The Generalized Valence Bond Description of the Low-Lying States of Formaldehyde

The ground and valence excited states (n → π * and π → π *) were studied using ab initio generalized valence bond (GVB) and configuration interaction (GVB-CI) wave functions. The character and properties of the states are analyzed in terms of the GVB wave functions. The calculated adiabatic excitation energies to the ³A″(n → π *) and ¹A″(n → π *) states are 3.03 and 3.42 eV, in excellent agreement with the experimental values of 3.12 and 3.50 eV. The calculated vertical excitation energies of 3.62 and 4.09 eV, respectively, are in excellent agreement with the values from recent electron impact experiments (3.5 and 4.1 eV, respectively). With the best basis (double ζ plus d functions) the calculated dipole moment is 2.36 D (the double ζ basis leads to 2.59 D), in excellent agreement with the experimental value (2.32 D). For the double ζ basis the ¹A″ and ³A″ states are found to have dipole moments of 1.71 and 1.59 D in reasonable agreement with the experimental values of 1.56 and 1.29 D, respectively. The ³A₁ (π → π *) state is found to have a vertical excitation energy of 5.95 eV, in excellent agreement with the result (5.9 eV) from recent electron impact experiments. The singlet π → π * state is found to involve a diffuse π* orbital and lies at 10.77 eV. This transition is not identified in the experimental studies; however, a broad peak in high energy electron impact spectra is observed around 10.5 eV.