An a b i n i t i o determination of the potential-energy surfaces and rotation–vibration energy levels of methylene in the lowest triplet and singlet states and the singlet–triplet splitting

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Abstract
The potential‐energy surfaces and rotation–vibration energy levels of the ground (X̃ 3B1) and first excited (ã 1A1) electronic states of the methylene radical have been determined by purely ab initio means. The potential‐energy surfaces were determined by multireference configuration interaction calculations, using a full‐valence complete‐active‐space reference space, with an atomic‐natural‐orbital basis set of size [5s4p3d2f1g/3s2p1d]. The configuration interaction (CI) calculations were carried out at 45 points on the triplet surface and 24 points on the singlet surface. The Morse oscillator rigid bender internal dynamics (MORBID) procedure was used to calculate vibrational and rotational energy levels for 12CH2, 12CD2, 13CH2, and 12CHD. Also calculated were the zero‐point vibrational energies, the singlet–triplet splitting, and the dissociation energy. The zero‐point energy of 12CH2 is found to be 127 cm−1 (0.363 kcal/mol) greater in the triplet state than in the singlet. The singlet–triplet splitting in 12CH2 is computed as T0=3116 cm−1 (8.909 kcal/mol), compared with the experimentally derived value of 3156±5 cm−1 (9.024±0.014 kcal/mol). The dissociation energy of the ground state is obtained as D0=179.06 kcal/mol, compared to an experimental value of 179.2±0.8 kcal/mol. The fundamental frequencies for the triplet state are obtained as ν1=3015, ν2=974, and ν3=3236 cm−1 (the experimental value of ν2 is 963.10 cm−1). The corresponding values for the singlet (experimental values in parentheses) are ν1=2787 (2806), ν2=1351 (1353), and ν3=2839 (2865) cm−1.