5-aminoisophthalic acid hemihydrate.

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) A and N-H...O angles ranging from 155 (2) to 163 (2) degrees. Two intermolecular O-H...O hydrogen bonds have O...O distances of 2.536 (2) and 2.746 (2) A, and O-H...O angles of 178 (2) and 176 (2) degrees. 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.