Molecular crystals are formed by one or more kinds of molecules tiled up in the microscopic space with regular patterns.

How does melting a molecular crystal affect its alignment?

1. The model below shows a crystal formed by A₃B₈ at 100 K. The green particles represent A and the white ones represent B. Run the model, and then slowly raise the temperature, one tick (marker) at a time.
2. After the molecular crystal is molten, try lowering the temperature to see if you can get it recrystallized. It is almost impossible to recrystallize this as it was originally just by decreasing the temperature, or even through a simulated annealing procedure (i.e., raising and lowering temperature alternately).

The ability of molecules to crystallize into more than one structure is known as polymorphism (many-shapes.) Each different structure is called a polymorph. Some molecules have only one polymorph, while others can have multiple polymorphs.

Polymorphism is particularly important in the pharmaceutical industry where safety and reliability are critical. Many drugs are administered in crystalline forms. Different polymorphs of the same molecule have different chemical and physical properties (such as solubilities), and therefore different therapeutic effects. If, in an extreme case, drug molecules crystallize from solution into an insoluble form, the drug product will have no curative effect. So drug companies want to know which “morph” they are using. Polymorphism is also important in intellectual property laws, e.g., the highly profiled case of the polymorphism of Zantac.

3. The model below shows two possible crystals (polymorphs) that can be formed by the molecule AB₃ at 100 K. The green particles represent A and the white ones represent B. Both polymorphs are stable at 100 K. Before running the model, click on the radial button (below the model) to switch between Polymorph-1 and Polymorph-II. Look at the two models and observe their difference in structure.

4. Degree of solidity is an indicator of structure strength. But which way? Observe which polymorph is more dense than the other. Then make a prediction: Which structure do you think would lose its solidity the most quickly? What do you think would be the relationship between density and structure loss?

5. Then run the models and see which polymorph loses its solidity first.

1. In the first model, what temperature does the molecular crystal appear to melt? What is the first sign of melting?
2. Which Polymorph structure did you think would lose its solidity the most quickly? Were you correct? Why or why not?
3. What do you think would be the relationship between density and structure loss?

What is a molecular crystal? If molecular crystal C is more porous than a molecular crystal D, would you expect C to melt at a lower or higher temperature than D?

Use the model below to perform a hands-on experiment in which you are invited to crash the molecular crystal and convert it into another form. Follow the instructions in the green box within the model window.

You may notice that the resulting crystal is not perfect. Can you observe the imperfections? You may remember that the loose structure had a higher melting point than the compact structure. So here is the interesting point: the loose structure can withstand heat, but not pressure. This is unusual because we intuitively expect harder materials to melt at higher temperature (of course, we know our intuition is not correct in this case).