ETHANE CATION-RADICAL ISOMERS AND THEIR INTERCONVERSION PATHWAYS - ELECTRON SHIFT ISOMERISM IN CATION RADICALS

An extensive computational exploration of the C2H6+. surface has been performed with electron correlation methods beyond MP2, up to the leve l of quadratic configuration interaction with single, double and tripl e substitutions, using the 6-311G* basis set. Five ground-state speci es, two excited-state species and five transition states, for intercon versions and internal rotation, are located. The results at the highes t levels show the existence of three isomers, which in order of decrea sing stability are: 2A(g) (C2h)DB, 2A1g (D3d) and 2A'' (C(s)), where t he most stable isomer has a diboranoid (DB) character. Two interconver sion pathways are found to link the diboranoid 2A(g) (C2h)DB isomer to the other two isomers. The lowest energy mechanism appears to be the one linking 2A(g) (C2h)DB and 2A1g (D3d). Thus, the work identifies a low-energy mechanism which funnels the dynamics through the original p oint-group symmetry, away from the traditional Jahn-Teller pathway. Ea ch event of 2A1g (D3d) formation is, in turn, followed by a faster rev erse process back to 2A(g) (C2h)DB which results in scrambling of the hydrogens in the bridging positions. These results offer the basis for an interpretation of the observed EPR spectrum (reference 8) in the l ow-as well as the high-temperature studies. A qualitative analysis of the origins of the various isomers and their interconversion pathways is presented. It is shown that a useful way of understanding the resul ts is in terms of 'electron-shift isomerism' in which single electron- shifts amongst different fragments of the atomic skeleton generate bot h the C2H6+. isomers as well as their intervening transition structure s.