2. • Introduction
X-Ray crystallography is a powerful technique for visualizing the structure of protein.
It is a tool used for identifying the atomic and molecular structure of a crystal.
In crystallography the cryastalline atoms cause a beam of incident x-rays to diffract into many
specific direction.
Then crystallography can product a 3-dimensional picture of the density of electrons within the
crystals.
From the electron density the mean position of the atoms in the crystals can be determined.
X-Ray crystallography can locate every atom in a Zeolit, an Aluminosillicate.
3. History
The English Physicist Sir William Henry Bragg pioneered the determination of
cryastal structure of X-Ray diffraction methods.
Using X-Ray crystal data, Dr.James Watson and Dr. Francis Crick were able to
determine the helix structure of DNA in 1953.
In 1998,Dr.Peter Kim, a scientist, was able to determine the structure of a key
Protein responsible for the HIV infection process.
4. Principle
Data is collected by Difracting X-Ray from a single cryastal, which has an
ordered, regularly repeating arrangement of atoms. Based on the diffraction
pattern obtained from X-Ray scattering off the periodic assembly of molecule
or atoms in the cryastals, the electron density can be reconstructed.
5. X-Ray Diffraction
X-Ray crystallography uses the uniformity of light diffraction of crystal to
determine the structure of molecule or atoms.
Then X-Ray beam is used to hit the crystalised molecule.
The electron surrounding the molecule difffract as the X-Ray them.
This forms a pattern, this type of pattern is known as X-Ray diffraction
pattern.
6. Bragg’s Law
X-Ray diffraction is a technique used for structural determination of any
cryastalline substance.
It is based on the concept of Bragg’s law.
Bragg‘s law states that X-Ray Refleted from different parallel plane of a
cryastal interfere constructivity when the path difference is integral multiple
of wavelength of X-Ray.
9. X-Ray Diffraction Methods
These are generally used for investigating the international structure and
cryastal structure of various solid compunds.,
They are,
1.Laue photographic method.
A.transmission method.
B. Back reflection method.
2.Bragg’ X-Ray spectrometer method.
3.Cryastal method.
4.Powder method.
10. 1.Laue method-
-Orientation single cryastal polychromatic beam fixed angle.
-mainly used to determine the orientation of large single crystals. White
radiations is reflected from or transmitted through a fixed cryastal.
2.Rotating Cryastal Methods-
-lattice constant single cryastal monochromatic beam variable angle.
3.Powder Method-
- lattice parameters polycryastals monochromatic beam variable angle.
12. A beam of X-Ray is passed through the crystals, after passing through the crystals, X-rays are
diffracted and recorded on a photographic plate.
Big crystals are required.
B) Back reflection method-
13. This method provides similar information as the transmission method.
The beam which are diffracted in a backward direction are recorded.
One side of the cone of laue reflection is defined by the transmitted beam.
The film intersects the cone with the diffraction spot generally lying on a hyperbola.
Can be used to orient crystals for solid state experiment.
Also used in determination of symmetry of single cryastal.
2) Bragg’s X-Ray spectrometer method-
14. Laue beam of X-Ray crystals- emitted X-Ray obtained on photographic plate using photograph
brag analysed structures of crystals of Nacl. Kuch, Zns Bragg’s equation.
3)
Rotating crystal method-
15. Photographs can be taken by:
1) Complete rotation method-
In this method Series of complete revolution occurs.
Each set of plane in a crystal diffract four times during rotation.
Four diffracted beam are distributed into a rectangular pattern in the central point of photograph.
2) Oscillation method-
The crystal is oscillated at an angle of 15 oc or 20oc.
The Photographic plate is also moved back and forth with the crystal.
The position of the spot on the plate indicate the orientation of the crystal at which the spot was
formed.
17. Fine powder is a struck on a hair with a gum, it is suspended vertically in the axis of a
cylindrical camera.
When monochromatic beam is allowed to pass different possibilities may happen,
1.There will be some particles out of random orientation of small cryastal in the fine powder.
2.Another fraction of grain will have another set of planes in the correct positions for the
reflection to occurs.
3.Reflection are possible in different orders for each set.
If the angle of incident is theta then the angle of reflection will be 2 theta.
If the radius is r the circumference 2 pi r corresponds to a scattering angle of 360 0
From the above equation the value of theta can be calculated and substituted in Bragg’s
equation to get the value of d.
18. Applications of XRD
1.Structure of crystals.
2.Polymer characterization.
3.Particle size determination.
a. Spot counting method.
b. Broadening of diffraction lines.
c. Low angle scattering.
4.Application of diffraction methods to complexes.
a. Determination of Cis-trans isomerism.
b. Determination of linkage isomerism.
5.
19. 5.Miscellaneous application-
a. Soil classification based on Crystallanity.
b. Analysis of industrial dust.
c. Assessment of weathering and degradation of minerals and polymers.
d. Study of corrosion products.
e. Examination of tooth enamel and dentine.
f. Examination of bone state and tissue state.
g. Structure of DNA and RNA.
