The Ethylene Dication: A Theoretical Study of the C₂H₄₂+ Potential Energy Surface

The C₂H[formula omitted] potential energy surface was examined by ab initio molecular orbital theory corrected for electron correlation by means of Mφller-Plesset perturbation theory to third order (MP3/6-31G^*) using 6-31G^* (and 3-21G) optimized geometries. The perpendicular (D_2d) ethylene dication, 1 (D_2d), is the global and only singlet C₂H[formula omitted] minimum with an estimated heat of formation of 654 kcal/mol. The rotational transition structure, 2 (D_2h), is 28.1 kcal/mol higher in energy. This rotational barrier is remarkably large for such 14-electron species (compare H₂BBH₂ and H₂B-CH₂+, 10.5 and 20.1 kcal/mol, respectively). The C3v ethylidene dication, 4, 21.3 kcal/mol higher in energy than 1, is probably not a minimum, but may facilitate hydrogen scrambling. Although 1 is unstable thermodynamically toward proton loss (by 16 kcal/mol), the barriers for deprotonation (and homolytic cleavage (into two CH₂+ cations)) are 68.8 and about 88.4 kcal/mol, respectively. The transition structure for cleavage of 4 into CH₃+ and CH+ lies 86.5 kcal/mol above 1 in energy. These large barriers are consistent with the experimental observation of C₂H[formula omitted] in the gas phase.