The potential surface of X̃ 3B1 methylene (CH2) and the singlet–triplet splitting

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Abstract
The data in the two immediately preceding papers, when combined with the extant microwave, infrared, and photodetachment spectroscopic data, provide 152 rotation and rotation-bending energy level separations in X̃ 3B1 methylene (involving 12CH2, 13CH2, and CD2). In the present paper we fit all this data using the two nonrigid bender Hamiltonians NRB1 and NRB2. The more refined model (NRB2) leads to the following results for triplet methylene: re=1.0766±0.0014 Å, αe=134.037°±0.045°, and the barrier height to linearity=1931±30 cm−1 (the uncertainties are three times the standard errors). Rotation-bending energy levels for CH2, CD2, and CHD are calculated for v2≤4 and N≤6. The determination of the rotation-bending energy levels in CH2 leads to an improved determination of the singlet–triplet splitting T0(ã1A1) in methylene as 3156±5 cm−1 (9.023±0.014 kcal/mol, 0.3913±0.0006 eV). Although the rotation-bending energy levels are accurately predicted it is not possible to predict the stretching frequencies of CH2 very accurately, since the data to hand are not very sensitive to the stretching force field.

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