Theab initiopotential energy surfaces of some low-lying states of acetylene

Abstract

Excitation energies, fluorescence wave numbers, vibrational frequencies, and ionization potentials of singlet valence states of acetylene were calculated by means of MRBW (multireference Brillouin–Wigner) perturbation, CI (configuration interaction), and GF matrix methods with minimal and extended basis sets. The behavior of the main configurations with respect to the CCH bending angle was elucidated with the π electron approximation. The fluorescence of acetylene recently found with strong Stokes shift was assigned to ¹A_u. The mechanism of the fluorescence in acetylene and propyne was explained with the use of potential energy maps. The vibrational structure of the acetylene cation in the lowest ²Π_u state was well simulated with the potential energy curve and by the harmonic oscillator approximation. The breakdown of the Koopman’s theorem was analyzed by the CI method.