Xray Crystallography - Research Paper Example

?X-ray Crystallography Prior to the discovery of X-rays in 1895 (Nelson), the general concept about the crystals was that these consist of an orderlyarrangement of atom, ions or molecules. As there was no definite clue, only a general imaginary concept about these crystals prevailed. With the advancement in technology and discovery of X-rays, crystallographers found a powerful source of obtaining complete information about any type of crystal. With a wavelength of the order of 1010, comparable to that of the diameter of an atom, X-rays have the ability to penetrate into the crystal and get diffracted by atom, ions or molecules in their way. With the discovery of this technique in 1940s, the scientists used the specific wave-particle nature of X-rays to determine the arrangement of the constituent specie in a crystal. Crystal and its pattern: Crystal is a three dimensional pattern obtained by the repetition of unit cell, the smallest possible, arranged volume of any crystalline solid. In crystals, the atoms, ions or molecule (the constituent species) are held into their orderly arranged positions by inter atomic, inter ionic or inter molecular forces respectively. The scientists were searching for a way to determine the pattern of their arrangement. Once the pattern could be known, all the other information about the substance was easy to get. Crystals were not studied, deeply, until the 17th century. “Crystal symmetry was first investigated experimentally by Nicolas Steno (1669), who showed that the angles between the faces are the same in every exemplar of a particular type of crystal, and by Rene Just Hauy (1784), who discovered that every face of a crystal can be described by simple stacking patterns of blocks of the same shape and size.” (“X-ray Crystallography. Wikipedia”) With the invention of X-rays, the scientists found it suitable to target X-rays to a crystal for knowing the arrangement of the constituent particles. The X-rays can be diffracted by the electronic cloud, present around an atom, ion or molecule, because their wavelength is comparable to that of it. Ordinary light can, definitely, not be used as it has a wavelength, many times greater than the order of atomic diameter. Therefore, it is impossible for ordinary light to be diffracted by a crystal. Only X-rays have the ability to penetrate into a crystal and determine the three dimensional pattern by getting diffracted by the constituent particles. The technique of X-ray Crystallography: X-ray Crystallography uses a focused X-ray beam to reveal the structure of a crystal. X-rays strike the particles in a crystal and spread into many specific directions. Censors present around the crystal then cense the angle of diffraction and the strength of the beam reaching them. The pattern produced by the diffraction of X-rays through the closely spaced lattice of atoms is recorded and analyzed to reveal the structure of the crystal. The very basic fact exploited by this technique is that X-rays are diffracted by crystals. With the invention of this technique, Crystallography was completely revolutionized and improved. X-rays and their production: “X-rays are electromagnetic radiation with wavelengths between about 0.02 A and 100 A (1A = 10-10meters).” (Nelson) They are produced when electrons from a cathode strike the electrons in the inner shells of transition elements. As these electrons are hit, the energy from moving electrons is transferred to them. Thus, these electrons excite and during de-excitation, these electrons emit radiations of high energy, whose wavelength lies in the invisible region of electromagnetic spectrum. These high energy, less wavelength possessing waves can penetrate into most of the crystals. X-ray Diffraction and Bragg’s law: As X-rays hit a row of particles in a crystal, they are diffracted. Actually, the diffraction is the interaction of separate waves of X-ray beam. It can be considered as the reflection of X-ray beam from the row of constituent particles that are arranged in a crystal. There are certain conditions necessary for diffraction. The beam of radiation undergoing diffraction should be focused and monochromatic. When such a monochromatic beam of X-ray strikes a plane of particles in a crystal at an angle ?, it reflects back from this plane at the same angle. Let us suppose that the spacing between the planes is d. As the X-ray beam consists of different rays, each of them strikes a separate layer of particles. Thus, each ray is reflected separately. The ray reflecting from the layer present a little distance (a) farther from the source of X-ray, than the first layer, travels a total distance of 2a greater than the first ray reflecting from the first layer. If this distance 2a is an integral multiple of the wavelength of the X-rays, the resulting waves will be in phase and will interfere constructively? In this case, n?