The vinylidene-acetylene rearrangement. A phantom minimum on the MP2 potential energy surface

Abstract

At the MP2 level of theory there exists a third minimum on the C 2H 2 potential energy surface in addition to acetylene and vinylidene. This new hydrogen-bridged structure, however, is only an artifact of the MP2 procedure; it disappears at higher levels of theory. High-level calculations (QCISD(T)/6-311++G(3df,2pd)//QCISD(T)/6-311G (d,p)) predict a classical barrier for the isomerization of vinylidene to acetylene of 11.0 kJ mol −1 and an energy difference between vinylidene and acetylene of 185 kJ mol −1. After the approximate incorporation of zero-point vibrational energy vibrational energy corrections, these energies become 3 and 177 kJ mol −1, respectively.