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X-Ray Crystallography Assignment

 X-RAY CRYSTALLOGRAPHY

Definition:


X-ray crystallography (XRC) is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X- Rays to diffract into many specific directions..

PRINCIPLE:

The underlying principle is that the crystalline atoms cause a beam of X-rays to diffract into many specific directions. By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a 3D picture of the density of electrons within the crystal.

INSTRUMENTATION OF X-RAY CRYSTALLOGRAPHY


X-Ray Crystallography Assignment



 

X-ray diffractometers consist of three basic elements:


Ø  X-ray tube.

Ø  A sample holder.

Ø  X-ray detector.

 

WORKING:

       The first step is the most difficult which is to obtain an adequate crystal of the material under study. 

       The crystal should be sufficiently large, pure in composition and regular in structure.

       The crystal is placed in an intense (monochromatic) beam of X-ray producing the regular pattern of reflections. 

       As the crystal is gradually rotated, previous reflections disappear and new ones appear, the intensity of every spot is recorded at every orientation of the crystal.

       These data are now combined computationally with complementary chemical information to produce and refine a model of the arrangement of atoms within the crystal.

       After the pattern is obtained the data is then processed by a computer and the structure of the atom or molecule is deduced and visualized.

 

APPLICATIONS OF X-RAY CRSTALLOGRAPHY:

 

Ø  New mineral identification, crystal solution and refinement

Ø  Determination of unit cell, bond-lengths, bond-angles and site-ordering

Ø  Characterization of cation-anion coordination

Ø  Variations in crystal lattice with chemistry

Ø  With specialized chambers, structures of high pressure and/or temperature phases can be determined

 

 

 

ADVANTAGES OF XRAY CRYSTALLOGRAPHY:

Ø  X-Ray crystallography provides a two-dimensional view that gives an indication of the three-dimensional structure of a material

Ø  Relatively cheap and simple

Ø  Useful for large structures: Not limited by size or atomic weight.

Ø  Can yield high atomic resolution.

DISADVANTAGES OF XRAY CRYSTALLOGRAPHY:

Ø  The sample must be crystallizable

Ø  The types of sample that can be analyzed are limited. In particular, membrane proteins and large molecules are difficult to crystallize, due to their large molecular weight and relatively poor solubility

Ø  An organized single crystal must be obtained to produce the desired diffraction

Ø  Non-dynamic method due to preparation of samples and crystallization. Only a static three-dimensional analysis is produced.


X-Ray crystallography has limited applications for studies of biological samples due to the aforementioned issues.


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