Ab initio characterization of the S₁–S₂ conical intersection in pyrazine and calculation of spectra

Potentialâ€�energy functions of the three lowest singlet states of pyrazine have been computed on the selfâ€�consistent field, secondâ€�order Mo/ller–Plesset (SCF/MP2) level as a function of ab initio determined groundâ€�state normal coordinates. The electronicâ€�structure calculations confirm the existence of a conical intersection of the S₁(nÏ€*) and S₂(ππ*) surfaces near the minimum of the latter surface. A vibronicâ€�coupling model involving four spectroscopically active vibrational modes is constructed on the basis of the ab initio data. Absorption, resonanceâ€�Raman, fluorescence, and femtosecond timeâ€�resolved pump–probe spectra are computed for this model using previously developed methods. The results are compared with experimental data where available.