The molecular structure of water is the essence of all life.

Albert Szent-Gyorgyi, 1937 Nobel Prize Winner

Outline

1. Overview
   1. Water Molecule
   2. Hydrogen Bond
   3. Tetrahedral Geometry
2. The Hydrogen Bond is Flexible
   1. Ice
   2. Liquid Water
3. Melting of Ice

Overview

Water

The hydrogen bond can be understood by taking a close look at the water molecule itself. The water molecule is composed of two hydrogen atoms and one oxygen atom. The oxygen atom has six valence electrons in its outer shell. These six electrons are distributed into four sp³ orbitals.

Two lone pairs of electrons will occupy the first two sp³ orbitals, leaving an unpaired bonding electron in each of the other two sp³ orbitals. The one and only electron from the hydrogen atom and an unpaired electron from an sp³ orbital occupy a σ-bonding molecular orbital.

The bonding electrons in each σ-covalent bond are localized to the region directly between oxygen and hydrogen nuclei, as indicated by a line, O–H, connecting the two nuclei. The nucleus of the oxygen atom exerts a greater coulombic attraction on the bonding electrons than does the hydrogen nucleus, leaving the single proton that makes up the nucleus of the hydrogen atom partially unshielded. The oxygen atom swells and carries a partial charge of -0.8, while each hydrogen atom carries +0.4 charge. Toggle map of charges on and off.

Overview

The Hydrogen Bond

The hydrogen bond is ubiquitous in biological systems. It plays an essential role in explaining the physical properties of water and in understanding of the three-dimensional structure of biological macromolecules.

An attractive interaction exists between the lone pair of the oxygen atom and the hydrogen atom in an O–H covalent bond. The attraction is strongest when the O–H bond is directed along the lone-pair orbital axis. The hydrogen bond is always long and weak compared to a covalent bond. [Notes]

Toggle the hydrogen bond between two water molecules. The hydrogen bond's length (O···H) is given in angstroms (1 Å = 0.1 nm). Compare it to the length of the covalent O–H bond.

Toggle between spacefilling and ball-and-stick models.

Toggle map of charges on and off.

Overview

Tetrahedral Geometry

The geometry of water molecules in ice. Toggle tetrahedron.

When water freezes at 0°C, its volume increases by about 9%. The packing density of ice at 0°C is 0.34. This means that 66% of the volume is unoccupied because the packing is restricted by the hydrogen-bonding geometrical constraints. Water has the highest density at 4°C, where the packing density is 0.37. As the temperature is lowered, hydrogen bonds break less frequently and become stronger. This causes a decrease in the packing density from 0.37 at 4° to 0.36 in liquid water at 0°C.

Toggle spacefill.

The Hydrogen Bond is Flexible

Ice

The model contains 432 water molecules arranged as sheets of interlocking hexagons. Toggle spacefill.

In liquid water, hydrogen bonds are constantly stretching, bending, and breaking as the molecules rotate and jump around. The average lifetime of a hydrogen bond is about one picosecond (10^-12 seconds) in liquid water at 25°C. It is this network of flickering hydrogen bonds that gives liquid water its unique properties. This flickering also accounts for the fact that while water is more dense than ice, the "collapsed" structure is still "open" because of the highly directional character of the hydrogen bonds.

The Hydrogen Bond is Flexible

Liquid Water

A snapshot of a drop of water at 25°C based on molecular dynamics simulation. There are 432 molecules in the cluster, taken from the center of a box containing 2,000 molecules at thermodynamic equilibrium.

Toggle between the previous ice model and drop model. Notice the presence of large cavities, even though the overall volume occupied by 432 molecules in liquid water is less than that for the same number of water molecules in the ice model.

It's important to remember that the model is static. As mentioned above, water is less ordered in the liquid state due to chaotic thermal motion of the molecules. Let's look at a smaller ensemble (30 molecules) from this set.

The Hydrogen Bond is Flexible

Liquid Water

Toggle stick model and examine the geometry of the hydrogen bonds carefully. About half of the hydrogen bonds have non-ideal orientations.

In a given set of hydrogen bonds, the strongest hydrogen bonds are those closest to perfect geometry. While hydrogen bonds that are longer than normal are definitely weaker, those with bond lengths shorter than normal are not necessarily stronger. Find several strong and very weak hydrogen bonds.

Melting of Ice

Melting of ice is a cataclysmic event. At 0.0000°C ice and water co-exist but at 0.0001°C the ice completely disappears. As the ice melts, the average hydrogen bonds length increases slightly. In the reverse direction no cataclysmic event occurs. Liquid water can exist without any ice at temperatures well below 0°C. The coldest temperature at which supercooled water has been shown to exist is -40°C. This reflects the difficulty of disrupting the hydrogen bonds network in the flickering clusters during the liquid-to-ice transition at low temperatures.

There is still considerable hydrogen bonding in water at 100°C. Since all the hydrogen bonds to a water molecule must be broken for it to escape into the gas phase, a great deal of energy is required to convert water to steam.