanding of the mechanisms of methyl transfer reactions. As a prelude to the construction of a database of O–CH3 BDEs, the present work examines the reliability of a series of theoretical methods for the prediction of O–CH3 BDEs using a set of 25 compounds that included both aromatic and non-aromatic molecules. The BDEs calculated by density functional theory (DFT) with traditional exchange–correlation functions exhibited much larger errors than those obtained by either the M06-2X or G4 methods. For the non-aromatic compounds, M06-2X/def2-TZVP performed slightly better than G4, but G4 was more accurate for the aromatic molecules. As a result, we recommend G4 as the preferred method for the theoretical estimation of O–CH3 bond dissociation enthalpies, although M06-2X may be a good alternative for large complex molecules when the use of G4 is impractical.