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.
- Absorption and fluorescence spectra
- Franck-Condon factors
- Harmonic and anharmonic simulation
- Reorganization energy
- Vibronic spectra