Theoretical study of the structure and stability of silyl cations with lewis bases using DFT and NBO analysis

This study investigates the intricate interplay between electronic and steric factors in shaping Si-P bond lengths, bond angles, binding energies, charge distribution, and interaction energies in silicon-centered cation-phosphine base systems. A detailed analysis reveals that (CH₃)₃Si⁺ exhibits the shortest Si-P bond lengths (2.01–2.03 Å), attributed to minimal steric hindrance and favorable electronic overlap. In contrast, (C₂H₅)₃Si⁺ displays slightly longer bond lengths (2.03–2.05 Å), while the longest bond lengths are observed in Ar₃Si⁺ (2.08–2.10 Å), due to the combined electron-withdrawing nature and steric effects of aromatic groups. Variations in bond angles further demonstrate the influence of steric bulk and electronic effects, with (C₂H₅)₃P consistently exhibiting larger bond angles (e.g., 120°–121° for Ar₃Si⁺) compared to (CH₃)₃P. Binding energy calculations identify the most favorable interaction (-28 kcal/mol) between (CH₃)₃Si⁺ and (CH₃)₃P, driven by an optimal balance of geometry and charge distribution. Charge transfer analysis indicates that (C₂H₅)₃P donates more electron density than (CH₃)₃P, consistent with trends expected from steric and electronic considerations. Natural Bond Orbital (NBO) analysis confirms a strong negative correlation between σ-donation and interaction energy, with increased σ-donation leading to stronger interaction, which are energies that are more negative. In contrast, Ar₃Si⁺ consistently exhibits weaker interactions due to the electron-withdrawing effects of its aromatic groups. These findings underscore the critical role of steric and electronic factors in defining geometry and interaction strengths, providing valuable insights for designing tailored silicon-phosphine complexes with potential applications in catalysis and material sciences.