Ab initio MR-CISD study of gas-phase basicity of formamide in the first excited singlet state

MR-CISD and MR-CISD+Q calculations have been carried out to investigate geometries, energies, and electronic absorption spectra of formamide and its O- and N-protonated forms. The vertical excitation energies for formamide are in good agreement with available experimental data and with the results of best calculations reported so far. Analysis of the calculated electronic absorption spectra reveals that the lowest excited state in the parent molecule and in its N-protonated form corresponds to the n-π* valence excited state, whereas the O-protonated form shows the π-π* excited valence state as the lowest one. The second excited valence states in the neutral molecule and the N-protonated ion are the π-π* state, whereas it is the n-π* state for the O-protonated formamide. Adiabatic excitation energies are reported for the first excited valence state of all three species with structures optimized at the MR-CISD level. All structures exhibit strong deviation from planarity characterized by pyramidalization of the C and the N atoms and rotation of the NH₂ with respect to the plane of the COH group. It appears that oxygen is the most basic site of formamide both in the ground state and in the first singlet excited state. Its calculated gas-phase basicity (GB) and proton affinity (PA) in the latter are smaller than in the ground state by 2.1 kcal mol^-1 and 2.4 kcal mol^-1, respectively. The difference in basicity between oxygen and nitrogen positions drops from 16 kcal mol^-1 (ground state) to 6 kcal mol^-1 (first singlet excited state). These results are analyzed within the Förster thermodynamic cycle.