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Acta Cryst. (1968). B24, 23The Crystal Structure of L-Ascorbie Acid, 'Vitamin C'. I. The X-ray Analysis BY JAN HVOSLEFUniversitetets Kjemiske Institutt, Blindern-Oslo, Norway (Received 7 January 1967) The crystal and molecular structure of L-ascorbic acid has been determined and the parameters refined to a final R index of 5.0 ~o by means of 1640 X-ray reflexions. The space group is P21 with the axes a= 17. 299, b=6.353, c=6.411 A and ,8= 102 ° l l'. The four molecules in the unit cell are related in pairs by pseudo screw axes, and each molecule consists of an almost planar five-membered ring plus a side chain. The atomic distances are in general accordance with the accepted values for different hybridizations. The enediol group is planar, and one of the hydrogen atoms attached to an oxygen atom in this group is picked out as the probably protolytic one. All hydrogen bonds are intermolecular and consist partly of helices running along [001] and partly of isolated bonds. Evidence is found of systematic shortening of the C-H and O-H bonds when determined by X-ray diffraction compared with those found by neutron diffraction. IntroductionHerbert, Hirst, Percival, Reynolds & Smith (1933) determined the chemical constitution of the substance isolated by Szent-Gy6rgyi (1928) called hexuronic acid. Later the name was changed to ascorbic acid, as the substance was proved to be identical with the active factor in Hoist & Fr61ich's (1907) found that an almost planar molecule would account satisfactorily for the crystallographic properties. This view was later supported by Cox & Goodwin (1936) in an attempt to give a model of the crystal structure. However, only 15 intensities were used, and this is far too small a number to give any conclusive choice between different alternatives. The proposed structure is in error, but their conclusion that the two molecules in the asymmetric unit are connected by a pseudo symmetry element is in principle correct. There are many reasons for a renewed inspection of this structure. The particular interest in L-ascorbic acid lies in its biological importance and also in its acidity and reducing power. It was hoped that it would be possible to decide which of the two hydrogen atoms in the enediol grouping protolyses most easily. A crystallographic investigation of the sodium salt is in progress in order to see what actually happens to the molecule under salt formation. Other items of interest are the various C-C and C-O bonds of different hybridizations, and the hydrogen bond system. A neutron diffraction analysis of two projections has been undertaken in order to determine the hydrogen positions with some precision, and to compare these with the results of the X-ray analysis. An account of the neutron diffraction analysis will be given in a separate paper. ExperimentalThe nature and habit of single crystals of L-ascorbic acid have been discussed by Cox & Goodwin (1936), by Hendricks (1934) and by Niggli (1942). The present investigation includes axial determinations by rotation...

The syntheses and X-ray crystal structures of Li and Cr" complexes of the tri-t-butylmethoxide ligand are described, these are [{Li (OCBut,) (thf))J and [Cr( OCBut,) .Lie1 (thf) 2] (thf = tetra hydrofuran) ; the latter compound has a rare distorted T-shaped geometry at chromium and when dissolved in n-hexane gives LiCl and a complex that analyses as [Cr(OCBut,),(thf),].

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The crystal structure of sodium ascorbate, C6HvO6Na, has been determined and the parameters refined to an R index of 5.8% for the 854 observed X-ray reflexions. The space group is P2a2121 with a= 19.051, b=4.490, c=8.516/~ and with four molecules in the unit cell. The sodium ion is surrounded by six oxygen atoms forming a distorted octahedron where the Na-O distances vary between 2-416 and 2.725 .~. Bonding and conformational changes from those in ascorbic acid have been observed, and the anion may be regarded as a mesomer'between two structural formulae. The hydrogen atoms are located, and the H atom attached to 0(3) is proved to be the protolytic one.

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