Weak molecular interactions of 1:1 Nitrile–Lewis base complexes: A theoretical perspective on nature of chemical bonding

Our study focused on understanding how certain Lewis bases like H${}_2$O, NH${}_3$, HCl, and C${}_2$H${}_2$, interact with specific nitriles such as, Glycolonitrile (GLY), Cyanic acid (CYA), and 3-Hydroxy-2-propenenitrile (HPN). These interactions form 1:1 complexes through hydrogen bond. Here we observed weak hydrogen bonding interactions such as, C$\equiv$N$\cdot$ $\cdot$ $\cdot$H, O-H$\cdots$N, O-H$\cdots$O, or O-H$\cdots \pi$ in these complexes. In C$\equiv$N$\cdot$ $\cdot$ $\cdot$H interaction, when a nitrile molecule interacts with a Lewis base, it acts as a proton acceptor, while the corresponding Lewis base acts as a proton donor. However, this is reversed when the molecule possesses an O-H$\cdots$X (X = N, O, Cl, $\pi$) interaction. Among these, in most cases, the complexes with O-H$\cdots$X interactions were the most stable, representing the global minima. Detailed Atom in Molecule (AIM) and Natural Bonding Orbital (NBO) analyses confirmed the presence of these hydrogen bonds. Intermolecular bond critical points (BCPs) were identified. The electron density at BCPs $\rho \left(r_c\right)$ was within the range of 0.002 to 0.035 a.u. as described in the Koch and Popelier study. The Laplacian of electron density ${\nabla }^2\rho$ was positive, further proving hydrogen bonding interaction. NBO analysis showed that O-H$\cdots$N interactions had higher second order perturbation energies compared to O-H$\cdots$O, O-H$\cdots \pi$, and O-H$\cdots$Cl interactions.