An insight into Csp–H⋯π hydrogen bonds and stability of complexes formed by acetylene and its substituted derivatives with benzene and borazine

Abstract

Theoretical calculations at the MP2/aug-cc-pVDZ level are used to investigate the C_sp–H⋯π interactions of C₂HX (X = H, F, Cl, Br, CH₃, NH₂) with C₆H₆ and B₃N₃H₆ molecules. Twelve stable complexes with similar structures are observed, in which the (C₂HX, C₆H₆) complex is always found to be more stable than the corresponding (C₂HX, B₃N₃H₆) complex. The C₂HBr⋯C₆H₆ complex is the most stable, whereas the weakest one is C₂HNH₂⋯B₃N₃H₆. When replacing one H atom in C₂H₂ by different X groups, the stability of the complexes increases in the order of NH₂ < CH₃ < H ≈ F < Cl < Br and is directly proportional to the polarization of the C_sp–H bond in isolated monomers and π electron density of aromatic rings. The C_sp–H⋯π hydrogen bonds in all the complexes belong to the red-shifting hydrogen bond, and the magnitude of the C_sp–H stretching frequency red shift increases when one H atom of C₂H₂ is replaced by different X groups, except for the (C₂HCH₃, C₆H₆) complex. Remarkably, the SAPT analysis indicates that the contribution of dispersion energy towards the total stabilization energy is more important than the electrostatic interaction and other energy components. Substitution of one H atom in C₂H₂ by an electron-donor or withdrawing X group negligibly affects the role of the electrostatic and dispersion components in stabilizing (C₂HX, C₆H₆) and (C₂HX, B₃N₃H₆) compared to (C₂H₂, C₆H₆) and (C₂H₂, B₃N₃H₆) pairs, respectively.