A CASSCF-CI study of the ground and low-lying excited electronic states of C 2H +2

Abstract

Complete-active-space SCF and multi-reference configuration interaction calculations employing large Gaussian basis sets of the general contraction type have been carried out to determine the equilibrium structures and relative stabilities of the ground electronic state X˜ 2 Π u of the acetylene cation C 2 H + 2 and its low-lying excited electronic states, the à 2 Σ + g an as their HCC H dissociation behavior. While the X˜ and B˜ states have linear equilibrium structures, the first excitedA˜state has a trans-bent equilibrium geometry with a bond angle of approximately 112°. The C H bond dissociation energy of the ion and parent neutral ground states were obtained in good agreement with experimental findings, whereas the first ionization potentials of C 2 H 2 and C 2 H derived from the differences between the energies of the ion and the parent neutral were found to be too small by about 0.2 eV. The present results were used to set a theoretical lower limit of 17.30 eV for the appearance energy of the C 2 H + radical ion by dissociative photoionization of C 2 H 2 and to discuss possible reaction pathways for this process.