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The Squeeze Is On

How do molecules behave at extremely high pressure?

Roald Hoffmann


General%20curve%20for%20interactions%20between%20atoms%20not%20bonded%20to%20each%20otherClick to Enlarge ImageAs the pressure rises, there is only one imperative: denser, denser. Two response strategies on the part of the matter in question are pretty obvious—the conversion of gases and liquids into solids, and the shift of any equilibrium that involves gases away from the side of the chemical equation that contains that state of matter.

Squeeze some more. A molecular solid contains well-defined molecules with weak attractive forces (called dispersion or van der Waals forces) between them. A standard potential energy curve describes how the energy varies with distance between two such molecules (see the second figure, above right).

Apply pressure, and the individual molecules come closer to each other. Actually, let’s think of a specific molecule, one my group has studied—silane (SiH4), the silicon analogue of methane. At ambient pressure the crystal has distances between nonbonded hydrogens of two different molecules of around 3.2 Ångströms. The analog computer that the molecule is itself tells us that this is the minimum of the potential energy curve—it’s as close as two hydrogen atoms of different SiH4 molecules wish to be.

As pressure is applied, the volume per SiH4 has to decrease. This can be accomplished by decreasing the distances between bonded silicon and hydrogens within each molecule, or by having neighboring molecules get closer to each other than they would at 1 atmosphere. Or both. Matter will do what hurts least, so to speak. It turns out that for SiH4 and most molecular solids we have studied, it’s the non-bonded intermolecular hydrogen distances that decrease, while the silicon-hydrogen distances remain pretty constant. “Van der Waals space” is squeezed out first.

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