Hydrogen bonds with pi and sigma electrons as the multicenter proton acceptors: high level ab initio calculations.

Pi-electrons of acetylene and sigma-electrons of molecular hydrogen were investigated as Lewis bases in different complexes. Hence high level ab initio calculations were performed up to the MP2/6-311++G(3df,3pd) level of approximation. It was found that species analyzed possess characteristics typical for H-bonded systems. The Bader theory was additionally applied; bond paths between proton and pi-electrons of acetylene or sigma-electrons of molecular hydrogen were detected with the corresponding bond critical points attributed to the proton-acceptor interactions. Numerous correlations between topological, geometrical and energetic parameters were also found. For example, the H...pi or H...sigma interaction is stronger for the shorter corresponding distance between the proton and the middle of C[triple bond]C or H-H bond. It is connected with the greater elongation of C[triple bond]C or H-H bonds and the greater transfer of electron charge from the Lewis base to the Lewis acid.