Structure of suberic acid at 18.4, 75 and 123 K from neutron diffraction data.

Neutron diffraction data for suberic acid [HOOC(CH2)6COOH] were collected at 18.4, 75 and 123 K using a twinned crystal. The neutron data reduction included derivation of a complete set of corrected intensities, as if from a single crystal. This was followed by full-matrix structure refinement in the usual way. The molecule has an almost fully extended conformation with the hydrocarbon chains packed in an orthorhombic mode. Molecules form infinite hydrogen-bonded chains with crystallographic inversion centers occuring at the center and at the ends of each molecule. We suggest that at the twin boundary the hydrogen bonding is different, involving only the hydroxyl groups as both donors and acceptors. Accurate bond lengths have been obtained with corrections for thermal vibration (harmonic for C--C, C--O; harmonic and anharmonic for C--H). Values at the three temperatures agree well in terms of their e.s.d.'s (0.001 A for C--C and C--O and 0.005 A for C--H). Similar agreement is obtained for the corrected bond angles (e.s.d.'s 0.1 degrees for C--C--C, 0.2 degrees for H--C--H). For the methylene groups, the observed m.s. displacement parameters at each temperature are significantly greater at the middle of the molecule than at the ends. This indicates that the molecular backbone is vibrating internally. The thermal vibrations of the molecule have been analysed in terms of a simple segmented body model.