Theoretical studies of the formation of quaternary pyridinium mesylates

Theoretical studies of the reaction between suitable mesylate derivatives and pyridine by which quaternary pyridinium salts are formed were performed using the B3LYP/6-31+G** method. Activation barriers were also calculated at the MPW1K/6-31+G** level. All the structures were fully optimized in the gas phase, whereas single point calculations were done using the polarizable continuum model (PCM) in three media of different polarity: chloroform, ethanol and water. Full optimization in water has been carried out for the reaction between methyl mesylate and pyridine. It is shown that the B3LYP/6-31+G** activation barrier increases when a hydrogen atom in the methyl group is replaced by a tetrahydrofuran ring. MPW1K/6-31+G** level calculations show that activation barriers are higher than those from B3LYP/6-31+G**. Transfer from the gas phase to the aqueous solution causes the activation barrier to decrease significantly.