METHANE ELIMINATION FROM IONIZED PROPANE THROUGH AN ION-NEUTRAL COMPLEX - AN AB-INITIO STUDY

In connection with the major unimolecular decompositions of ionized pr opane, namely, methane elimination and methyl radical loss, ab initio molecular orbital calculations at the UMP2/6-31G(d) (molecular geometr ies) 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 pot ential energy surface (PES). The calculations demonstrate that at inte rnal energies about 15 kJ/mol below the thermochemical threshold for m ethyl radical loss, partial bond dissociation to an ion-neutral comple x consisting of the methyl radical coordinated to the H-bridged ethyl cation and methane elimination therefrom both occur. The contours of t he C3H8.+ ground-state PES are such that at the threshold for methane elimination the methyl radical partner of the complex is directed to t he hydrogen atom it will abstract without being able to move freely ar ound the ethyl cation partner. A higher energy configuration of the io n-neutral complex, lying about 11 kJ/mol below the threshold for simpl e dissociation, mediates the carbon skeletal rearrangement of the prop ane radical cation. The observed predominance of the methane losses in volving terminal carbon atoms over those involving the central carbon atom are rationalized on the basis that such a degenerate rearrangemen t precedes the methane elimination.