X-Ray Crystallography
X-ray diffraction has perhaps been one of the most critical and exigent discoveries of the 20th century. Early X-ray diffraction images for tobacco mosaic virus had been collected before World War II. By 1954, Watson had discovered from his X-ray diffraction images that the tobacco mosaic virus had a helical structure, and was able to apply this to his famous DNA structure research. It is so valued because it allows the structure of a crystalline material can be determined, based on the diffraction of x-rays due to electron density within the crystal. The mystery of the arrangement of atoms can be solved when the electrons in the crystal, which has a particular, repeating arrangement of atoms, scatter a beam of x-rays. It can be applied to study many crystalline materials, from ionic molecules to biomolecules. The technique can also be used to determine the degree of crystallinity of a sample. Due to increased data and knowledge, identification of compounds can now be performed using x-ray diffraction data. X-ray crystallography uses the physical phenomenon of diffraction of electromagnetic radiation. Diffraction occurs when light passes through a slit with size comparable to its wavelength. When the light passes through the slit it is spread out. When several slits are present light from different slits may hit the target no longer in phase. When the beams of light are in phase it causes high intensity to be observed. When the light is out of phase by 180 degrees than the waves destructively interfere with each other and causes no intensity to be detected. This constructive and destructive interference is observed when light passes through a series of slits. When this situation occurs the pattern of differing intensities is called a diffraction pattern. This phenomenon can be used to determine the placement of objects by determining the space in between. Since crystals contain an ordered arrangement of atoms, a diffraction pattern can be created.
Cited: [1] Brown, T. L.; Lemay, H. E.; Bursten, B. E.; Burdge, J. R. Chemistry: The Central Science; Prentice Hall: Upper Saddle River, NJ, 2003; ninth ed., p. 436. [2] Robinson, J. W.; Skelly Frame, E. M.; Frame II, G. M. Undergraduate Instrumental Analysis, 6th ed.; Marcel Dekker: New York, 2005, pp. 576-585. [3] PDF-4 / Organics 2008 http://www.icdd.com/products/PDF4Org08flyer.pdf (assessed 12/12/07). [4] A View of the Future: X-Ray and Ultrafast Science. http://www.lbl.gov/Publications/annual-report/2005-2006/files/05-x-ray-1.html (12/18/2007).