The Generalized Valence Bond Description of the Valence States of Formamide

The wave functions for the valence states of formamide (NH₂CHO) are reported 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 vertical excitation energies to the ³A″(n → π *), ¹A″(n → π *), and ³A″(π → π *) states are 5.39, 5.65, and 6.19 eV, respectively. The only experimental result known is the ¹A″ absorption which peaks at 5.65 eV, in excellent agreement with our results. The calculated ground state dipole moment is 3.79 D, in good agreement with the experimental value of 3.71 D. Calculated dipole moments of the ³A′,A″, and ³A″ states are 3.29, 2.19, and 2.05 D, respectively. The calculated barrier to rotation about the C-N bond is 18.2 kcal, in good agreement with the experimental liquid phase values (16-20 kcal).