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This programme deals with crystal structure, molecular structure and their determination by X-ray diffraction techniques. After a briel introduction by Prof. Haynes, Dr. Harding uses models to show... how a crystal may be described in terms of its Unit Cell, and how planes through the crystal can be described in terms of Miller Indices. Dr. Ross then describes a typical laboratory X-ray diffraction camera. He shows how three types of photograph are obtained from the same crystalline material by presenting the sample to the X-ray beam as : (i) a single crystal, randomly rotated (ii) a powder (iii) a single crystal rotated about a. principle axis of symmetry. The photographs are then compared by smperimposition on one another. Dr, Ross continues with a laser experiment to show the basis of The Fourier Fringe method for determining molecular structures from X-ray diffraction data. The programme finishes by showing how this method results in an electron density contour map of the molecule.
Metadata describing this Open University video programme
Item code: S24-; 02; 1973
First transmission date: 1973
Published: 1973
Rights Statement:
Restrictions on use:
Duration: 00:24:18
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Producer: Barrie Whatley
Contributors: Charles Harding; Len Haynes
Publisher: BBC Open University
Keyword(s): Crystal structure; Molecular structure; X-ray diffraction techniques
Footage description: Shot of a crystal. Shot of a molecular model of a crystal showing several planes. Len Haynes introduces the programme. Charlie Harding with some large NaCl crystals. He points out the crystal structure and cleaves them to show the cleavage lines. Harding explains why, although this experiment gives the angle between some of the faces, X-ray spectroscopy is necessary to determine the separation of faces. Harding with a model of the NaCl crystal. He points out the components including the unit cell. Harding with another molecular model of the NaCl crystal points out the planes of the crystal (2 planes). He takes the model apart to show the planes. Harding uses a third molecular ball and stick model of the NaCl crystal to index 2 of the planes. He explains each step of the way. Ross begins his discussion on the principles of X-ray crystallography. He has with him an X-ray camera and he explains how it works. Ross uses a model of a crystal to show how it is randomly rotated in the camera. Shots of the diffraction pattern of a randomly oriented crystal. Ross next explains how a crystal is prepared for a powder photograph. (This shows the pattern of the crystal as rotated through all possible positions). Shots of a powder photograph. The randomly oriented photograph is then superimposed over the powder photograph. Ross explains how information on the content of the crystal unit cell is obtained by rotating the crystal about a principle axis of symmetry. Shots of the resultant rotation photograph. This is superimposed over a powder photograph as well. Ross with a laser analogue of the X-ray camera. He points out the components of the model. He uses the pattern of a tri-atomic molecule for his diffraction grating. Shots of several images Ross then explains why in X-ray spectroscopy. Fourier synthesis must be used rather than a lens to get the image of the diffraction grating. More images are shown. Charlie Harding builds up a picture, using X-ray spectroscopy, of the fringe systems of a molecule. He has a diagram of the unit cell of the molecule and the X-ray diffraction shots are superimposed on this. This method does not, however, locate the hydrogen atoms in the unit cell. Len Haynes explains how the Fourier synthesis is done by computer. He has a computer printout with him and explains how the full picture of the unit cell is built up in the form of an electron contour map. Shot of the finished map with credits over.
Master spool number: 6HT/70854
Production number: 00525_1062
Videofinder number: 1853
Available to public: no