An insight into C-sp-H center dot center dot center dot pi hydrogen bonds and stability of complexes formed by acetylene and its substituted derivatives with benzene and borazine

摘要

Theoretical calculations at the MP2/aug-cc-pVDZ level are used to investigate the C-sp-H center dot center dot center dot pi 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 center dot center dot center dot C6H6 complex is the most stable, whereas the weakest one is C2HNH2 center dot center dot center dot 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 approximate to F < Cl < Br and is directly proportional to the polarization of the C-sp-H bond in isolated monomers and pi electron density of aromatic rings. The C-sp-H center dot center dot center dot pi 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.