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

Abstract

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.