Ab initio studies of excited electronic state S2 of pyrazine and Franck-Condon simulation of its absorption spectrum

The equilibrium geometry and 24 vibrational-normal-mode frequencies of the excited state S-2(B-1(2u))of pyrazine are calculated and characterized using the complete active space self-consistent field method in the adiabatic representation. The displaced harmonic oscillator approximation is used to simulate the absorption spectrum of the S-2(B-1(2u)) state along with the Franck-Condon approximation. It is found that the totally symmetric mode v(1) plays the most important role and this exactly agrees with the experimental observations. The simulated absorption spectrum agrees well with those experimentally observed. This indicates that the present S-2(B-1(2u)) state calculated in the adiabatic representation effectively includes contribution from the diabatic vibronic coupling through the conical intersection. (C) 2009 Elsevier B.V. All rights reserved.