Structure of acetyl chloride molecule isotopomers CH3COCI and CD3COCl in the ground and lowest excited singlet and triplet electronic states:: a quantum-mechanical study

The structures of isotopomers of conformationally flexible acetyl chloride molecule, CH3COCl and CD3COCl, in the ground (SO) and lowest excited singlet (SI) and triplet (T-1) electronic states were calculated by the RHF, MP2, and CASSCF methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these electronic states were estimated. According to calculations, electronic excitation causes considerable conformational changes involving rotation of the CH3 (CD3) top and a substantial deviation of the CCOCl fragment from planarity. The results of calculations agree with experimental data. Two-dimensional torsional-inversion sections of the potential energy surface were calculated and analyzed. Vibrational problems for large-amplitude vibrations (torsional vibration in the S-0 state and both torsional and inversion vibrations in the T-1 and S-1 states) were solved in one- and two-dimensional approximations.