Quantum chemical calculations of bond dissociation energies for COOH scission and electronic structure in some acids∗

Quantum chemical calculations are performed to investigate the equilibrium C–COOH bond distances and the bond dissociation energies (BDEs) for 15 acids. These compounds are studied by utilizing the hybrid density functional theory (DFT) (B3LYP, B3PW91, B3P86, PBE1PBE) and the complete basis set (CBS–Q) method in conjunction with the 6311G** basis as DFT methods have been found to have low basis sets sensitivity for small and medium molecules in our previous work. Comparisons between the computational results and the experimental values reveal that CBS–Q method, which can produce reasonable BDEs for some systems in our previous work, seems unable to predict accurate BDEs here. However, the B3P86 calculated results accord very well with the experimental values, within an average absolute error of 2.3 kcal/mol. Thus, B3P86 method is suitable for computing the reliable BDEs of C–COOH bond for carboxylic acid compounds. In addition, the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of studied compounds are estimated, based on which the relative thermal stabilities of the studied acids are also discussed.