Rubidium hydrogen glutarate and ammonium hydrogen glutarate: X-ray studies of quasi-isostructural crystals involving very short hydrogen bonds

Rubidium and ammonium hydrogen glutarates (M ^{+ –}CO₂.C₃H₆.CO₂H) each crystallize with four molecules in orthorhombic unit cells of almost identical dimensions: (M = Rb)a = 18·55,b = 7·57,c = 5·29 Å; (M = NH₄)a= 18·59,b= 7·56,c= 5· 27 Å. They have very similar structures, but they are not isomorphous: space groupsCmma (Rb) andPmaa (NH₄).The crystal structures have been determined by three-dimensional X-ray analysis and refined with moderate precision. Both have infinite chains of glutarate residues, linked end-to-end by very short, symmetrical hydrogen bonds, with O···H···O = 2·40(2) (Rb) and O···H···O = 2·460(6) Å (NH₄).In the Rb salt, the glutarate residue has strict, crystallographicm symmetry. The difference in the NH₄ salt arises from small movements of the carbon and (especially) the oxygen atoms out of this symmetry plane. These are due to N-H···O bonding: two equivalent cations in the Rb salt, each making contact with eight oxygen atoms, become differentiated in the NH₄ salt by linkage to different sets of four oxygens.We wish to acknowledge financial support from the Science Research Council, and our indebtedness to Drs. J. G. Sime, K. W. Muir and others for help with KDF9 programs.