X-ray crystallography

2017-05-09T10:29:05Z

62.254.17.242: /* X-ray analysis of crystals */

[[File:X-ray diffraction pattern 3clpro.jpg|thumb|An X-ray diffraction pattern of a crystallized [[enzyme]]. The pattern of spots (''reflections'') and the relative strength of each spot (''intensities'') is used to work out the structure of the enzyme]]

'''X-ray crystallography''' is a way to see the [[3D|three-dimensional]] [[structure]] of a [[molecule]]. The [[electron]] cloud of an atom bends the [[X-ray]]s slightly. This makes a "picture" of the molecule that can be seen on a screen. It can be used for both [[organic chemistry|organic]] and [[inorganic chemistry|inorganic]] molecules. The sample is not destroyed in the process.

The technique was jointly invented by Sir [[William Bragg]] (1862–1942) and his son Sir [[Lawrence Bragg]] (1890–1971). They won the [[Nobel Prize in Physics]] for 1915. Lawrence Bragg is the youngest to be made a Nobel Laureate. He was the Director of the [[Cavendish Laboratory]], [[Cambridge University]], when the discovery of the structure of [[DNA]] was made by [[James D. Watson]] and [[Francis Crick]] in February 1953.

The oldest method of X-ray [[crystallography]] is X-ray [[diffraction]] (XRD). X-rays are fired at a single crystal and the way they are scattered produces a pattern. These patterns are used to work out the arrangement of atoms inside the crystal.<ref>{{cite web |url= http://www.panalytical.com/index.cfm?pid=135 |title=Introduction to X-ray Diffraction (XRD) |first= |last= |work=panalytical.com |year=2012 [last update] |accessdate=6 November 2012}}</ref>

== X-ray analysis of crystals ==
[[File:Bragg diffraction 2.svg|thumb|left|The incoming beam (from upper left) causes each scatterer (e.g. [[electron]]) to re-radiate a part of its energy as a spherical wave. <br/>If atoms are arranged [[symmetry|symmetrically]] with a separation ''d'', these spherical waves will add up only where their path-length difference 2''d'' sin θ equals a multiple of the [[wavelength]] λ. In that case, a ''reflection'' spot occurs in the [[diffraction]] pattern]]

Crystals are regular arrays of atoms, meaning that the atoms are repeat over and over in all three dimensions. X-rays are waves of [[electromagnetic radiation]]. When X-rays meet atoms, the electrons in the atoms cause the X-rays to scatter in all directions. Because the X-rays are emitted in all directions, an X-ray striking an electron produces secondary spherical waves emanating from the electron. The electron is known as the ''scatterer''. A regular array of scatterers (here the repeating pattern of atoms in the crystal) produces a regular array of spherical waves. Although these waves cancel one another out in most directions, they add up in a few specific directions, determined by '''Bragg's law''':

:<math>2d \sin \theta = n \lambda</math>

Here ''d'' is the spacing between diffracting [[plane (geometry)|planes]], <math>\theta</math> is the incident [[angle]], ''n'' is any [[integer]], and λ is the [[wavelength]] of the beam. These specific directions appear as spots on the diffraction pattern called ''reflections''. Thus, X-ray diffraction results from an electromagnetic wave (the X-ray) hitting a regular array of scatterers (the repeating arrangement of atoms within the crystal).

==Related pages==
*[[Crystallography]]
*[[Electron crystallography]]
*[[Spectroscopy]]

==References==
{{reflist}}

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X-ray crystallography