Ab Initio Calculations of the Excited States of Formamide

The excited states of formamide in the gas phase have been calculated using multireference configuration interaction methods. The nπ* transition is calculated to be at 5.85 eV, and the ππ* transition is placed at 7.94 eV. The corresponding experimental energies are 5.65 and 7.32 eV, respectively. In addition, a number of states with significant intensity are calculated to occur in the region of the experimental spectrum designated as the V 1 band. The experimental ππ* transition dipole moment, 3.71 D, is estimated by assuming that the intensity of the V 1 band is due solely to the ππ* transition. The calculated ππ* transition dipole moment is 3.70 D, if only one unoccupied a‘‘ orbital is included in the active space. If the number of unoccupied a‘‘ orbitals in the active space is increased, the calculated ππ* transition moment drops to 2.40 D, and another transition, the π3p π, appears at 7.40 eV, but with only low to moderate intensity. The calculated orientation of the ππ* transition dipole moment agrees with the experimental data, lying along the axis defined by the N and O atoms.