Abstract: Theoretical study on the structure of hydrogen bond was carried out by using density functional theory. Geometries were optimized at B3LYP/6-31G^* and single point calculations were performed at B3LYP/6-31+G^* level. Stabilization energy was obtained by taking basis set superposition error correction into account. After the investigation of various minimum-energy geometries, single-ring structures of water pentamer and water hexamer were found most stable in liquid water. The calculations show that these single-ring structures have no imaginary frequency. Adding the methanol molecules to the ring like water cluster, geometries of water-methanol complexes were optimized. No imaginary frequencies were found in these structures. A significant and novel feature revealed firstly in this work is the perfect double hydrogen bond between methanol molecules and the ring-like water complexes. In water and methanol solvent, the polymer chain, PNIPAM, can form hydrogen bond with solvent molecules. It can stretch to coil shape and be soluble. But when the two solvents are mixed, water and methanol molecules will form stable comlexes because of strong double-hydrogen bonding. The active point of hydrogen bonds of mixed solvent will be shielded; and the solvent becomes non-polar. So, the polymer chain will overlap immediately because of the formation of intra molecular hydrogen bonds and consequently perform a coil-to-globule transition.

Key words: Stabilization energy, Basis set superposition error(BSSE), Density functional theory, Complex-Hydrogen bond