Tautomerization reactions of thiobarbituric acid: A detailed kinetic study using combined canonical variational transition state theory and QTAIM approach

The kinetics of tautomerization reactions of thiobarbituric acid were thoroughly investigated at the B3LYP/6-311+G(3df,2p) level of theory and elevated temperatures from 260 to 1000 K. The kinetic parameters were determined by employing canonical variational transition state theory (CVT). The quantum tunneling effect was included by centrifugal-dominant small-curvature semiclassical adiabatic ground-state approximation method. Intrinsic reaction coordinate (IRC) analysis was performed to certify a unique connection from an obtained transition state configuration to optimized structures of reactant and products. The reaction forces and reaction force constants were computed across the IRC of each reaction. Then, using extremums of reaction forces, each reaction was partitioned into three stages containing reactant, transition state, and product stages. The extent of reactions was followed along with three reaction stages. The dependence of kinetic parameters and curvature of Arrhenius plots on temperature were investigated and discussed in the framework of CVT. To further unveil the mechanism of the reactions, the quantum theory of atoms in molecules was employed. To do this aim, QTAIM descriptors on the bond critical point (BCP) of involved bonds including the density of electrons, Laplacian of the density of electrons, Lagrangian kinetic energy, potential energy, and energy density were computed and surveyed along with reaction extent through IRC.