Theory and experiment in accord: the vinylidene radical cation

Using ab initio calculations based on electronic structure theory, the activation barrier of the ²B₁ ground state of the vinylidene cation radical 3 for rearrangement to the ²Π_u acetylene ground state ion 2 is predicted to be 9.4 kcal mol^-1 [MP4/6-311G(d,p)//HF/6-31G(d)]. At the same level of theory, the ²A₁ excited state of H₂CC^{+{radical dot}} 4 is calculated to rearrange without a barrier to 2. In contrast to the doublet states, the activation barrier for hydrogen migration of the lowest-lying ⁴A₂ quartet state of H₂CC^{+{radical dot}} 5 is predicted to be very high (57.2 kcal mol^-1). It is concluded that the experimentally observed long-living vinylidene cation radical is probably 5.