Evidence of a lattice instability during the thermally enhanced reaction of MSE studied by calorimetry, 13C-NMR, Raman scattering and birefringence

A methyl p-dimethylaminobenzenesulfonate (MSE) polycrystalline powder or single crystal is converted to the zwitterionic product p-trimethylammonium benzenesulfonate (ZWT) by a chemical thermally induced reaction. This reaction is associated to a first-order ferroelastic phase transition in partially reacted MSE crystals. A 13C-NMR study made during the in situ transformation MSE → ZWT shows that the reaction is of first order. A strong change appears in the low-frequency Raman spectrum of MSE monocrystals when passing the transition, but only a continuous evolution in the intensity of the high-frequency modes is observed. Birefringence observations permit the evolution of the optical axes during the chemical reaction to be specified. The importance of stress induced by the reaction is discussed in relation to the ferroelastic phase transition.