A case study of antiaromaticity: Carbomethoxy cyclopropenyl anion

The simplest ideas of antiaromaticity refer to regular monocyclic systems and the eigenfunctions of the Hückel Hamiltonian for 4n π electrons in such systems. The antiaromaticity is expressed in the energy penalty for such idealized systems relative to the Hückel energy for 2n noninteracting π pairs. Observed systems seldom achieve the regular planar geometry assumed in this picture, owing to their ability to ease the antiaromaticity penalty by departures from the regular geometry and also by export of the 4n π electrons’ charge to substituents. In this report we estimate numerical values for the stabilization derived from such departures from the structure and the charge distribution of the idealized antiaromatic cyclopropenyl anion for a specific case, 3-dehydro-3-methyl carboxylate cyclopropenyl anion 1(–) using the thermochemical scheme CBSQB3 supplemented by CCSD(T) calculations.
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A case study of antiaromaticity: Carbomethoxy cyclopropenyl anion