Abstract
New ab initio potential energy surfaces (PESs) for the two lowest-lying singlet 11A′ and 11A′′ electronic states of CH2, coupled by the Renner-Teller (RT) effect and meant for the spectroscopic study, are presented. The surfaces are constructed using a dual-level strategy. The internally contracted multireference configuration interaction calculations with the Davidson correction, using the aug-cc-pVQZ basis set, are employed to obtain 3042 points at the lower level. The core and core-valence correlation effects are taken into account in the ab initio calculations with a modified optimized aug-cc-pCVQZ basis set for the higher-level points. The analytical representations of these PESs, with the inclusion of the nonadiabatic RT terms, are obtained by the nonlinear least-squares fit of the calculated points to three-body expansion. Quantum dynamical calculations are performed on these PESs, and the computed vibronic energy levels for the two singlet electronic states are in excellent agreement with experiment.
Highlights
The CH2 biradical has been the subject of many theoretical and experimental studies, due to its distinct electronic characteristics and chemical and physical properties
Our results are in slightly better agreement with the experimental values than the results of Gu et al [19], and it should be noted that the semiempirical potential energy surfaces (PESs) used by Gu et al were adjusted according to the experimental values while ours are fully ab initio ones
We report fully ab initio PESs for the RT coupled 11A and 11A states of CH2 suitable for the spectroscopic study, based on the internally contracted multireference configuration interaction (icMRCI)+Q method using the AVQZ and a kind of optimized ACVQZ basis sets
Summary
The CH2 biradical has been the subject of many theoretical and experimental studies, due to its distinct electronic characteristics and chemical and physical properties. The two lowest-lying singlet electronic states of CH2, which become a degenerate Δ pair at linearity, interact strongly with each other and are coupled dynamically [17, 24]. Bussery-Honvault and coworkers [27] computed an ab initio global PES for the first singlet state of CH2, where a mixed numerical and analytical method was employed in the PES construction This surface shows no barrier for the C2V insertion, while a barrier of 4319 cm−1 (12.35 kcal/mol) is present for the collinear approach. Appropriate active space and an optimized basis set with additional functions for describing core and corevalence correlation effects (called CV) is employed in our ab initio calculations, which can guarantee that the two lowest-lying singlet PESs are degenerate at linearity.
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