Their project has several goals. The first is to see how reliable current computational methods are for certain molecules that have very weak intermolecular bonds, such as hydrocarbons. Hydrocarbons are simple compounds that contain only hydrogen and carbon. Since these molecules have weak interactions that stabilize their orientations, many of them, when surrounded by other molecules just like them, form crystals that retain the arrangement of atoms that they had as isolated molecules.
In those cases, scientists usually think of the crystalline solid as composed of molecular building blocks linked together. If the molecules don't change their structure when surrounded by other like molecules, scientists treat them as an oriented gas, a collection of molecules with fixed relative orientation, spacing, and properties. Because the arrangement of the individual molecules is not altered by strong chemical interactions with surrounding molecules, their traits will be expressed in the whole crystalline solid. Scientists can then predict the properties of the crystalline solid based on the traits of the individual building blocks by using quantum chemical methods called oriented gas approximations.
On the other hand, some molecules--even some hydrocarbons--do change their structure when surrounded with other like molecules. The properties of those crystalline solids are different than the properties of the individual molecules, and oriented gas approximations fail. In those cases, Hudson says, "the assembly of the crystalline solid with the desired properties may not be as simple as the independent packing of the building blocks but rather a cooperative process of different chemical reactions working together when each molecule interacts with the ones surrounding it."
The team found that the best way to look for failure in quantum theoretical methods like the oriented gas approximation was to experimentally test the vibrations in the molecular structure. The bonds between atoms in a molecule are a lot like springs. By exciting a molecule with energy, you can learn a lot about its structure and the strength of its bonds based on the way those springs vibrate.
Hudson's team did this by shooting a beam of neutrons into a highly symmetric hydrocarbon molecule called dodecahedrane, using an experimental method called inelastic neutron scattering (INS) spectroscopy. The intensity with which the neutrons scattered upon interaction with the nuclei of particular molecules was computed from knowledge of which atoms were moving like they normally would absent the beam of neutrons. The team then compared the results of this experimental method on both isolated crystal molecules and a sample of the crystalline solid.
Damian Allis, a graduate student working on the project, says, "Molecules feel the same physical constraints from crystal packing that someone on the subway would from other passengers. If you take that person on the subway and put them on a bumpy track, where they move and how far they move will be determined by where everyone else is. Their restricted motion on a crowded subway will look very different to an observer than their motion on a bumpy track if the car was empty of other people. We see those differences when comparing vibrational spectra of isolated molecules with crystalline solids and, therefore, learn about the environment of the molecule in the crystal."
The team found that quantum theoretical methods currently used by scientists described the vibrations observed in the isolated molecule fairly well. However, when looked at in detail, the methods used for isolated molecules did not accurately describe the molecular structure of many molecules forming a solid, showing that the arrangement of dodecahedrane atoms relative to each other changes slightly when surrounded by other like molecules. New quantum chemical methods applicable to solids correctly predict this molecular deformation.
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