Neutron diffraction study of sodium hydrogen maleate trihydrate, NaH[C4H2O4].3H2O, at 120 K

Abstract

M r = 192.10, triclinic, P1, a = 5.927 (2), b=6.288(2), c= 11.222(4)A, it= 104.00(2), /~= 91.47 (2), ~,= 100.26 (2) ° , V= 398.3 (3)/k 3, Z= 2, D x = 1.60 (2) g cm -3, 2 = 0.8323 (4)/~, /t = 1.62 (5) cm -1, F(000) = 118, T= 120 (1.5) K. Final R = 0.033 for 2508 independent reflexions. The hydro- gen maleate ion forms a close to planar ring structure containing a short but still very asymmetric hydrogen bond: O...O - 2.445 (1), O-H = 1.079 (2) and H...O = 1.367 (2)/k. The three water molecules are crystaUographically independent, each with a distorted tetrahedral environment. A comparison is made with intramolecular hydrogen-bond schemes in other hydro- gen maleates. Introduction. In the current hydrogen-bond project an emphasis is presently laid on detailed studies of geometry and electron density of strong hydrogen bonds and of water molecules in different crystal environments. Nail maleate.3H20 would appear a particularly suitable choice as it contains a strong hydrogen bond, on which no crystallographic sym- metry restrictions are imposed, as well as three independent water molecules. Furthermore, for the purposes of electron-density studies, it has the advan- tage that it is centrosymmetric (P1) and contains only light elements. X-ray diffraction studies of the title compound at room temperature by Weissenberg film methods have been reported briefly by Gupta, Prasad & Yadav (1972) and by Gupta & Yadav (1974). The prime purpose of the present neutron diffraction work at 120 K has been to study the detailed geometry of the short intramolecular hydrogen bond and to obtain accurate thermal parameters and hydrogen positions for a subsequent electron density study using the X-N technique. * Hydrogen-Bond Studies. 146. Part 145: Hermansson & Lunell (1982). Experimental. Crystal growth: Nail maleate.3H20 prepared from sodium hydroxide and maleic acid; recrystallized several times from aqueous solution. Single crystals grown by slow evaporation of a saturated aqueous solution at room temperature. A crystal of volume 15 mm a (polyhedron --,1.6 × 2.9 x 4.5 mm) was mounted on an aluminium pin and covered with polystyrene film by dipping in chloroform solution to prevent water loss. Crystal data: Cell dimensions determined at 120 K from single-crystal X-ray diffraction data (Olovsson, Kvick & Olovsson, 1984). Linear absorption coeffi- cient determined experimentally by measuring the attenuation of a narrow beam of neutrons through different Nail maleate.3H20 crystals. From the coefficients for true absorption for Na, H, C and O (corrected to our wavelength, 0.8323 (4),~) and the incoherent scattering cross-sections for Na, C and O as tabulated in International Tables for X-ray Crystal- lography (1968, 1974), a value of 36 (1) × 102 fm 2 was calculated for the mean incoherent scattering cross- section for H. Data collection and reduction: Intensity data col- lected at 120 K on a four-circle diffractometer (D9) at the ILL (Institut Laue-Langevin) High-Flux Reactor in Grenoble. Primary beam monochromatized by a Cu crystal ((200) plane in transmission). Detector: 3He proportional counter. A closed-cycle Displex® system (Model CS-1003, Air Products and Chemicals Inc.) was used for cooling. The crystal was enclosed in a vanadium cylinder which was further surrounded by an evacuated aluminium can. A phase transition in KH2PO 4 at 122.5 K was used to check the tem- perature. A difference of 1.0 K between the thermo- couple and crystal was noted. Temperature generally constant within +1.5 K. Flux at sample 3 ×106 neutrons cm-2s -1 at a wavelength of 0.8323 (4) A. A 0.25 mm erbium foil was placed in front of the monitor to filter out the second-order Q,/2) component in the beam. 0-2t~ step-scan technique, scan interval for the peak 0.6, 0.6, 1.0, 1.5 and 2.4 ° for 0 values 0, 15, 25,