ABINITIO CHARACTERIZATION OF THE S1-S2 CONICAL INTERSECTION IN PYRAZINE AND CALCULATION OF SPECTRA

Potential-energy functions of the three lowest singlet states of pyrazine have been computed on the self-consistent field, second-order Moller-Plesset (SCF/MP2) level as a function of ab initio determined ground-state normal coordinates. The electronic-structure calculations confirm the existence of a conical intersection of the S1 (n-pi*) and S2 (pi-pi*) surfaces near the minimum of the latter surface. A vibronic-coupling model involving four spectroscopically active vibrational modes is constructed on the basis of the ab initio data. Absorption, resonance-Raman, fluorescence, and femtosecond time-resolved pump-probe spectra are computed for this model using previously developed methods. The results are compared with experimental data where available.