Kinetics of Tautomerization Reactions of Trithiocyanuric Acid at Low Temperatures

Photochemical irradiation of tri-thiocyanuric acid at low temperatures results in the formation of a higher energy isomer, the tri-thiol 1,3,5-triazine-2,4,6-trithiol [Rostkowska Journal of Physical Chemistry A 109 (2005): 2160]. The question arises as to how, and whether, the latter can revert to its parent 1,3,5-triazine-2,4,6-trithione in a reasonable time at temperatures of 10–300 K. Quantum chemical calculations using both density functional and wave function theories attempt to compute the kinetics of the return voyage. We show that reactivity is dominated by hydrogen atom tunneling and conformational changes, calculated from variational transition state theory allied to small-curvature tunneling effects, rather than purely thermal reactions, and that there is no way back during the usual laboratory lifetimes. Both water and methanol are shown to act as catalysts by both accepting and donating hydrogen atoms, reducing the barrier to reaction by more than one-half and enhancing the reaction rate, with methanol being the more effective agent.