On the 1A1-3B1 separation in CH2 and SiH2

We have determined the ¹A₁-³B₁ separation (T_e) in both CH₂ and SiH₂ using very large Gaussian basis sets (including g functions) and second-order CI wave functions. Complete geometry optimizations have been performed, and relativistic effects have been included using first-order perturbation theory. This treatment yields T_e values for the ¹A₁-³B₁ separation of 9.07 kcal/mol in CH₂ and -20.58 kcal/mol in SiH₂. Using a combination of theoretical and experimental values to estimate the contribution of zero-point vibration to the separation yields T₀ values of 8.9 kcal/mol for CH₂ and -20.9 kcal/mol for SiH₂, in excellent agreement with the experimental values of 9.02 and -21.0 kcal/mol. A corollary to the small zero-point vibrational contribution to the separation is that the symmetric stretching fundamental in CH₂(³B₁) must be near 3100 cm^-1, much less than a recently suggested value of around 3400 cm^-1. Our accurate T_e value for SiH₂ establishes the ionization potential of the ¹A₁ state as 9.15 eV, the higher of two recent experimental values.