Spectroscopic Studies of the Thermal Rearrangement Reaction of Dimethyl 3,6-Dichloro-2,5-dihydroxyterephthalate in the Solid State

The thermal rearrangement of the yellow (Y) isomer of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate in the solid state, which yields a white (W) isomer, has been investigated by a combination of Raman phonon spectroscopy and electronic emission spectroscopy. The rearrangement is found to start as a homogeneous process but becomes heterogeneous as the product forms. The Raman spectra of the Y and the W isomers both in the phonon and in the internal vibration region show differences that relate to the difference in the nature of the hydrogen bonds of these two isomers in the solid state. A rigid-motion analysis of the published X-ray thermal parameters has been used to make assignments of the molecular nature of the phonon transitions observed in the Raman spectra. A temperature-dependence study of the Raman phonon spectra suggests that the rearrangement may be defect controlled rather than phonon assisted.