Structure and conformational dynamics of oxiranecarboxaldehyde in the ground and excited electronic states

Geometrical parameters, fundamental frequencies, conformer energy differences and barriers to inversion and internal rotation of the oxiranecarboxaldehyde molecule (OXCA) in the ground S0 and lowest excited triplet (T1) and singlet (S1) electronic states were obtained by means of various ab initio methods. The OXCA molecule displayed two stable conformations in the S0 state. The electronic excitation of OXCA caused pyramidalization of the carbonyl fragment and rotation of the oxirane fragment relative to the carbonyl group. The number of stable OXCA conformations in the T1 and S1 states strongly depended on the basis and method of calculation and varied from three to six. One- and two-dimensional PES sections, corresponding to the rotation of the oxirane ring and (or) inversion of the carbonyl fragment, were calculated to study the conformational dynamics of OXCA in the S0, T1 and S1 states. Vibrational energy levels and wave functions were obtained variationally from the PES. We found a strong coupling of the large-amplitude motions in some regions on the PES.