An ab initio investigation of alkali–metal non-covalent bonds B⋯LiR and B⋯NaR (R = F, H or CH3) formed with simple Lewis bases B: the relative inductive effects of F, H and CH3

Abstract

The alkali–metal bonds formed by simple molecules LiR and NaR (R = F, H or CH₃) with each of the six Lewis bases B = OC, HCN, H₂O, H₃N, H₂S and H₃P were investigated by ab initio calculations at the CCSD(T)/AVTZ and CCSD(T)/awCVTZ levels of theory with the aim of characterising this type of non-covalent interaction. In some complexes, two minima were discovered, especially for those involving the NaR. The higher-energy minimum (referred to as Type I) for a given B was found to have geometry that is isomorphous with that of the corresponding hydrogen-bonded analogue B⋯HF. The lower-energy minimum (when two were present) showed evidence of a significant secondary interaction of R with the main electrophilic region of B (Type II complexes). Energies D^CBS_e for dissociation of the complexes into separate components were found to be directly proportional to the intermolecular stretching force constant k_σ The value of D^CBS_e could be partitioned into a nucleophilicity of B and an electrophilicity of LiR or NaR, with the order E_LiH ≳ E_LiF = E_LiCH3 for the LiR and E_NaF > E_NaH ≈ E_NaCH3 for the NaR. For a given B, the order of the electrophilicities is E_LiR > E_NaR, which presumably reflects the fact that Li⁺ is smaller than Na⁺ and can approach the Lewis base more closely. A SAPT analysis revealed that the complexes B⋯LiR and B⋯NaR have larger electrostatic contributions to D_e than do the hydrogen- and halogen-bonded counterparts B⋯HCl and B⋯ClF.