THE C4H8.-ENERGY SURFACE .2. THE (C2H4)2.+ COMPLEX CATION AND ITS REACTION TO THE RADICAL CATIONS OF CYCLOBUTANE AND 1-BUTENE( POTENTIAL)

The reaction of ethylene (Et) and its radical cation (Et.+), which has received much attention from the experimental side, was studied at th e QCISD(T)/6-31G//UMP2/6-31G* level of theory. According to these cal culations the primary product of this reaction is a pi complex cation [Et...Et].+, where the two Et molecules are linked by a single bond of almost-equal-to 1.9 angstrom and which is bound by 18.2 kcal/mol rela tive to Et + Et.+, in good agreement with experiment. Starting from th is complex, two transition states were located, one leading to the rad ical cation of cyclobutane (E(a) = 9.0 kcal/mol) and the other involvi ng a 1,3 H-shift leading to the radical cation of 1-butene (E(a) = 5.9 kcal/mol). No equilibrium structure corresponding to the frequently p ostulated tightly bound tetramethylene radical cation intermediate cou ld be found, and therefore no such species seems to be involved in the well-documented rearrangements of [Et...Et].+ and/or ionized cyclobut ane to butene radical cations. An intriguing discrepancy between the m inimum-energy structures of the [Et...Et].+ complex on the UHF and the UMP2 potential energy surfaces is discussed in terms of a bias introd uced by the poor convergence of the Moller-Plesset perturbation series for species whose UHF wave function is strongly contaminated by highe r spin states. A revised picture of the potential energy surfaces on w hich the various C4H4.+ rearrangements which were observed both in the gas phase and in condensed phase is proposed.