Methane elimination from ionized propane through an ion-neutral complex: An ab initio study

In connection with the major unimolecular decompositions of ionized propane, namely, methane elimination and methyl radical loss, ab initio molecular orbital calculations at the UMP2/6-31G(d) (molecular geometries) and QCISD(T)/6-311+G(2d,2p) (energies) levels of theory have been used to investigate the relevant parts of the C3H8.+ ground-state potential energy surface (PES). The calculations demonstrate that at internal energies about 15 kJ/mol below the thermochemical threshold for methyl radical loss, partial bond dissociation to an ion-neutral complex consisting of the methyl radical coordinated to the H-bridged ethyl cation and methane elimination therefrom both occur. The contours of the C3H8.+ ground-state PES are such that at the threshold for methane elimination the methyl radical partner of the complex is directed to the hydrogen atom it will abstract without being able to move freely around the ethyl cation partner. A higher energy configuration of the ion-neutral complex, lying about 11 kJ/mol below the threshold for simple dissociation, mediates the carbon skeletal rearrangement of the propane radical cation. The observed predominance of the methane losses involving terminal carbon atoms over those involving the central carbon atom are rationalized on the basis that such a degenerate rearrangement precedes the methane elimination.