Bond Paths Show Preferable Interactions: Ab Initio and QTAIM Studies on the X-H•••π Hydrogen Bond

MP2/6-311++G(d,p) calculations on complexes of C4H4, C5H5-, and C6H6, acting as Lewis bases, and C2H2 and HF species, being Lewis acids, were carried out. Additionally the calculations were performed on C3H3+center dot center dot center dot C2H2 and C3H3+center dot center dot center dot HF complexes. For the latter complexes, HF and C2H2 molecules are Lewis bases while C3H3+ ion is the Lewis acid. All systems were optimized, and no imaginary frequencies were found, showing that the complexes analyzed are energetic minima. The quantum theory of "atoms in molecules" (QTAIM) was also applied to analyze occurring interactions. The X-H center dot center dot center dot pi hydrogen bonds were found for complexes, except of those where C3H3+ ion acts as the Lewis acid. QTAIM shows that for all systems analyzed the bond paths correspond to preferable interactions.