Theoretical studies of the formation of quaternary ammonium mesylates

► The reaction pathway of the formation of four quaternary ammonium salts was investigated at the DFT level. ► Structures were optimized in the vacuum, single point PCM calculations were done in chloroform, ethanol and water. ► B3LYP/6-31 + G** activation barrier increases when a hydrogen atom in the methyl group is replaced by a tetrahydrofuran ring. ► A differentiation of barriers relative to the bifurcation occurring at a γ carbon is observed. ► MPW1K/6-31 + G** level activation barriers are at least 4 kcal mol−1 higher than those from B3LYP/6-31 + G**.