4.4: Sample and data collection

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Crystallization

In order to run an X-ray diffraction experiment, one must first obtain a crystal. In organometallic chemistry, a reaction might work but when no crystals form, it is impossible to characterize the products. Crystals are grown by slowly cooling a supersaturated solution. Such a solution can be made by heating a solution to decrease the amount of solvent present and to increase the solubility of the desired compound in the solvent. Once made, the solution must be cooled gradually. Rapid temperature change will cause the compound to crash out of solution, trapping solvent and impurities within the newly formed matrix. Cooling continues as a seed crystal forms. This crystal is a point where solute can deposit out of the solution and into the solid phase. Solutions are generally placed into a freezer (-78 ºC) in order to ensure all of the compound has crystallized. One way to ensure gradual cooling in a -78 ºC freezer is to place the container housing the compound into a beaker of ethanol. The ethanol will act as a temperature buffer, ensuring a slow decrease in the temperature gradient between the flask and the freezer. Once crystals are grown, it is imperative that they remain cold as any addition of energy will cause a disruption of the crystal lattice, which will yield bad diffraction data. The result of an organometallic chromium compound crystallization can be seen below.

Mounting the Crystal

Due to the air-sensitivity of most organometallic compounds, crystals must be transported in a highly viscous organic compound called paratone oil. Crystals are abstracted from their respective container by dabbing the end of a spatula with the paratone oil and then sticking the crystal onto the oil. Although there might be some exposure of the compounds to air and water, crystals can withstand more exposure than solution can before degrading. On top of serving to protect the crystal, the paratone oil also serves as the glue to bind the crystal to the mount.

Rotating Crystal Method

To describe the periodic, three dimensional nature of crystals, the Laue equations are employed:

\[a(\cos\ \alpha_n – \cos \alpha_o) = h\lambda \]

\[b(\cos\ \beta_n – \cos \beta_o) = k\lambda \]

\[c(\cos\ \gamma_n – \cos \gamma_o) = l\lambda \]

where \(a\), \(b\), and \(c\) are the three axes of the unit cell, \(\alpha\)_o, \(\beta\)_o, and \(\gamma\)_o are the angles of incident radiation, and \(\alpha\)_n, \(\beta\)_n, and \(\gamma\)_n are the angles of the diffracted radiation. A diffraction signal (constructive interference) will arise when \(h\), \(k\), and \(l\) are integer values. The rotating crystal method employs these equations. A crystal is exposed to X-ray radiation as it rotates around one of its unit cell axis. The beam strikes the crystal at a 90 degree angle. Using Bragg's Law, we see that if \(\theta_o\) is 90 degrees, then \(\cos \theta_o = 0\). For the equation to hold true, we can set h=0, granted that \theta= 90. The above three equations will be satisfied at various points as the crystal rotates. This gives rise to a diffraction pattern.

Contributors and Attributions

Roman Kazantsev (UC Davis) and Michelle Towles (UC Davis)