Resonance Stabilization in Allyl Cation, Radical, and Anion

The equilibrium structures, transition states for rotation, and selected distorted structures of the allyl cation (1), radical (2), and anion (3) are calculated at the HF/6-3 lG(d) and MP2/6-3 lG(d) level of theory. The electronic structure of the molecules is investigated using the topological analysis of the electron density distribution and the natural bond orbital partitioning scheme. The detailed analysis of the changes in the electronic structure upon rotation of a methylene group shows clearly that the planar forms of 1, 2, and 3 are strongly stabilized by π resonance. The significantly higher rotational barriers of 1 and 3 than that of 2 is explained by the charge distribution associated with the conjugation in the planar allyl ions. The statement made by Wiberg (J. Am. Chem. Soc. 1990, 112, 61) that the allyl anion has little resonance stabilization is repudiated. The rotation of a methylene group in 1 and 3 needs nearly the same energy when the CH₂ group is kept planar. The (C_2ν) equilibrium structures 1, 2, and 3 need little energy to be distorted toward planar structures with alternating C-C bond lengths. The calculations prove that resonance stabilization is strong in the C_2ν, equilibrium structures and in the bond alternating planar forms. It is clearly shown that the C_2ν forms of 1-3 are enforced by the σ frame; the π energy favors distorted planar structures with one long and one short C-C bond.