Ab Initio Study of 2-Silaallyl Cation and Anion. Some Attributes of Silicon-Containing π-Conjugated Systems

2-Silaallyl cation and anion, closed-shell ¹A₁ states, have been examined by using ab initio single determinantal restricted Hartree-Fock wave functions. Integrated projected electron populations (IPP) indicate significant polarization of both σ and π SiC bonds. The anion shows greater charge alternation (Si, +2.6; CH₂, -1.4) than the cation (Si, +2.6; CH₂, -0.4). Such charge alternation dominates allylic resonance conjugation. The cation is more susceptible to breaking the allylic C_2ν symmetry as indicated by the smaller diagonal force constant for the SiC bond (0.28 au) than for the anion (0.33 au). The cation possesses a low-lying triplet state (UMP3/6-31G//6-31G, 1.5 kcal mol^-1 above the MP3/6-31G//6-31G ¹A₁ singlet after ZPVE correction at the HF/3-21G level). The rotational barrier for the cation (25.6 kcal mol^-1) is larger than the barrier for the anion (15.5 kcal mol^-1) and is reflected in the mininum IPPs along the SiC bond axis for the bonding allylic π-MO. The remarkable CSiC angles, 107.0° for the cation and 135.3° for the anion, are explained in terms of 1, 3-HOMO interactions and Coulomb repulsion between the terminal methylene groups.