Insights Into the Nature of Bifurcated Intermolecular Hydrogen-Bonds in Symmetrical Complexes of 1,8-Naphthyridine-4,5(1H,8H)-Dione With Small Molecules (H2O, H2S, Furan, Thiofuran, and Pyridine).

We present a quantum chemical investigation into the nature of bifurcated intermolecular hydrogen-bonds (HBs) in symmetrical complexes of 1,8-naphthyridine-4,5(1H,8H)-dione (ND) with small molecules (ND:H2O, ND:H2S, ND:Furan, ND:Thiofuran, and ND:Pyridine). The natural bond orbital (NBO) analysis accounts for the formation of redshifting HBs (X-H⋅⋅⋅Y) in the complexes by hyperconjugative charge transfer from the filled lone-pair orbital of atom-Y to the antibonding σ ∗ ( X - H ) $\sigma ^{*}(\mathrm{X}-\mathrm{H})$ orbital. It is demonstrated that hyperconjugation outweighs the rehybridization effect underlying the formation of the HBs, which is in parity with the Bent rule. The quantum theory of atoms-in-molecules (QTAIM) is employed to rationalize the nature of the HBs. From the topological properties at the bond critical points (bcps) along the bond paths for the HBs, the nature of the HBs is identified as predominantly closed-shell (electrostatic) interaction in all the studied complexes. The noncovalent interaction (NCI) analysis reveals that the HB energies are relatively weak in the sense that they typically fall in the range of van der Waals interactions. The observed nonhydrogen bonding O⋅⋅⋅O NCIs in ND also fall in the range of van der Waals type of interactions, and are even stronger than the HB interactions for ND:H2S and ND:Thiofuran complexes.