An ab initio quantum chemical study of vertically excited singlet states of pyrimidine (original) (raw)

Ab imtio calculations of a number of singlet electronic states of pyrirmclme are presented. In addltlon to the ground 'A, state, ten valence excited states were studied: three 'A, and three 'B, states, which are mt* excited states, and two 'B, and two 'AZ states, wluch are nn* excited. Rydberg states were encountered, but since these were irrelevant to the present study, the basis set employed was not designed to describe these states well, and their energy relative to the valence states will be incorrect. Such spurious states may perturb the valence states by accidental near-degeneracy, and care was taken to effectively remove them to prevent such perturbation. The calculations were performed by the CASSCF and MRCI methods, using a modest basis set (C, N: 3s3pld; H: 2s: 116 basis functions) of AN0 type. Energies, dipole and quadrupole moments, transition moments, and oscillator strengths are reported. All calculations were made using the experimental equilibnum geometry, and all excitation data are therefore vertIcal. The MRCI excitation energies, corrected for unlinked clusters, give results in good agreement with known experimental data. The error in the computed excitation energy is m no case larger than 0.4 eV. In some cases, alternative assignments of experimental energies are indicated.