Bond paths show preferable interactions: ab initio and QTAIM studies on the X-H...pi hydrogen bond.

MP2/6-311++G(d,p) calculations on complexes of C(4)H(4), C(5)H(5)(-), and C(6)H(6), acting as Lewis bases, and C(2)H(2) and HF species, being Lewis acids, were carried out. Additionally the calculations were performed on C(3)H(3)(+)...C(2)H(2) and C(3)H(3)(+)...HF complexes. For the latter complexes, HF and C(2)H(2) molecules are Lewis bases while C(3)H(3)(+) 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...pi hydrogen bonds were found for complexes, except of those where C(3)H(3)(+) ion acts as the Lewis acid. QTAIM shows that for all systems analyzed the bond paths correspond to preferable interactions.