Resonance Stabilization in the Allyl Systems CH₂CHXH₂^+/-(X = C, Si, Ge, Sn, Pb)

The equilibrium structures and barriers for rotation around the C-X bond are calculated for the allyl cations and anions CH₂CHXH₂^+/- (X = C, Si, Ge, Sn, Pb) using quantum mechanical ab initio methods. Effective core potentials are employed for the heavy atoms. The allyl cations are predicted with a planar geometry. All allyl cations are stabilized by π conjugative interactions. The strength of the resonance interactions as measured by the rotational barrier decreases from 37.8 kcal/mol (X = C) to 14.1 kcal/mol (X = Si), 12.0 kcal/mol (X = Ge), 7.2 kcal/mol (X = Sn), and 6.1 kcal/mol (X = Pb). The allyl cations are additionally stabilized by σ bonding and through-space charge interactions, which have the same magnitude as the resonance stabilization. The equilibrium geometries of the heavy atom allyl anions have strongly pyramidal XH₂ groups. The planar forms are much higher in energy. The calculations suggest that there is no resonance stabilization in the allyl anions, except in the parent anion CH₂CHCH₂-. The electronic structure of the molecules is investigated using the Laplacian of the electron density distribution.