=2a (where a=d sin??) n?=2d sin ? If this distance, 2a, is not an integral multiple of the wavelength ?, the resulting waves will be out of phase and interfere destructively. In this case, d=n?/2 sin ? This equation represents Bragg’s law for electromagnetic diffraction. Now, these equations tell us that we can find the spacing, d, if we know the wavelength of the X-rays and the angle of diffraction. Thus we can have an exact idea about the distance between the consecutive layers within a crystalline structure. In the same way, spacing from another dimension is measured and the records are analyzed to build a three dimensional pattern of the crystal. Methods of X-ray Crystallography: It is clearly visible that focusing a beam of X-ray on to a crystal, measuring the angle and analyzing data is a very difficult and time consuming practice. Thus a faster and easy way to carry out X-ray crystallography is “The X-ray Powder Method”. The instrument used for this technique is the X-ray powder diffractrometer. The apparatus consists of an X-ray tube capable of producing X-rays at angles from 0- 90o. A powdered mineral is placed on the sample stage to be analyzed. Am electronic detector, capable of rotating, is placed on the other side of the sample stage. An instrument known as Gonimeter, rotates the X-ray tube and the detector and keeps track of the angle. After the experiment, a chart is produced which displays the record of angle and the relative spacing. Thus it is quick and easy to measure the layer spacing in a crystal. Applications of X-ray Crystallography: 1. Chemistry and Biochemistry: X-ray Crystallography is widely used in chemistry and biochemistry to determine the basic structure of elements and compounds. Today, every new element and compound is analyzed by the use of this technique. This technique has been used to determine the structures of many inorganic and organic substances including diamond, graphite, sulfur, DNA, proteins and many other substances. 2. Mineralogy and Metallurgy: Since 1920s, this technique has been the principal method of determining the structure of minerals and metals. Today, every mineral and metal is known by its crystalline structure. “The application of X-ray crystallography to mineralogy began with the structure of garnet, which was determined in 1924 by Menzer.” (X-ray Crystallography. Wikipedia) The study of silicates by this technique started in 1920s. In the same way, X-ray Crystallography was used in Metallurgy, most notably, when Linus Pauling developed a theory of stability and structure of complex ionic crystals by studying the structure of the alloy Mg2Sn. (X-ray Crystallography) 3. Macromolecules: The first organic compound to be studied through this technique was hexamethylenetetramine. Its structure was resolved in 1923. (X-ray Crystallography. Wikipedia) Later, many long chain fatty acids were studied. In 1950s, the study of the structure of proteins was initiated. Since then, over 61840 structures of proteins, fats, nucleic acids and other biological molecules have been determined. Today, X-ray Crystallography is an important part of the study of a compound. 4. Study of Diseases caused by mutations: X-ray Crystallography has enabled us to determine the basis of diseases caused by mutations. Mutations cause the structure of proteins to deteriorate. Thus with the help of X-ray Crystallography, scientists are able to study the defect in the structure and can abolish it by using other genetic techniques. Not only the above mentioned fields. But also many other fields exploit this technique to know about the structure of compounds under consideration. Thus X-ray Crystallography is an important technique in modern scientific research. Future Prospects: X-ray crystallography has great future prospects. It has widely contributed to chemistry and material science and is going to contribute in the study of newer compounds. In this era, where thousands of compounds are manufactured every year, X-ray Crystallography will be a part and parcel of modern scientific research. “In an article for Drugs, Discovery and Development in 2006, James Netterwald asserted that the future of x-ray crystallography is beaming.” (“X-ray Crystallography. Wikis”) Despite the facts that undermine the importance of this technique, many scientists expect a rise of X-ray Crystallography in future. Works Cited X-ray Crystallography. Wikipedia, the free encyclopedia. Last updated, October 27, 2011. Web. November 02, 2011 Crystallography 101. Ruppweb.org. n. d. Web. November 02, 2011 X-ray Crystallography. Stolaf.edu. n. d. Web. November 02, 2011 Nelson, Stephen A. X-ray Crystallography. Mineralogy, Tulane University. Last updated, October 19, 2011. Web. November 02, 2011 X-ray Crystallography. 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