Density functional theory calculations with different functionals have been performed to investigate the kinetics and mechanism of the thermal decomposition of oxalic acid in the gas phase and in solution in aniline, N-methylaniline, quinoline and DMSO. Five major possible mechanisms for this reaction have been proposed. Comparison between the theoretical data and experimental results shows that in the most probable path of this reaction CO, CO 2 and H 2O are produced. Moreover, this reaction is a little faster in the gas phase than in solvents. Quantum theory of atoms in molecules analysis indicates that C–C and O–H bond formations in the transition states have a covalent nature, because the ratios of kinetic energy density to potential energy density at the corresponding bond critical points are smaller than 0.5. Based on natural bond orbital analysis, during transition state formation, the interaction energy between the lone pair of electrons of the oxygen atom and the π* molecular orbital of the critical C–O bond is reduced and solvents affect the charge transfer process.

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