A comparative study of the spectroscopic features of the low-lying electronic states of H2F+ and H2Cl+ ions

Abstract

Configuration interaction studies of H2F+ and H2Cl+ ions, using 6–311G (3d, 3p) basis sets, have revealed several interesting differences in their spectral behaviour. Both of them are having bent ground state (1A1) equilibrium geometries, but there is a huge difference (∼1·93 eV) between their energy barrier to linearity. Their first two excited states are found to be linear 3Γu and 1Γu states, correlating to 3B1/3A1 and 1B1/1A1 Renner-Teller pairs, respectively, in the C2v symmetry. Considering only the allowed singlet-singlet transitions at the ground state equilibrium geometry, the lowest energy transitions found to have transition moment values of 0·65 D and 0·48 D for H2F+ and H2Cl+, respectively, appearing in the far UV region. Conical intersections take place during the symmetrical stretching of two H-Cl bonds in the chloronium ion for the first two pairs of excited states (13A2 /13B1 and 11A2 /11B1) in the C2v symmetry. This intersection may initiate pre-dissociation from the upper bound adiabatic 1A″ state to the lower repulsive 1A″ state in the Cs symmetry. Fluoronium ion is expected to dissociate via a single electronic state due to the absence of such intersection.