The structures of the stable cis-enol forms of BA and p-X-BA, X=F,Cl,CH3,OCH3 and their relative stabilities (calculated at the B3LYP/6-311++G**) are shown in Fig. 1. According to this Figure, the energy differences between the stable chelated enols in the gas phase are negligible (0.59-0.83 kcal/mol). So, different substituents have small effect on the stability and relative energies. The computational calculation of equilibrium constants are obtained from Gibbs free energy calculations (equation ΔG0= - RT LnK) at 298 K, and shows in Table.1, Which ΔG0 is the Gibbs free energy differences between two cis-enol forms in p-X-BA. According to this table, the solvents and calculation basis sets have small effect on the equilibrium constant and the value of the equilibrium constant is near that of experiment.

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