THEORETICAL INVESTIGATION OF THE ETHENE ETHENE RADICAL-CATION ADDITION-REACTION

The addition reaction of an ethene radical cation and a neutral ethene molecule, as previously observed in low-temperature matrix isolation ESR measurements, has been studied using a number of computational methods, ranging from the semiempirical AMI and PM3 methods to ab initio UHF, MP2, and MP4 calculations using different basis sets, with and without spin projection included. An intermediate addition complex is observed as a local minimum on the potential energy surface of the reaction. The transition state between this local minimum and the final product, the 1-butene radical cation, is identified as having a hydrogen partly transferred between the carbons in the 2- and 4-positions, the activation energy calculated at the PMP4/6-31G**//MP2/6-31G** level being 6 kcal/mol. Spin projection is found to be important for the addition complex, which has one very long (1.9 angstrom) carbon-carbon bond, but not at the other stationary points of the potential energy surface.