Configuration interaction studies on the S2 surface of H2CO: 21A′(σ, π*/π, π*) as perturber of 1 1B2(n, 3s)

Abstract

MRD-Cl calculations reveal that the third singlet surface S₂ of H₂CO exhibits two energetically close-lying minima. The more stable state at 7.09 eV is the experimentally known 1 ¹B₂(n, 3s). The second minimum at 7.53 eV corresponds to the pyramidal state 2 ¹A′, of mixed σ, π* and π, π* character near equilibrium. This S₂ valence minimum, which apparently has not been experimentally observed, is suggested to cause the blue shift of the S₂â†� S₀ system of H₂CO in condensed media (as well as in related aldehydes and ketones), ruling out n →σ^*_co(¹B₂) as responsible for this blue shift. The 2¹A′, state is non-adiabatically coupled with the ground state, explaining the observed predissociation of 1¹B₂ into the CH₂+ O continuum. The planar (C_2v) and pyramidal (C_s) minima can be connected only through a C₁ intermediary, requiring participation of the antisymmetric CH-stretch and CH₂ bend vibrations, besides the CO-stretch and out-of-plane modes. A dipole moment µ=+3.45 D (1D ≈ 3.335 64 × 10^–30 C m) with polarity (H₂C)^–O⁺ is predicted for 1¹B₂(n, 3s) at equilibrium, at variance with an experimental µ of +0.33 D. An experimental route for studying the valence region of S₂ is proposed.