Nature of the Lithium Bond in H₃N···LiHal Complexes (Hal = F, Cl, Br) from Quantum Chemical Calculations

Properties of lithium- and hydrogen-bonded complexes formed by ammonia molecules, lithium halides (LiHal, A-complexes), and hydrogen halides (ННal, B-complexes) are aligned using quantum chemical MP2/aug-cc-pVTZ calculations. NBO analysis shows energy E(2) of inter-orbital interaction between the monomers grows upon the transition to heavier and less electronegative halogen, along with an increase in the contribution from the p-orbital to the hybrid orbital of lithium atom in A-complexes and the hybrid orbital of the halogen atom in B-complexes. The calculated value of E(2) correlates to the elongation of covalent Li−Hal and Н−Hal bonds as the complex forms. Analytical investigation of the topology of electron density predicts noteworthily higher values of the electron and potential energy densities at the critical point of intermolecular contact in B-complexes, relative to A-complexes, and a growing mutual penetration of atoms that form the intermolecular contact. The higher thermodynamic stability of lithium-bonded complexes could be due to the stronger positive electrostatic potential on the lithium atoms in molecules of lithium halides and the weaker exchange repulsion of the monomers that form an A-complex.