Theoretical study on S1(1B3u) state electronic structure and absorption spectrum of pyrazine

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(B-1(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(B-1(3u)) state of pyrazine. The assignment of main vibronic transitions is made for S-1(B-1(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(B-1(3u)) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S-1(B-1(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(B-1(3u)) absorption spectrum is in reasonable agreement with the experimental spectra.