Gas-Phase Structures and Conformational Properties of Malonic Acid Derivatives, [CY₂(COX)₂], as Studied by Electron Diffraction and Ab Initio Calculations

Abstract

The geometric structures and conformational compositions of malonyl difluoride, COF - CH₂ - COF, difluoromalonyl difluoride, COF - CF₂- COF, and malonyl dichloride, COCI - CH₂ - COCI, were studied by gas electron diffraction and ab initio calculations (HF/3–21G and HF/6–31G^*(*)). The experimental scattering intensities of all three compounds are consistent with mixtures of two conformers with skew skelétons and the C = O bonds eclipsed to vicinal single bonds (C - C, C - H or C - F). In conformers possessing C₂ symmetry, both double bonds eclipse the C - H or C - F bonds of the methylene group, and in conformers with C₁ symmetry, one C=O bond eclipses a methylene bond and the other the opposite C - C bond. The theoretical potential curves for rotation around the C - C bonds for various conformations are discussed and compared to the experimental results. Corresponding ab initio calculations for dichloromalonyl dichloride, COCI - CCl₂- COCI, for which no experimental data are available, predict - for both basis sets - a conformer with C₂ symmetry to be lowest in energy. In addition, the potential functions for rotation around the C - C bonds for the diketo form of the parent compound malondialdehyde, COH - CH₂- COH, were studied at the HF, MP2 and MP4 level of theory using various basis sets (3–21G, 4–31G and 6–31G^**).