Theoretical study on S(1)((1)B(3u)) state electronic structure and absorption spectrum of pyrazine

Abstract

Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S(0)((1)A(g))) and the first (S(1)((1)B(3u))) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorption spectrum of S(1)((1)B(3u)) state of pyrazine. The assignment of main vibronic transitions is made for S(1)((1)B(3u)) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode v(6a). For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S(1)((1)B(3u)) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S(1)((1)B(3u)) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode v(10a). The calculated S(1)((1)B(3u)) absorption spectrum is in reasonable agreement with the experimental spectra.