Anharmonic Franck-Condon simulation of the absorption and fluorescence spectra for the low-lying S ₁ and S ₂ excited states of pyrimidine

Intensities and profiles of vibronic spectra of the low-lying singlet excited states were investigated with anharmonic and harmonic Franck-Condon simulations for pyrimidine. The first-order anharmonic correction shows dynamic shift of spectra that is exactly same as difference of reorganization energy between ground and excited states. The first-order correction show intensity enhancement of absorption and intensity weakening of fluorescence for S ₁ state, and dynamic shift is also significant. On the other hand, the first-order correction is negligible for S ₂ state. The main spectral progressions are well described by totally symmetry modes ν _6a, ν ₁ and ν ₁₂. One mode from non-total symmetry ν _16a contributes to the weak band at 16a ² transition for S ₁ state. Four ab initio methods were employed in simulation; CASSCF, CASPT2, DFT and TD-DFT, and coupled-cluster singles-doubles (CCSD) and the equation-of-motion (EOM-CCSD) methods. They all work well, but CASSCF method show the best agreement with experiment for the weak-band intensities.