STRUCTURE OF THE ADIPATE COMPLEX [NA2(C6H9O4)2(C6H10O4)].2H2O FROM NEUTRON-DIFFRACTION AT 220 AND 295 K

The crystal structure of sodium hydrogen adipateadipic acid (2/1) dihydrate, [Na2(C6H9O4)2(C6H10O4)].2H2O, M(r) = 518.4, has been determined from neutron diffraction data collected at 220 and 295 K. Crystals are monoclinic, space group C2/m, with Z = 2. At 295 K, a = 9.378 (2), b = 13.379 (5), c = 10.247 (3) angstrom, beta = 95.93 (3)-degrees, V = 1278.8 (7) angstrom 3, D(n) = 1.346, D(m) = 1.348 (1) g cm-3 (in dibromomethane/bromobutane), lambda = 1.1588 (2) angstrom, mu = 2.186 cm-1, F(000) = 176.4 fm, R(F2) = 0.108 for all 1995 non-equivalent reflections with sin-theta/lambda < 0.71 angstrom -1. The crystal structure is similar at 220 K except for reduced nuclear mean-square displacements. Hydrogen adipate subunits (called A) are linked end-to-end in infinite chains by very short O...O (2.44 angstrom) hydrogen bonds where the H nuclei are on centers of symmetry within the experimental error. The Na cation is octahedrally coordinated by O atoms from molecules A and also by non-ionized adipic acid molecules (called B). The B molecules have large mean-square nuclear displacements which are described in terms of anharmonicity and disorder.