Typical Diffraction Patterns: Single-crystal Versus Powder:
Phase diffraction using crystaldiffract series#
The resulting light pattern is made of a series of lines, where the light areas represent constructive interference while the dark areas are a result of destructive interference. When the waves interact, they can add together (if the waves have the same wavelength and phase) or cancel each other out (if the waves have the same wavelength, but have different phases), which is called constructive and destructive interference, respectively. When monochromatic light enters two slits, the wave-like property of light results in light emanating in a spherical fashion from each slit. Recall that light has both wave- and particle-like properties. During data collection, the sample is rotated with respect to the X-ray source and detector. The sample (single- or polycrystalline) is irradiated with X-rays and the diffracted X-rays hit a detector. For powder XRD, a polycrystalline sample is ground into a fine powder and mounted on a plate. For single-crystal XRD, a crystal is mounted and centered within the X-ray beam. Single-crystal and powder XRD have similar instrumentation setups. By repeating the unit cell in space, one can generate a 3D representation of the solid. The unit cell allows chemists to describe the contents of a crystal using a fraction of or a small number of atoms or molecule(s). It is defined as a 3D "box" with lengths a, b, and c, and angles α, β, and γ ( Figure 1). The unit cell of a crystal is the least volume containing a repeating unit of a solid. The complexity of an object is simplified when it exists as a crystal, where the contents of a unit cell can be used to describe the entire structure. If one approximates that it's comprised of 6.02 × 10 23 molecules (or 1 mole), it would seem nearly impossible to describe that object on the molecular level. Imagine trying to describe all of the molecules on the tip of a pen. X-rays have wavelengths in the Å range, which matches perfectly with typical bond distances (1-3 Å). Therefore, in order to study bonds in molecules, it is important to use a wavelength of light that matches the length of those bonds. Similarly, if one wanted to measure the length of a car, it would be inappropriate to use a 12-inch ruler with cm marks. For example, to measure the length of a pencil, one would not want to use a yard stick that only has feet gradations. When measuring distance, it is important to select a unit of measure that is on the scale of the object being measured. We will then collect both single-crystal and powder data on Mo 2(ArNC(H)NAr) 4, where Ar = p-MeOC 6H 5. Here we will learn the principles behind XRD.
Phase diffraction using crystaldiffract how to#
Previously, we have seen how to grow X-ray quality crystals (see video in Essentials of Organic Chemistry series). Powder XRD can also be used to establish bulk purity of molecular species. Typically, powder XRD is used to study minerals, zeolites, metal-organic frameworks (MOFs), and other extended solids. The powder pattern is considered a "fingerprint" for a given material it provides information about the phase (polymorph) and crystallinity of the material. Unlike single-crystal XRD, powder XRD looks at a large sample of polycrystalline material and therefore is considered a bulk characterization technique. Therefore, additional bulk characterization methods must be utilized to prove the identity and purity of a compound. This technique provides the structure within a single crystal, which does not necessarily represent the bulk of the material. With single-crystal XRD data, the exact atomic positions can be observed, and thus bond lengths and angles can be determined. Single-crystal XRD allows for absolute structure determination. X-ray diffraction (XRD) experiments are routinely carried out with either single-crystal or powdered samples. X-ray crystallography is a technique that uses X-rays to study the structure of molecules. Powers, Department of Chemistry, Texas A&M University
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