CNDO Molecular-Orbital Theory of Molecular Spectra. I. The Virtual-Orbital Approximation to Excited States

A semiempirical theory of transition energies and molecular excited-state geometries is developed. The excited-state electronic wavefunction is approximated by a single determinant constructed using a virtual orbital obtained from the minimization of the energy of the ground-state electron configuration. This theory represents a semiquantitative study of the accepted qualitative description of molecular spectra. Results are presented for HNO, HCF, H2CO, H2C2, and NH3. The calculated bond angles, with the exception of H2CO, are in good agreement with experiment, whereas the excitation energies are in general found to be rather too high. In all cases the symmetries of the lowest predicted excited states are found to be consistent with the observed spectra. These results indicate that the qualitative description of the observed transitions of these molecules has a quantitative basis.