An ab initio study of the molecular properties of the propyne–water hydrogen-bonded complex

We have employed ab initio MP2 and DFT/B3LYP calculations with the 6-31++G(d,p) basis set to obtain structural, electronic and vibrational properties of the H-bonded complex between propyne and water. This study has revealed that H2O can doubly complex with propyne forming a quasi five-membered ring. The first complexation occurs through the hydrogen bond between the acid hydrogen of H2O and the CC triple bond, whereas the second complexation involves the oxygen atom of H2O and the in-plane hydrogen atom of the methyl group in propyne. Our calculations have shown that the H-bond lengths between H⋯π and O⋯Hin-plane(H3C)O⋯Hin-plane(H3C) are 2.419 and 2.707 Å, respectively, employing the DFT/B3LYP calculation whereas the corresponding MP2 values are 2.373 and 2.651 Å. The binding energies including both BSSE and ZPE corrections are −6.16 and −6.72 kJ mol−1, respectively, using the DFT/B3LYP and MP2 calculations. For example, the O–H stretching frequencies of water are decreased by −60 and −29 cm−1 using the DFT/B3LYP calculation, whereas the bending frequency is increased by +15 cm−1. As expected, the infrared intensities for the stretching modes are increased after complexation, especially involving the O–Hb bond forming the hydrogen bond with the CC triple bond.