Ab initio studies of the low-lying electronic states of ketene

Features of the potential energy surfaces of the X 1A1, 3A2(3A''), 1A2(1A''), 3A1(3A'), 1B1, and 2 1As1 low-lying electronic states of ketene have been investigated using self-consistent-field (SCF) and configuration interaction singles and doubles (CISD) methods with double zeta plus polarization (DZP) and DZP+Rydberg (DZP+R) basis sets. The DZP+R CISD vertical excitation energies are in excellent agreement with observed transition energies and suggest assignments for the X 1A1→1B1 and X 1A1→2 1A1 transitions in the electronic spectrum of ketene. Stationary points have been located at the DZP SCF level of theory for the first four states listed above, and SCF quadratic force constants and harmonic vibrational frequencies have been computed analytically at these stationary points. The X 1A1 geometry and vibrational frequencies compare favorably with experimental values, the agreement being typical of DZP SCF results. Due to curve crossings and conical intersections of potential surfaces, the four lowest theoretical excited state surfaces have only two valid (double) minima, corresponding to 3A'' and 1A'' electronic states. At the DZP SCF geometries, Davidson-corrected CISD adiabatic excitation energies of 16 700 and 19 000 cm-1 have been obtained for the 3A'' and 1A'' states, supporting the previous experimental T0 upper bounds of Laufer and Keller