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At the recent BCA meeting in Leeds, John Robertson provided a historical exhibit with many photographs of early Leeds crystallographers, the Braggs’ original camera and notebook containing his early X-Ray photographs, and a crystallographic carpet accompanied by the story below. Several retired crystallographers visiting the Poster Session reminisced about the times when Gordon Cox had called them into his office for ‘discussion’ while they stood on this very carpet. Over the years parts had become worn; the whole carpet was about to be consigned to oblivion in a waste skip during office renovations in the Chemistry Department at Leeds when John Robertson managed to rescue it, cut out the worn parts and have it cleaned and rebound to make the much admired carpet we saw at the Poster Session.

The carpet at the poster session of the BCA in Leeds
Photograph by Richard Glazer of Oxford Cryosystems
First digital photograph published in the BCA newsletter
Issue 61 June 1997

This carpet depicts the crystal structure of Pentaerythritol,    C(CH₂OH)₄. Sixty years ago, pentaeryritol made an interesting contribution to the correct understanding of the tetrahedral valency of carbon. During the 1920’s there had been controversy over the spatial distribution of the bonds around an aliphatic carbon atom. Direct measurement of C-C bonds at that time had been possible only for some aromatic compounds, plus a few oxalic acid derivatives. No reliable study of an aliphatic system had been made. Pentaerythitol was a test case for this study.

Today, we know its three-dimensional structure to be:

However, in those early days of stereochemistry, a paper had been published in 1923 which had claimed that the four bonds in this molecule were planar, directed to the four corners of a square. This controversy was discussed in the British Chemical Society’s Annual Reports of 1929.

It was Gordon Cox and his co-workers (Llewellyn and Goodwin) at the University of Birmingham who put this matter right. (This was prior to Cox’s appointment, in 1945, to the Chair here in Leeds) They carried out a 3-dimensional X-ray crystal structure analysis of pentaerythritol, showing conclusively that the four C-C bonds were not planar at all but were directed towards the four vertices of a regular tetrahedron. This was long before computers, and before even Beevers-Lipson strips were available. In those days it was one of the first 3-D analyses of crystal diffraction. The paper was published in J.Chem.Soc., in 1937 (page 883-). Cox’s Fourier projection, a view down the 4₂ symmetry axis, based on the x,y coordinates of all the atoms, is reproduced overleaf.

The central carbon atoms lie on S₄ axes. Around a different set of S₄ axes the four hydroxyl groups, from four separate molecules, are hydrogen-bonded to one another in squares. The four C-C bonds from the central atom look planar in this projection but, in fact, two opposing bonds point upwards while the other two point down. The essentially perfect tetrahedral symmetry of these bonds was derived from the 3-D coordinates, x, y, z, of the five carbons.

So how did all this chemistry get onto a carpet?

In 1951, during the prestigious ‘Festival of Britain’, when leading industrialists from all parts of the UK were displaying their latest and best creations, crystal structure patterns were much-sought after, as exciting new designs for textiles, ceramics and other materials. JAMES TEMPLETON, Ltd, a carpet manufacturer in Glasgow, produced this unique carpet with its historic crystal structure pattern, specially for the exhibition in London. Then, following its display at the Festival, the carpet was proudly acquired by Professor Cox for his room in the School of Chemistry here (from 1952-60).

* This article first appeared in Crystallography News, the quarterly journal of the British Crystallographic Association, Number 61, June 1997. It has been reproduced here with the permission of the Editor.

page last updated 24 November2012

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