The C₄H₆^•+ potential energy surface. 1. The ring-opening reaction of cyclobutene radical cation and related rearrangements

Abstract

Ab initio MO and density functional calculations indicate that the ring opening of the cyclobutene radical cation (CB^•+) follows two competitive pathways, whose energy barriers differ by less than 1 kcal/mol at the highest level of theory employed, RCCSD(T)/cc-pVTZ//UQCISD/6-31G*. The first corresponds to a conrotatory rearrangement to the cis-butadiene radical cation (cis-BD^•+). The second one leads to trans-BD^•+ via a very flat potential energy plateau which comprises cyclopropylcarbinyl-type structures of the type proposed some time ago by Bauld, but the controtatory stereochemistry is preserved also along this process. State correlation diagrams indicate that the rearrangement leading to trans-BD^•+ may occur adiabatically along a C₂ reaction coordinate. Despite this, the transition state has no symmetry. This seemingly “unnecessary” loss of symmetry is traced back to the proximity of the ²A and ²B surfaces in the vicinity of the C₂ stationary points, where the two states encounter a strong vibronic interaction which leads to breaking of the C₂ symmetry. These vibronic interactions are also responsible for the general flattening of the potential energy surface in this area.