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

Abstract

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.