Ethane cation-radical isomers and their interconversion pathways. Electron shift isomerism in cation radicals

Abstract

An extensive computational exploration of the C₂H₆⁺˙ surface has been performed with electron correlation methods beyond MP2, up to the level of quadratic configuration interaction with single, double and triple substitutions, using the 6–311 G** basis set. Five ground-state species, two excited-state species and five transition states, for interconversions and internal rotations, are located. The results at the highest levels show the existence of three isomers, which in order of decreasing stability are: ²A_g(C_2h)_DB, ²A_1g(D_3d) and ²A″(C_s), where the most stable isomer has a diboranoid (DB) character. Two interconversion pathways are found to link the diboranoid ²A_g(C_2h)_DB isomer to the other two isomers. The lowest energy mechanism appears to be the one linking ²A_g(C_2h)_DB and ²A_1g(D_3d). Thus, the work identifies a low-energy mechanism which funnels the dynamics through the original point-group symmetry, away from the traditional Jahn–Teller pathway. Each event of ²A_1g(D_3d) formation is, in turn, followed by a faster reverse process back to ²A_g(C_2h)_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 low-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 results is in terms of ‘electron-shift isomerism’ in which single electron-shifts amongst different fragments of the atomic skeleton generate both the C₂H₆⁺˙ isomers as well as their intervening transition structures.