Electronic structure of pyrazine: a study of the geometries in the lowest excited singlet π *← n and π*←π states

Electronic, excitation energies, charge distributions and geometries of pyrazine in the lowest excited singlet π^*←π and π^*←n states have been studied by the VE—PPP, CNDO/2 and CNDO/s-CI molecular orbital methods. Study of the change of geometry in the π^*←n excitation requires localization of the density matrices in the ground and excited states, and with the help of these σ-bond orders are defined. Charge distributions and bond orders in the lowest excited singlet π^*←π and π^*←n states are compared. Whereas in the lowest singlet π*←π excitation the pyrazine ring expands uniformly, in the case of the π^*←n excitation C-C bonds contract whereas C-N bonds elongate. The predictions of theory are in agreement with experimental results, showing that the method used can be employed to obtain reliably the trends of geometry changes following a π^*←π excitation of a molecule before a more complete theoretical or experimental study is performed.