Configuration Interaction Studies of Ground and Excited States of Polyatomic Molecules II. The Electronic States and Spectrum of Pyrazine

Abstract

Ab initio SCF and CI treatments of the ground state and n → π* and π → π* excited states of pyrazine, C4H4N2, are reported for the molecule in its ground state equilibrium geometry. In the case of n → π* transitions, the two symmetry combinations of nitrogen lone pair orbitals n1 and n2 are found to correspond to the description n+ ≈ n1 + n2 + λσ and n− ≈ n1 − n2 in which the n+ orbital has the higher orbital energy and is significantly more delocalized than n− by interaction with the pyrazine ring sigma system. This result leads to a significant splitting at the CI level of description of 1.4 eV between the B3u and B2g excited states which are derived from the orbital promotions n+ → π* and n− → π*, respectively. Calculated transition energies to the lowest n → π* excited states B3u3 and B3u1 of 3.56 and 4.22 eV, respectively, are in good agreement with experimental values. A low-lying B1u3(π → π*) state for which there exists indirect experimental evidence is found to occur between the B3u3 and B3u1 states. Also, an allowed transition to the B2u1(π → π*) state is calculated at 5.29 eV, slightly above the observed range of absorption. Numerous other excited states are calculated, but of these only the B1u1(π → π*) and B2u1(π → π*) states can be identified with observed transitions and in these cases the calculated transition energies are too high by approximately 2.5 eV.