Cooperativity between two types of hydrogen bond in H(3)C-HCN-HCN and H(3)C-HNC-HNC complexes.

Hydrogen-bonded clusters, H(3)C-HCN, HCN-HCN, H(3)C-HCN-HCN, H(3)C-HNC, HNC-HNC, and H(3)C-HNC-HNC, have been studied by using ab initio calculations. The optimized structures, harmonic vibrational frequencies, and interaction energies are calculated at the MP2 level with aug-cc-pVTZ basis set. The cooperative effects in the properties of these complexes are investigated quantitatively. A cooperativity contribution of around 10% relative to the total interaction energy was found in the H(3)C-HCN-HCN complex. In the case of H(3)C-HNC-HNC complex, the cooperativity contribution is about 15%. The cooperativity contribution in the single-electron hydrogen bond is larger than that in the hydrogen bond of HCN-HCN and HNC-HNC complexes. NMR chemical shifts, charge transfers, and topological parameters also support such conclusions.