STATIONARY-POINTS ON THE S(1) POTENTIAL-ENERGY SURFACE OF C2H2

Recent theoretical developments that facilitate characterization of ex cited-state potential energy surfaces are applied to study five statio nary points on the S-1 surface of C2H2. Although previous calculations have focused on the acetylenic trans and cis forms, the present resea rch predicts that the vinylidene isomer is the global minimum on the p otential surface. However, a high activation barrier calculated for re arrangement to the trans isomer suggests that S-1 vinylidene is not li kely to be formed via photoexcitation of ground-state acetylene. The t rans and cis acetylenic forms of the S-1 state are shown to interconve rt along an in-plane reaction coordinate with an activation energy of similar to 4000 cm(-1), a value which is significantly lower than usua lly assumed in spectroscopic analyses.