Structure of sodium para-hydroxybenzoate, NaO₂C–C₆H₄OH by powder diffraction: application of a phenomenological model of anisotropic peak width

The ab initio structure solution of sodium para-hydroxybenzoate from high-resolution X-ray powder diffraction data is reported. The compound is of interest with respect to understanding the mechanism of Kolbe–Schmitt type reactions. It crystallizes in space group P2₁, Z = 2, with unit-cell parameters a = 16.0608 (3), b =  5.38291 (9), c =  3.63834 (6) Å, β =  92.8692 (5)° and V =  314.153 ų. The compound consists of layers of distorted NaO₆ prisms perpendicular to the a axis and phenol rings perpendicular to these layers pointing up and down. The molecular structure is held together by van der Waals forces between the phenyl groups of different layers and additional hydrogen-bridge bonding between the phenolate oxygen atoms. The sample showed powder peak widths which are not a smooth function of diffraction angle; a recently implemented phenomenological model was able to describe this effect sufficiently well to obtain excellent Rietveld fits to the data. The accuracy of modeling the data makes this one of the rare cases where the position of a hydrogen atom could be unambiguously determined by powder techniques.