| Abstract
| - Resonance energies of the trimethylenemethane dication (1) and the butadienyl dication (4) wereevaluated using two independent computational methodologies to provide insight into the validityof Y-aromaticity. One methodology employed density functional theory calculations and examinedthe resonance contribution of the CC double bond toward the double hydride abstraction enthalpiesof methylpropene (6) and 2-butene (8), yielding 1 and 4, respectively. These resonance contributionsby the double bond were determined by calculating the double hydride abstraction enthalpies ofboth the parallel and perpendicular conformations of vinylogues of 6 and 8, in which n = 1−4vinyl units were inserted between the central carbon−carbon double bond and each of the reactioncenters. Extrapolation of the resonance contribution in each vinylogue to n = 0 yielded the resonancecontribution in the respective parent molecules. The second methodology employed an orbitaldeletion procedure (ODP), which effectively allowed us to examine the energies of individualresonance structures. The resonance energy of each dication is computed as the difference betweenthe most stable resonance structure and that of the delocalized species. The two methodologies arein agreement, suggesting that the resonance energy of the trimethylenemethane dication issubstantially greater than that of the butadienyl dication. The origin of this difference in resonancestabilization is discussed.
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