The Structure and Properties of Water

Water is the major constituent of cells and a remarkable solvent whose chemical and physical properties affect almost every aspect of life. Many of these properties are a direct reflection of the fact that most water molecules are in contact with their neighbors entirely through hydrogen bonds.Water is the only known substance for which this is true.
In ordinary ice all of the water molecules are connected by hydrogen bonds, six molecules forming a hexagonal ring resembling that of cyclohexane. The structure is extended in all directions by the formation of additional hydrogen bonds to adjacent molecules. As can be seen in this drawing, the molecules in ice assume various orientations in the hexagonal array, and frequently rotate to form their hydrogen bonds in different ways. This randomness remains as the temperature is lowered, and ice is one of few substances with a residual entropy at absolute zero. Ice is unusual also in that the molecules do not assume closest packing in the crystal but form an open structure. The hole through the middle of the hexagon and on through the hexagons lying below it is ∼0.06 nm in diameter.
The short hydrogen-bond length (averaging 0.276 nm) in ice indicates of strong bonding. The heat of sublimation (ΔH°) of ice is –48.6 kJ/mol. If the van der Waals dispersion energy of –15 kJ/mol is subtracted from this, the difference of –34 kJ/mol can be attributed entirely to the hydrogen bonds—two for each molecule. Their average energy is 17 kJ/mol apiece. However, some of the hydrogen bonds are stronger and others weaker than the average.

Six water molecules in the lattice of an ice crystal. The hydrogen bonds, which connect protons with electron pairs of adjacent molecules, are shown as dashed lines.

In a gaseous water dimer the hydrogen bond is linear, a fact that suggests some covalent character.

Its length is distinctly greater than that in ice. This is one of a number of pieces of evidence suggesting cooperativity in formation of chains of hydrogen bonds. Consider the following three trimers for which theoretical calculations have predicted the indicated hydrogen bond energies. In the first case the central water molecule donates two protons for hydrogen-bond formation; in the second it accepts the protons. In the third case it is both an electron acceptor and a donor. The OH dipoles are oriented “head to tail” and the hydrogen bonds are stronger than in the other



cases. Long chains of similarly oriented hydrogen bonds exist in ice and this may account for the short hydrogen bond lengths. Closed rings of hydrogen bonds oriented to give a maximum cooperative effect also exist in liquid water clusters and within proteins, carbohydrates, and nucleic acids.
The nature of liquid water is still incompletely understood,but we know that water contains icelike clusters of molecules that are continually breaking up and reforming in what has been called a “flickering cluster” structure. Judging by the infrared spectrum of water, about 10% of the hydrogen bonds are broken when ice melts.41 A similar conclusion can be drawn from the fact that the heat of melting of ice is –5.9 kJ/mol. It has been estimated that at 0°C the average cluster contains about 500 water molecules.41 At 50°C there are over 100 and at the boiling point about 40. Although most molecules in liquid water are present in these clusters, the hydrogen bonds are rapidly broken and reformed in new ways, with the average lifetime of a given hydrogen bond being ∼10(–12) s.