Electronic excitation and ionization spectra of azabenzenes: Pyridine revisited by the symmetry-adapted cluster configuration interaction method

Electronic excited and ionized states of pyridine were reinvestigated by th e symmetry-adapted cluster configuration interaction (SAC-CI) method using an extended basis set and a wide active space. The present SAC-CI results f or the singlet and triplet excited states are greatly improved and agree we ll with the experimental observations, providing a firm assignment of all l ow-lying n --> pi (*) and pi --> pi* valence excited states observed in the vacuum ultraviolet spectrum and electron energy-loss spectrum. The ionizat ion potentials were reexamined by the SAC-CI general-R (R represents excita tion operator) method. The first four ionization potentials are greatly imp roved compared with our previous results obtained by the SAC-CI single- and double-R (SD-R) method. The present theoretical ionization potentials are in good agreement with the experimental values in high-resolution synchrotr on photoelectron spectrum for energy regions up to 25 eV (which contain out er- and inner-valence regions), and give a detailed theoretical assignment for the photoelectron spectra. (C) 2001 American Institute of Physics.