Ab initio calculations using the improved virtual orbital formalism
are reported for acetylene, vinylidene, and the vinyl cation,
C2H,+, in classical and bridged geometries. Electronic transition
energies and equilibrium geometries for ground and lower lying
electronically excited states have been calculated. A modified
Walsh diagram for acetylene and simple molecular orbital considerations
explain excited state structures and energy orderings.
While acetylene in the ground state is much more stable than
vinylidene, the energies of several corresponding excited states are
comparable. The stabilities of bridged and classical structures of
the vinyl cation are very similar in the ground state, but in the
various excited states either strcture can predominate. The proton
affinity of acetylene in the ground state should be appreciably
lower than in excited states
