Hydrogen Bonding Between two H2O Molecules

In the vapour, free H2O molecules have 0-H bond-lengths of 0.976 A. When H2O molecules associate to form clusters in the gaseous, liquid or solid states, these O-H bonds lengthen and weaken a little, and the intermolecular 0-H hydrogen bonds are long and weak. Some bond-lengths are reported in (4) for (DjO)2 and (DjO). The D-0 stretching frequency is reduced from 2727 cm in HDO vapour to 2454 cm in ice IX. 

We shall assume here that the two H2O molecules of the water dimer form a charge-transfer complex, and describe the intermolecular bonding by means of Eqn. (1). To do this, we require one molecule to be the electron donor, and the other to be the electron acceptor. Thus, we have D = H2O, A = H2O, D = H2O and A = H20 , and write the wave function for the complex as Eqn. (3). 

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Explain what is meant by hydrogen bonding. Describe the hydrogen bonding between two H2O molecules. 

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