C 2υ Potential Energy Surfaces for Seven Low-Lying States of CH2

Abstract

Ab initio calculations have been carried out at 28 C2υ geometries for the lowest A11, 1A2, 3A2, 1B1, 3B1, 1B2, and B23 states of CH2. The basis set used was of the contracted Gaussian type with four s and two p functions on carbon and two s functions on hydrogen. In all calculations except A11 the SCF configuration plus all singly and doubly excited configurations were included (holding the K shell frozen), and the iterative natural orbital procedure was used to obtain an optimum set of orbitals. For the A11 state a two configuration SCF calculation was used as the starting point for the configuration interaction calculations. In a preliminary communication we predicted the triplet ground state of CH2 to be bent, and this prediction has since been justified experimentally by Bernheim et al. and by Wasserman et al. For the A11 state the ab initio geometry is r = 1.13 Å, θ = 104°, compared to experiment, r = 1.11 Å, θ = 102°. For the B11 state the predicted geometry is r = 1.09 Å, θ = 144°, as opposed to experiment, r = 1.05 Å, θ ∼ 140°. The four other states investigated, A21, 3A2, 1B2, and B23, are all unstable with respect to a carbon atom plus a hydrogen molecule, and it is concluded that there are only four bound non-Rydberg states of CH2. The B13–1A1 splitting, unknown experimentally, is predicted to be 7770 cm−1 (0.96 eV). The calculated A11–1B1 splitting is 7860 cm−1 (0.97 eV), compared to the experimental value 7100 cm−1 (0.88 eV). The wave-functions are discussed and three-dimensional plots of the potential surfaces are presented.