Novel hydrogen bonding of a C(sp²) atom in planar tetracoordinate carbon molecules

In planar tetracoordinate carbon (ptC) molecules, the carbon atom is bonded to four other atoms via sp² hybridization and multi-center 2-electron bonding, and thus, the p-orbital has a lone pair of electrons. Hence, the carbon atom in ptC molecules is electron-rich and can act as a hydrogen bond acceptor. This work explores the theoretical investigation of the nature and scope of hydrogen bonding between molecules containing a ptC atom and main-group hydrides. To this end, various CAl₄Mg^-⋯HX (X = N, O, F, P, S, and Cl) dimers were examined using quantum chemical methods. Parameters such as geometry, bond energies, charge distributions, and vibrational redshifts were analyzed to assess the stability of these complexes. The nature of the bond-critical points (BCPs) assessed via the QTAIM analysis confirmed the presence of a non-covalent interaction. The electron density and its Laplacian at the BCP supported the closed-shell character of the interaction, consistent with non-covalent bonding. Natural bond orbital (NBO) analysis indicated stable donor-acceptor interactions between the ptC center and the hydride fragment, supporting the presence of a non-covalent interaction. Non-covalent interaction (NCI) analysis, based on the reduced density gradient (RDG) method, provided visual and topological insights into the nature of the non-covalent interaction.