Molecular Modeling of the Kinetics, Mechanisms and Thermodynamics of the Gas-Phase Thermal Decomposition of O-Isopropyl S-Methyldithiocarbonate

Abstract

The gas-phase elimination reaction of O-isopropyl S-methyl dithiocarbonate was studied using density functional theory with a hybrid B3LYP correlation with large 6-31G*, 6-311+G** basis sets. Calculated values of the activation and thermodynamics parameters for the thermal breakdown were estimated at 623.0K at interval of 25K. The entropy change was ∆S = (-29.842) and (-28.48) J/mol/K; free energy change ∆G = 181.491 and 131.164kJ/mol and enthalpy change ∆Hreaction = 162.808 and 113.720kJ/mol; activation energy Ea = 167.988 and 118.897kJ/mol; Arrhenius factor A = 3.56x1011 and 4.20x1011 and rate constant k = 1.4 x 10-2 and 2.9 x 10-3, 4.45x10-1S-1 compared well with the experimental results at 623K ∆S(-29.842J/mol/K) ∆G= (181.491kJ/mol) , ∆H = (162.808 kJ/mol), Ea = (167.988 kJ/mol) , A = (3.56x1011) rate constant k = (1.4 x 10-2). The results showed the influence of electron donating group on the kinetics and thermodynamics parameters of xanthates. It affirms concertedness of the elimination mechanism via a two-step reaction. The first being the liberation of ethylene an intermediate (methyl dithiocarbonate) through a 6-membered transition state (TS). The second step involves decomposing the intermediate through 4-membered cyclic TS to produce carbonylsulphide and thiol which involves a C-H and C-O bond breaking and S-H bond formation. Intrinsic reaction coordinate (IRC) calculation was done on each of the TS structures to verify that they each connect to their respective minima. Wilberg bond index was employed to monitor the reaction progress and it shows that the TS possess ‘an early’ character closer to the reactant than the products