The potential surface and stretching frequencies of X̃ 3B1 methylene (CH2) determined from experiment using the Morse oscillator-rigid bender internal dynamics Hamiltonian

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Abstract
The Morse oscillator‐rigid bender internal dynamics (MORBID) Hamiltonian [P. Jensen, J. Mol. Spectrosc. 1 2 8, 478 (1988)] has been used in a fitting to all extant rotation–vibration data for X̃ 3B1 methylene CH2. This fitting leads to an improved determination of the potential energy surface, and in particular to reliable predictions for the stretching frequencies. We predict ν1=2992 cm−1 and ν3=3213 cm−1 for 12CH2, and we hope that the new predictions will encourage the experimental search for these weak fundamentals. In the MORBID approach the rotation–vibration energies are obtained from the potential energy surface in a purely variational calculation, and consequently the present work is an improvement over previous determinations of the CH2 potential energy surface from experiment that used the nonrigid bender formalism [see P. R. Bunker et al., J. Chem. Phys. 8 5, 3724 (1986), and references therein]; this latter approach treats the stretching vibrations by second order perturbation theory. A fitting to the J=0 vibrational energy data for ã 1A1 methylene has also been made here using the MORBID Hamiltonian. Combining the results of these MORBID fittings to experimental data for the (X̃) and (ã) states of CH2 we obtain the singlet–triplet splittings T0(ã 1A1)=3147 cm−1 (8.998 kcal/mol) and Te(ã 1A1)=3223 cm−1 (9.215 kcal/mol).

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