The methylene saga continues: Stretching fundamentals and zero-point energy of X˜3B1 CH2

The vibrational fundamentals and the rotational levels up to J  =7 of the X˜3B1 and a˜1A1 electronic states of CH2 have been computed completely ab initio. The calculations were based on converged, variational nuclear motion calculations employing high-quality ab initio quartic force field approximations of the related potential energy surfaces (PES). The vibrational fundamentals obtained are compared to other computational results, namely those obtained from second-order vibrational perturbation theory (VPT2) and the nonrigid-rotation-large-amplitude-internal-motion Hamiltonian (NRLH) approach. The variationally computed rotational transitions are compared both to experimentally available results and to results obtained using empirical, fitted PESs. The comparisons suggest that while the fitted PESs of X˜3B1 CH2 reproduce excellently the available rovibrational transition wavenumbers, the corresponding stretching fundamental term values, which have not been determined experimentally, are less accurate than the ab initio values obtained in the present study. This means that the zero-point energy (ZPE) computed ab initio in the present work is an improvement over that computed from the fitted PES of X˜3B1 CH2. No similar problems are observed for the semirigid a˜1A1 state of CH2, where the computed, the fitted, and the experimental results all agree with each other. The symmetric and antisymmetric stretching fundamentals of X˜3B1 CH2 obtained in this study are 3035±7 and 3249±7 cm−1, respectively. The corresponding ZPE of X˜3B1 is 3733±10 cm−1, while that of a˜1A1 CH2 is 3605±15 cm−1.