Twinned α p-Aminobenzoic Acid at 106 K

Abstract

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) A, β = 93.790(3)°, V = 1281.65(8) A3, 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.