Ammonium and isopropylammonium salts of the fumaric acid dianion

The crystal structures of diammonium fumarate, 2NH(4)(+).C4H2O42-, (I), and bis(isopropylammonium) fumarate, 2C(3)H(7)NH(3)(+).C4H2O42-, (II), have been determined. The intermolecular N-H ... O hydrogen bonds form a three-dimensional network in (I), whereas two-dimensional sheets are formed in (II) separated by layers of isopropyl groups. The N ... O distances are 2.790 (1)-2.903 (1) Angstrom in (I) and 2.747 (1)-2.807 (1) Angstrom in (II). Differences between the solid-state photoreactivities of the fumaric acid ions in (I) and (II) are rationalized based on the crystal structures.