Unimolecular Decomposition of Oxalic Acid

Article abstract of DOI:10.1021/j100293a034

Ab initio Mo calculations have been carried out for the unimolecular decomposition of oxalic acid.We used the Hartree-Fock (HF) method with LCAO approximation mainly using the 3-21G basis set with standard parameters to optimize the geometries for the three conformers of oxalic acid and eight probable trnasition states.The energy gradient technique was employed.Normal modes and vibrational frequencies were calculated by using the 3-21G basis set.It was found that the lowest energy path was (COOH)2 -> CO2 + CO + H2O (2), having a five-center transition state.From the results of ab initio calculations, the first-order rate constant for channel 2 was evaluated as k₂ = 1E14.9 exp(-29.8 kcal mol^-1/RT) s^-1, over the temperature range 300-1300K, in terms of transition-state theory.The thermal decomposition of oxalic acid vapor diluted in Ar has been also briefly investigated behind reflected shock waves over the temperature range 850-1300 K.The decomposition was monitored by IR emission and vacuum-UV absorption from products.The decomposition product analysis was also done by gas chromatography.Although the rate constant could not be evaluated because of the very low reactant concentration and the too fast decomposition, major products observed were CO2, CO, and H2O, being consistent with the results of the ab initio calculations and the previous infrared multiphoton study by Yamamoto and Back.