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The crystal structure of benzoic acid, C6H5CO2H, has been redetermined by X-ray difiraction at room temperature. Extensive neutrondiffraction measurements have also been made: by single-crystal methods at room temperature and 130 K; and, at 130 K and 5 K, by powder-profile analysis on C6D5CO2H.The structure consists of centrosymmetric dimers [formula(l)], in which two molecules are linked by a pair of hydrogen bonds between their carboxyl groups. Better precision attaches to the X-ray results. Full-matrix refmement, on 1011 independent reflexions, converged at Ä = 3.7 %. This refmement was indeed based on a model that was formally ordered, so far as concerns all atoms except the acidic hydrogen. However the structural results implied an averaged molecule, with the C-O distances 1.258, 1.268(2) Ä and the C-C-O angles 118.7, 117.8(1)°; and the acidic hydrogen appeared as two "half atoms" on the hydrogen bond, 0.9 Ä from each oxygen atom. These findings are most simply interpreted as due to disorder: the two configurations, A and B (of Fig. 1), occur randomly and in nearly equal proportions.Owing to difficulties inherent in the crystal texture of benzoic acid, the neutron results were less satisfactory. Large Single crystals were affected by twinning. Though the powder method avoids this difficulty, the structure, further confused by modulation, is rather too complicated for profile refmement. At 5 K however, the structure may be ordered, consisting wholly of dimers in the /1-configuration.

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The crystal structure of potassium hydrogen bisphenylacetate, KH(C,H,O,) ,, has been studied by the X-ray-diffraction method. No discrete molecule of this formula exists in the solid state. The structure consists of infinite layers of potassium and hydrogen atoms (or ions) sandwiched between pairs of layers of phenylacetate residues, whose carboxyl graups link the potassium and hydrogen atoms together from either side. Each potassium ion is at the centre of an octahedron of six approximately equidistant oxygen atoms, and each hydrogen atom is situated between two oxygen atoms and constitutes a short hydrogen bond. This bond is remarkable in being effectively symmetrical, since the two oxygen atoms lie about a crystallographic centre of symmetry. The possible significance of this observation is discussed.MOST monocarboxylic acids (HX) form, not only normal salts (e.g., KX), but also acid salts, often of the type KHX,.

Such a picture is best seen with a stereoscopic viewer.After a little practice, however, many people can achieve stereopsis without instrumental aid. The picture should be viewed in a good, even light; it helps if a piece of card is held between the eyes, and normal to the picture, so that each eye can see only its appropriate half of the diagram.