The enhancement of X-H...π hydrogen bond by cooperativity effects - Ab initio and QTAIM calculations

The cooperativity effects for C2H2...(HF)n and C2H4...(HF)n (n = 1-4) complexes are analyzed using the results of MP2/6-311++G(d,p) calculations. It has been revealed that F-H...π and F-H...F hydrogen bonds exist for these complexes and those interactions are enhanced if the number of HF molecules increases. It is shown that cooperativity effect causes the shortening of H...F and H...π distances, simultaneously the electron density and its Laplacian at the corresponding bond critical point (BCP) increase. There is also the greater charge transfer corresponding to π(C=C) → σ*(F-H), π(C≡C) → σ*(F-H) and n(F) → σ*(F-H) interactions. One notices the greater elongation of H-F bonds within complexes if the number of HF molecules increases. The various correlations were found between geometrical, energetic and topological parameters. There are unique bond paths of the complexes analyzed that connect the hydrogen attractors with the BCPs of C=C and C≡C bonds of Lewis base sub-systems