OCS isomerization and dissociation kinetics from statistical models

Abstract

In this work computational modeling of the isomerization reactions of the linear OCS, CSO, COS, and the cyclic Δ-OCS structures, on the ground state Potential Energy Surface (PES), is performed using Rice–Ramsperger–Kassel–Marcus (RRKM), Semiclassical Transition State Theory (SCTST), and the Master Equation (ME). Adiabatic dissociations of the sulfur atom from the OCS, $$\Delta $$ -OCS and COS structures are also studied with the Variational Transition State Theory (VTST). Intramolecular Vibrational Energy Redistributions (IVRs), calculated within the adiabatic approximation framework, appear to be faster than the corresponding reactions, justifying the use of statistical methods to study the kinetics of the reactions. From the microcanonical rate coefficients, we establish that the lifetimes of isomers $$\Delta $$ -OCS and COS are insufficient to complete one vibrational period of the corresponding molecules. The energy-specific branching ratios lead us to conclude that only 0.08 and 0.07% of molecules reacting from the most stable isomer OCS produce CSO and COS, respectively, at 180 kcal/mol (when all the reaction channels are open). This can be attributed to the dominance of the dissociation over the isomerizations. From the SCTST treatment, we find that coupling and tunneling are very important in this system, as deviations from the Arrhenius-type behavior in the reaction kinetics are considerable, particularly at low temperatures.