The crystal structure of Benzoic Acid: a redetermination with X-rays at room temperature; a summary of neutron-diffraction work at temperatures down to 5 K

The crystal structure of benzoic acid, C₆H₅CO₂H, has been redetermined by X-ray diffraction at room temperature. Extensive neutron-diffraction 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 (1)], 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 refinement, on 1011 independent reflexions, converged at R = 3.7%. This refinement 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) Å 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 Å 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 refinement. At 5 K however, the structure may be ordered, consisting wholly of dimers in the A-configuration.