Ab Initio Structure Investigation of the Enol Forms of β-Diketones RCOCH2COR (R = H, CH3, CF3)

Abstract

The geometrical parameters, force field parameters, and vibrational frequencies of the enol forms of β-diketones RCOCH₂COR (R = H, CH₃, CF₃) were calculated using the ab initio MO LCAO SCF method involving broad bases of Cartesian Gaussian functions. For acetylacetone (R = CH₃), the potential surface of rotation of the two methyl groups was investigated in detail, and the minimal energy route of the transfer of the hydroxyl proton inside the chelate ring was calculated. For hexafluoroacetylacetone (R = CF₃) and malonic dialdehyde (R = H), calculations were carried out only for stationary points on the potential energy surface. For all the geometrical configurations under study, the relative energies were refined with inclusion of electron correlation in terms of second-, third-, and fourth-order Möller–Plesset perturbation theory; for molecules with R = H, CH₃, the configuration interaction procedure taking account of all single and double excitations was additionally employed. The experimental and theoretical data are compared with those available in the literature. The substituent effect on the geometrical and electronic structure of the chelate ring is studied.