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

Theoretical calculations at the MP2/aug-cc-pVDZ level are used to investigate the C sp –H⋯π interactions of C 2 HX (X = H, F, Cl, Br, CH 3 , NH 2 ) with C 6 H 6 and B 3 N 3 H 6 molecules. Twelve stable complexes with similar structures are observed, in which the (C 2 HX, C 6 H 6 ) complex is always found to be more stable than the corresponding (C 2 HX, B 3 N 3 H 6 ) complex. The C 2 HBr⋯C 6 H 6 complex is the most stable, whereas the weakest one is C 2 HNH 2 ⋯B 3 N 3 H 6 . When replacing one H atom in C 2 H 2 by different X groups, the stability of the complexes increases in the order of NH 2 < CH 3 < 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 C 2 H 2 is replaced by different X groups, except for the (C 2 HCH 3 , C 6 H 6 ) 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 C 2 H 2 by an electron-donor or withdrawing X group negligibly affects the role of the electrostatic and dispersion components in stabilizing (C 2 HX, C 6 H 6 ) and (C 2 HX, B 3 N 3 H 6 ) compared to (C 2 H 2 , C 6 H 6 ) and (C 2 H 2 , B 3 N 3 H 6 ) pairs, respectively.