Assessment of intermolecular interactions at three sites of the arylalkyne in phenylacetylene-containing lithium-bonded complexes: ab initio and QTAIM studies

The intermolecular interactions existing at three different sites between phenylacetylene and LiX (X = OH, NH(2) , F, Cl, Br, CN, NC) have been investigated by means of second-order Møller-Plesset perturbation theory (MP2) calculations and quantum theory of "atoms in molecules" (QTAIM) studies. At each site, the lithium-bonding interactions with electron-withdrawing groups (-F, -Cl, -Br, -CN, -NC) were found to be stronger than those with electron-donating groups (-OH and -NH(2)). Molecular graphs of C(6)H(5)C≡CH···LiF and πC(6)H(5)C≡CH···LiF show the same connectional positions, and the electron densities at the lithium bond critical points (BCPs) of the πC(6)H(5)C≡CH···LiF complexes are distinctly higher than those of the σC(6)H(5)C≡CH···LiF complexes, indicating that the intermolecular interactions in the C(6)H(5)C≡CH···LiX complexes can be mainly attributed to the π-type interaction. QTAIM studies have shown that these lithium-bond interactions display the characteristics of "closed-shell" noncovalent interactions, and the molecular formation density difference indicates that electron transfer plays an important role in the formation of the lithium bond. For each site, linear relationships have been found between the topological properties at the BCP (the electron density ρ(b), its Laplacian ∇(2)ρ(b), and the eigenvalue λ(3) of the Hessian matrix) and the lithium bond length d(Li-bond). The shorter the lithium bond length d(Li-bond), the larger ρ(b), and the stronger the π···Li bond. The shorter d(Li-bond), the larger ∇(2)ρ(b), and the greater the electrostatic character of the π···Li bond.