Vinylidene-acetylene cation isomerization investigated by large scale ab initio calculations.

The planar isomerization routes of the vinylidene/acetylene cation in the lowest electronic states are accurately examined for the first time, by using large scale MRCI and CCSDT calculations in a complementary way. They are compared with the similar calculations performed for the neutral ground state isomerization. An accurate value of the adiabatic ionization potential of vinylidene (11.26 eV) is predicted. The vinylidene cation lowest state, 1 2A1, follows an almost flat pathway with a shallow secondary minimum on the 1 2A' potential energy surface, before suddenly dropping to the stable acetylene cation ground state, X 2Piu. It is therefore confirmed to be completely unstable with respect to isomerization. The first excited state of the vinylidene cation, 1 2B1, which also correlates with the 2Piu ground state of acetylene cation along a 2A' isomerization route, has been studied at the same level of calculation. This 1 2B1 state is lying only 0.15 eV above the 1 2A1 state, and exhibits a potential energy barrier of 0.55 eV which explains the earlier assignment of this symmetry to the ground state of vinylidene cation. In addition to large scale calculations, a comprehensive description of the important steps of isomerization drawn from a very simple model involving monoconfigurational states is presented. In particular, the behavior of one unique orbital, namely, the 5a1 outer molecular orbital, is shown to completely govern the molecular geometry and energy evolution along the isomerization route of the ground state cation C2H2+.