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Water molecules condensing into a hydrogen-bonded droplet

Theoretical simulation by energy minimization

Once the molecular model has loaded into the black square:

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The atomic radii used in this display (1.0 Å for oxygen, 0.7 Å for hydrogen) are intermediate between van der Waals and covalent radii.

The dot surfaces represent van der Waals radii.

H bonds (not visible for spheres), when distance from H to O is under angstroms.

Ten water molecules are initially arranged arbitrarily in two rows of five, not quite contacting each other. Their mutual chemical attractions then pull them into a compact micro-droplet. Each H2O molecule reorients itself to optimize the intermolecular interactions.

Both van der Waals and electrostatic energies were used in the energy minimization that produced this simulation. The final conformation represents optimal hydrogen bonding for the small number of molecules involved. This simulation lacks the random motions that result from thermal energy. See further information about the methods below.

Challenge Questions for Students (opens in a new window, so you can keep working on the model while you think of the answers)


The model (molecular coordinates file in pdb format) was prepared by Eric Martz:

10 water molecules were arranged arbitrarily in two approximately straight rows of 5, a bit farther apart than van der Waals radii. This initial arrangement could be thought of as a gas-like conformation (lacking entropy!).

Energy minimization was then applied, causing the molecules to collapse into a small droplet with ample hydrogen bonding. Energy minimization involved van der Waals interactions and electrostatics, and was done with MDL Sculpt. There is no thermal energy in this simulation.

24 frames of atomic coordinates were saved from the minimization process, which can be played as an animation.

For the first 11 frames, only van der Waals energies were applied. Starting with the 12th frame, both van der Waals and electrostatic interactions were utilized. This was done in order to produce a single droplet. When electrostatics were invoked from the outset, two droplets were formed. In the animations, the transition at the 12th frame produces a momentary reversal of the general trend towards compaction.

In this page, the model and the animation are shown using Jmol capabilities. Covalent bonds are automatically determined by Jmol, while hydrogen bonds are generated between those oxygen and hydrogen atoms that are not covalently bonded and are separated by a distance below the value specified above.

Totally Valid HTML 4.01 Credits: Eric Martz, Angel Herráez
Versión en español
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