An a b i n i t i o calculation of ν1 and ν3 for triplet methylene (X̃ 3B1 CH2) and the determination of the vibrationless singlet–triplet splitting T e(ã 1A1)

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Abstract
Ab initio values of ν1, ν3 and the zero point energy for triplet methylene (X̃ 3B1 CH2) have been calculated. Complete second order configuration interaction calculations for the six valence electrons, with a Davidson‐type unlinked cluster correction, were done at 37 selected nuclear geometries. We have fitted an analytic potential through these points and used the nonrigid bender Hamiltonian, and a variational procedure, to calculate the rotation–vibration energies. We have obtained ν1=2985±20 cm−1, ν3=3205±20 cm−1, and the zero point energy as 3710±20 cm−1. The uncertainties represent our best estimate of the full uncertainties in the calculations. From the experimental data for singlet methylene (ã 1A1) we have determined the zero point energy of that state to be 3620±20 cm−1. Combining these two zero point energies with the experimental value of T0(ã 1A1)=3156±5 cm−1 [Bunker et al. J. Chem. Phys. 8 5, 3724 (1986)] we obtain the vibrationless singlet–triplet splitting Te(ã 1A1) =3246±30 cm−1 (9.28±0.1 kcal/mol).

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