The electronic structure of pyrazine. Configuration interaction calculations using an extended basis

Abstract

Extensive ab initio double zeta basis set configuration interaction calculations have been carried out on the nπ* and ππ* states of pyrazine as well as on the low-lying n and π cations. The calculations corroborate the validity of the valence bond (VB) model for the interaction of lone pair excitations proposed earlier by Wadt and Goddard. Good agreement (errors of ∼0.2 eV) with experiment is obtained (except for the higher-lying 1ππ* states that possess significant ionic character). The calculations indicate that the order of increasing ionization potentials is 2Ag(n), 2B1g(π), 2B1u(n), and 2B2g(π). The forbidden 1 1B2g(nπ*) state is predicted to be 1.30 eV above the allowed 1 1B3u(nπ*) state. Finally, the calculations indicate that the adiabatic excitation energies to the 1 3B1(nπ*) and 1 3A1(ππ*) states in pyridine should be nearly degenerate at ∼3.6 eV.