Alkali Metal Cation versus Proton and Methyl Cation Affinities: Structure and Bonding Mechanism

We have analyzed the structure and bonding of gas-phase Cl−X and [HCl−X]⁺ complexes for X⁺= H⁺, CH₃ ⁺, Li⁺, and Na⁺, using relativistic density functional theory (DFT). We wish to establish a quantitative trend in affinities of the anionic and neutral Lewis bases Cl⁻ and HCl for the various cations. The Cl−X bond becomes longer and weaker along X⁺ = H⁺, CH₃ ⁺, Li⁺, and Na⁺. Our main purpose is to understand the heterolytic bonding mechanism behind the intrinsic (i.e., in the absence of solvent) alkali metal cation affinities (AMCA) and how this compares with and differs from those of the proton affinity (PA) and methyl cation affinity (MCA). Our analyses are based on Kohn–Sham molecular orbital (KS-MO) theory in combination with a quantitative energy decomposition analysis (EDA) that pinpoints the importance of the different features in the bonding mechanism. Orbital overlap appears to play an important role in determining the trend in cation affinities.