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

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_e^"CBS" for dissociation of the complexes into separate components were found to be directly proportional to the intermolecular stretching force constant kσ The value of D_e^"CBS" could be partitioned into a nucleophilicity of B and an electrophilicity of LiR or NaR, with the order ELiH ≳ ELiF = ELiCH3 for the LiR and ENaF > ENaH ≈ ENaCH3 for the NaR. For a given B, the order of the electrophilicities is ELiR > ENaR , 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 De than do the hydrogen- and halogen-bonded counterparts BHCl and BClF.