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 C2ν 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry