Twinned α p-Aminobenzoic Acid at 106 K
The α crystalline form of p-aminobenzoic acid is monoclinic and prone to twinning by pseudo-merohedry (a ≈ c). A structure was reported by Lai and Marsh (1967) on an almost untwinned specimen where (a-c)/σ is 36. In revisiting this structure, a sample showing twinning was obtained from the metabolites of a Streptomyces strain collected from the soil at the ancient Roman city of Pollentia on the Balearic island of Mallorca. A 106(2) K determination shows pseudo-merohedral twinning, with a = 18.5452(6), b = 3.73406(15), c = 18.5484(7) Å, β = 93.790(3)°, V = 1281.65(8) Å3, and (a−c)/σ 5. The refinement included twin contributions [major domain 0.7435(15)] and produced a higher resolution structure determination. The cell contains two neutral molecules per asymmetric unit. Both form complementary carboxylic acid dimers across inversions centers of P21/n. Nitrogens are pyramidal and donate Hydrogen-bonds through one amine hydrogen; a stronger two-center N–H⋯O in one case, and a weaker three-center N–H⋯(O, O’) in the other. The molecular structure shows a pronounced quinoid distortion, and a comparison is drawn with the previous studies, an ordered β monoclinic phase, and with other structural studies.
The α monoclinic phase of p-aminobenzoic acid crystallizes commonly as pseudo-merohedral twins, and a specimen studied at 106 K had a ≈ c within 5 esd’s, allowing an improved refinement and a more accurate molecular model.
Keywordsp-Aminobenzoic acid Twinning Pseudo-merohedry Hydrogen-bonding
We thank the custodians of the Pollentia archeological site, Miguel Ángel Cau Ontiveros, Esther Chávez Álvarez, Fr. Richard Rutherford for access and guidance. We thank the University of Portland and the College of Arts and Sciences for summer research support for the Pollentia Undergraduate Research Expedition to Mallorca, Spain, and the National Science Foundation for support of crystallographic studies (MRI-0604188) and for LCMS equipment (MRI-0621648).
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Conflicts of interest
The authors declare no conflicts of interest.
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