X-ray crystallography
Introduction
X-ray crystallography is a widely used experimental technique that exploits the fact that X-rays are diffracted by crystals. It is primarily used to determine the atomic and molecular structure of a crystal, where the crystalline atoms cause a beam of incident X-rays to diffract into many specific directions.
History
The discovery of X-rays in 1895 by Wilhelm Conrad Röntgen gave scientists a tool that could penetrate into matter. The first X-ray diffraction experiment was carried out by Max von Laue in 1912, using a crystal of copper sulfate. This experiment proved that X-rays are a form of electromagnetic radiation, just like light.
Principles of X-ray Crystallography
X-ray crystallography is based on the principle of Bragg's law, which relates the angles at which X-rays are diffracted by a crystal to the spacing of the crystal's lattice planes and the wavelength of the X-rays. The diffraction pattern produced by a crystal is unique and can be used to identify the crystal or to determine its structure.
Experimental Techniques
There are several experimental techniques used in X-ray crystallography, including single-crystal X-ray crystallography, powder X-ray diffraction, and X-ray fiber diffraction.
Single-Crystal X-ray Crystallography
Single-crystal X-ray crystallography is the most common technique used to determine the structures of small organic and inorganic molecules. The crystal is mounted on a goniometer, and the diffraction pattern is recorded as the crystal is rotated.
Powder X-ray Diffraction
Powder X-ray diffraction is used when it is not possible to obtain a single crystal of the material. The sample is ground into a fine powder, and the diffraction pattern is recorded from many different orientations.
X-ray Fiber Diffraction
X-ray fiber diffraction is used to study the structure of fibrous materials, such as proteins and polymers. The fibers are aligned parallel to each other, and the diffraction pattern is recorded from the side.
Applications
X-ray crystallography has a wide range of applications in various fields, including chemistry, materials science, geology, biology, and physics. It is used to determine the structures of a wide range of materials, from simple inorganic compounds to complex biological macromolecules such as proteins and DNA.
Advantages and Limitations
While X-ray crystallography is a powerful technique for determining the structure of materials, it does have some limitations. One of the main limitations is the need for a high-quality single crystal, which can be difficult to obtain for some materials.