The electronic structure of the H2CO+ radical and higher Rydberg states of H2CO

Several properties of H2CO+ (e.g., equilibrium geometries, harmonic vibrational frequencies and infrared intensities, ionization potentials, electronic transition energies, and R (CO) potential curves) have been calculated at the ab initio level (MP2 and MRDCI). The electronic states X2B2, 12B1 and 12A1 are found to be planar, contrary to recent speculations by experimentalists. A low vibrational frequency in the X2B2 ionization spectrum, assumed in the literature to correspond to the out-of-plane motion of the cation, is reassigned to the asymmetric υ6(CH2) mode. The infrared absorption spectrum of the ground state is governed by the CH stretching modes, whereas υ2(CO) is practically infrared inactive. The lowest quartet states 14A" and 1,4A' are non-planar. Vertical excitation energies for several members of the π→Ryd series of H2CO converging to 12B1 of H2CO+ are presented. From the approximate R(CO) potential curves for low-lying π→Ryd and σ →Ryd states, the interactions between these H2CO states and X2B2 and 12B1 of H2CO+ are discussed in relation to observed autoionization processes.