LAUSR

Abstract The blue-shifting intramolecular C H⋯π hydrogen bonds (H-bonds) in conformer I of 1,3-hexadien-5-yne (13HD5Y) and its halogen-substituted derivatives are explored by theoretical calculations. The geometric analysis indicates that the C H bond in conformer I is shortened in comparison with the non-interacting C H bond, accompanied by the blue-shifts of the C H stretching frequency. The formation of H-bonds is verified by the atoms-in-molecules and noncovalent interaction analysis. The H-bond energies are estimated to be −0.5∼−2.2 kcal·mol−1 by isodesmic reaction method at MP2/aug-cc-pVTZ level, which agree with the trends predicted by the geometrical and topological parameters. The results of the natural bond orbital analysis indicate that the origin of the blue-shifting H-bond may be due to the weak π(C C) → σ*(C H) hyperconjugation interaction and the concomitant increasing in the s-character and polarization of C H bond. This research provides a rare theoretical evidence to identify an intramolecular blue-shifting CH⋯π H-bond system.