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The title acid, C8H7NO4.0.5H2O, crystallized in the centrosymmetric space group C2/c in a zwitterionic form (5-ammonioisophthalate), with the water molecule on a twofold axis. The three ammonio H atoms and the H atom on the remaining carboxyl group, which are involved in hydrogen bonding, are ordered. Three intermolecular N—H
O hydrogen bonds have N
O distances ranging from 2.762 (2) to 2.905 (2) Å and N—H
O angles ranging from 155 (2) to 163 (2)°. Two intermolecular O—H
O hydrogen bonds have O
O distances of 2.536 (2) and 2.746 (2) Å, and O—H
O angles of 178 (2) and 176 (2)°. A three-dimensional network of hydrogen bonds is present. Through basic second-level graphs involving acid-to-acid hydrogen bonds, chains are more numerous than rings.
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