| Summary: | The redetermined crystal structure of the title compound, C11H13N2O2+·Br−, is reported. Data collection at 100 K about three crystallographic axes resulted in a crystal structure with significantly higher precision in comparison to the two-dimensional data collected at 176 K [Takigawa et al. [(1966) Bull. Chem. Soc. Jpn, 39, 2369–2378]. The carboxyl group and indole ring system are planar, with maximum deviations of 0.002 (2) and 0.007 (2) Å, respectively, and make an angle of 70.17 (1)° with each other. The molecules are arranged in double layers of carboxyl and amino groups parallel to the ab plane, stabilized by an extensive network of N—H...Br and O—H...Br hydrogen bonds. The polar layer is held together by a network of three N—H...Br hydrogen bonds and one O—H...Br hydrogen bond. In the non-polar layer, the indole rings are linked mainly by electrostatic N—H...C interactions between the polarized bond N—H (H is δ+) of the pyrrole unit and two of the ring C atoms (δ−) of the benzene rings of adjacent molecules. The distances of these electrostatic interactions are 2.57 and 2.68 Å, respectively. C—H...O and C—H...π interactions are also present.
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