Ab initio Quantum-Mechanical Calculations of Molecular Structure and Conformations of 2,2-Dichloroethanal in the Ground and Excited Lowest Triplet States

Ab initio calculations were carried out to investigate the molecular structure of 2,2-dichloroethanal (DCE, CHCl₂CHO) in the ground (S ₀) and excited lowest triplet (₁) states. It is found that electronic excitation of DCE from the S ₀ to T ₁ state occurs with top rotations and a loss of planarity of the carbonyl fragments. Six minima corresponding to three pairs of enantiomers were found on the potential energy surface (PES) of the DCE molecule in the ₁ state. Based on the PES calculated (by the UHF and CASSCF methods in a 6-31G** basis) for DCE in the ₁ state, the one-dimensional torsional and inversion problems and the two-dimensional torsional-inversion problems are solved. A comparison of the results has revealed a relationship between the torsional and inversion motions.