A theoretical study of the solid-state thermal isomerization reaction of methyl 4-(dimethylamino)benzenesulfonate in the energetic aspect

The thermal isomerization reaction from methyl 4-(dimethylamino)benzenesulfonate (MDBS) to N,N,N-trimethylbenzenaminium-4-sulfonate (TBS) in the crystalline state has been studied to understand the origin of its exothermic nature. Ab initio MO calculations of these molecules with their molecular geometries realized in crystals and recalculations of coulombic contributions to the lattice energies based on those results show that the heat exhausted by the solid-state reaction of interest can be elucidated by the lattice energy of TBS, which is much higher than that of MDBS, although the energetic stability of a TBS zwitterion is lower than that of MDBS.