Theoretical investigation on the improper hydrogen bond in κ-carrabiose•••Y (Y = HF, HCl, HBr, NH3, H2O, and H2S) complexes

The nature of H-bonds in kappa-carrabiose center dot center dot center dot Y (Y = HF, HCl, HBr, NH3, H2O, and H2S) complexes was studied. For this aim, the structure of isolated kappa-carrabiose was optimized using three global hybrids functional: B3LYP, PBE0, and M06-2X combined with 6-311G** basis set. Subsequently, the kappa-carrabiose in the presence of HF, HCl, HBr, NH3, H2O, and H2S was optimized using the CBS-4 M method. NBO analyses were then carried out at the MP2/6-311G** level of theory. A particular interest was focused on C-(18)-H-(34)center dot center dot center dot Y bond. The results reveal that the C-(18)-H-(34)center dot center dot center dot Y bond is an improper H-bond since a significant contraction of C-(18)-H-(34) was observed during the complexation leading to a significant blueshifted stretching frequency. The NBO analyses have shown that the formation of the improper H-bonds C-(18)-H-(34)center dot center dot center dot Y (Y = F, Cl, Br, N, O, and S) is principally due to the increase of the s-character of the hybrid orbital in carbon atom (rehybridization) in kappa-carrabiose center dot center dot center dot Y complexes. Regarding the polarization, it was proved that more the H-bond center (carbon in C(18)-H(34)center dot center dot center dot Y) becomes less positive, the hydrogen more positive, and Y more negative; more the contraction of the C-(18)-H-(34) bond is important. It was also confirmed for intramolecular H-bonds in kappa-carrabiose center dot center dot center dot Y complexes that the rehybridization is responsible for H-bonds nature either proper or improper.

Automated summary

The study focused on the nature of improper H-bonds in kappa-carrabiose complexes with various molecules, revealing the key role of rehybridization in the formation of these bonds. Analysis showed that the polarization and contraction of specific bonds were crucial factors in determining the nature of these H-bonds.