The singlet-triplet separation in CF₂: State-of-the-art ab initio calculations and Franck-Condon simulations including anharmonicity

Geometrical parameters, vibrational frequencies and relative electronic energies of the X̃2B1state of CF2-and the X̃1A1and ã3B1states of CF2have been calculated. Core-electron effects on the computed minimum-energy geometries and relative electronic energies have been investigated, and relativistic contributions to the computed relative electronic energies calculated. Potential energy functions of the X̃2B1state of CF2-and the X̃1A1and ã3B1states of CF2have been determined, and anharmonic vibrational wavefunctions of these states calculated variationally. Franck-Condon factors including anharmonicity and Duschinsky rotation have been computed and used to simulate the ã-X̃ emission spectrum of CF2determined by S. Koda [Chem. Phys. Lett. 1978, 55, 353] and the 364 nm laser photodetachment spectrum of CF2-obtained by R. L. Schwartz et al. [J. Phys. Chem. A 1999, 103, 8213]. Comparison between theory and experiment shows that the theoretical approach benchmarked in the present study is able to give highly reliable positions for the CF2(X̃1A1) + e←CF2-(X̃2B1) and CF2(ã3B1) + e←CF2-(X̃2B1) bands in the photoelectron spectrum of CF2-and a reliable singlet-triplet gap for CF2. It is therefore concluded that the same theoretical approach should give reliable simulated CCl2(X̃1A1) + e←CCl2-(X̃2B1) and CCl2(ã3B1) + e←CCl2-(X̃2B1) bands in the photodetachment spectrum of CCl2-and a reliable singlet-triplet gap for CCl2. KGaA.