Exploring the Role of Consecutive Addition of Nitrogen Atoms on Stability and Reactivity of Hydrogen-Bonded Azine–Water Complexes

[Image: see text] The second-order Møller–Plesset perturbation theory (MP2) and density functional theory with dispersion function calculations have been applied to investigate the hydrogen-bonding interaction between azines and water. The study suggests that the ability of nitrogen present in azine to act as a hydrogen-bond acceptor decreases in the order of pyridine (PY) > diazine (DZ) > triazine (TZ) > tetrazine (TTZ) > pentazine (PZ) > hexazine (HZ). Natural bond orbital (NBO) analysis, atoms in molecules, symmetry-adapted perturbation theory (SAPT), and molecular electrostatic potential studies reflect the factors important for hydrogen-bond strength as well as for the structural, electronic, and vibrational changes occurring during complexation. NBO analysis reflects that upon gradual addition of nitrogen atoms, hyperconjugation leads to an increase in the population of antibonding O–H bond, thus causing elongation and weakening of O–H bond in complexes incorporating N···H–O(W) interaction, whereas rehybridization leads to an increase in the s character of the carbon hybrid orbital in C–H bond, thus causing contraction and shortening of C–H bond in complexes having C–H···O(W) interactions. From the topological analysis, an excellent linear correlation is found to exist between stabilization energy (ΔE(BSSE)), electron density (ρ(c)), and its Laplacian (∇(2)ρ(c)) at the bond critical points.