Singlet and triplet valence excited states of pyrimidine

The absorption spectrum of pyrimidine vapor at 75 °C in the region of the first singlet - triplet transition, encompassing hot bands of the first singlet - singlet transition, has been obtained and analyzed with the aid of extensive ab initio (EOM-CCSD, CASPT2, and CIS) and density functional (B3LYP and TD-B3LYP) vibrational analyses. The hot bands in these spectra give information about low-frequency vibrations, several of which are vibronically active but are not particularly effective at inducing intensity. Spectra obtained at 18 °C are also reported for up to 1100 cm^-1 above the singlet - singlet origin. Several singlet - singlet hot bands have been reassigned, giving excited-state vibrational frequencies for some modes. The calculations provide not only quantitative verification of perceived vibronic coupling and other features of the experimental assignments but also detailed maps of the complex lowest singlet and triplet manifolds. This includes vertical and adiabatic excitation energies, relaxation energies, excess spin densities, and normal-mode vibrational displacement and Duschinsky rotation analyses for up to eight singlet and eight triplet excited states as well as estimates for the structure and energy of some important interconnecting transition states and conical intersections. As a result, revised assignments for the majority of the triplet states are suggested. In addition, the pseudoparity selection rule, which forms the primary model for the (π*, π) spectroscopy of alternate conjugated hydrocarbons, is found not to apply to the ³A₁ manifold. The possibility of symmetry breaking in the (π*, n) states caused by vibronic coupling to a₂ vibrations is considered in detail, as is the possibility of the excited states taking the nonplanar "boat" configuration because of vibronic coupling in b₁ modes. Excited-state chemical properties such as reaction rates and hydrogen bonding are very sensitive to these effects.