MOLECULAR-ORBITAL CONSTRAINED GAS ELECTRON-DIFFRACTION STUDIES .1. INTERNAL-ROTATION IN 3-CHLOROBENZALDEHYDE

The geometries of the trans and cis forms of 3-chlorobenzaldehyde (I and II, respectively) were optimized using standard single determinant MO theory with the STO-3G basis. The calculated energy difference between I and II was 90 cal mole-1. Mean amplitudes of vibration were calculated for the system by vibrational analysis. Computed molecular properties - amplitude differences and ab initio differences between some geometrical parameters - were combined with the gas electron diffraction data for the compound to constrain least-squares data analysis. The model which was in best agreement with experiment represented a conformational equilibrium with a composition of 64(16)% trans and 36% cis for the vapor of 3-chlorobenzaldehyde at 120°C. This result contradicts a previous vapor phase far infrared investigation of the compound and the torsional potential derived from it. Potentially misleading factors may, therefore, generally have to be taken into account whenever torsional potentials are derived from vibrational data. Beyond the scope of the structural problem considered, the paper demonstrates the advantages of molecular orbital constrained electron diffraction (MOCED) studies. © 1979.