R. Feld, M. Lehmann, K. Muir
Dec 1, 1981
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Zeitschrift für Kristallographie - Crystalline Materials
Abstract
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. 216 R. Feld et al. : The crystal s tructure of Benzoic Acid