Anharmonic Franck-Condon simulation of the absorption and fluorescence spectra for the low-lying S1 and S2 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 v(6a), v(1) and v(12). One mode from non-total symmetry v(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. (C) 2012 Elsevier B.V. All rights reserved.