Quantum–chemical study of the structure of the acetyl fluoride molecule in the ground and lowest excited singlet and triplet electronic states

The structure of the conformationally flexible acetyl fluoride molecule (CH 3CFO and CD 3CFO) in the ground (S 0) and lowest excited triplet (T 1) and singlet (S 1) electronic states was calculated by different quantum–chemical methods (RHF, UHF, MP2, CASSCF). The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these electronic states were estimated. The calculations demonstrated that the electronic excitation causes considerable conformational changes involving the rotation of the CH 3(CD 3) top and a substantial deviation of the CCFO carbonyl fragment from planarity. For large-amplitude vibrations, namely, for the torsional vibration in the S 0 state and the torsional and inversion (nonplanar carbonyl fragment) vibrations in the T 1 and S 1 states, the quantum–mechanical problems were solved in one-dimensional (1D) and two-dimensional (2D) approximations. The results of calculations are in good agreement with experimental data.