Hydrogen bonds in small water clusters: A theoretical point of view.

The hydrogen bonds appearing between water molecules in a series of clusters have been analysed by a theoretical quantum chemical approach. Ab initio calculations at 6-31G level are realised and the geometry of optimised clusters is obtained by a gradient technique. Cyclic configurations are examined in a first approach. Structural results show that a single water molecule may be added on such a cyclic configuration only when the water molecule acts as proton acceptor. Transition structures are found when water acts as proton donor. An analysis of the energy released in the hydrogen bond formation shows that the first and second hydrogen bond releases much more energy than the third and fourth bond, suggesting that in bulk water, strong and weak hydrogen bonds coexist.