Theoretical investigation on the improper hydrogen bond in κ-carrabiose⋯Y (Y = HF, HCl, HBr, NH, HO, and HS) complexes.

The nature of H-bonds in κ-carrabiose⋯Y (Y = HF, HCl, HBr, NH, HO, and HS) complexes was studied. For this aim, the structure of isolated κ-carrabiose was optimized using three global hybrids functional: B3LYP, PBE0, and M06-2X combined with 6-311G** basis set. Subsequently, the κ-carrabiose in the presence of HF, HCl, HBr, NH, HO, and HS 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-H⋯Y bond. The results reveal that the C-H⋯Y bond is an improper H-bond since a significant contraction of C-H 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-H⋯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 κ-carrabiose⋯Y complexes. Regarding the polarization, it was proved that more the H-bond center (carbon in C-H⋯Y) becomes less positive, the hydrogen more positive, and Y more negative; more the contraction of the C-H bond is important. It was also confirmed for intramolecular H-bonds in κ-carrabiose⋯Y complexes that the rehybridization is responsible for H-bonds nature either proper or improper.