L. R. Andrade, Μ. M. R. Costa, P. Filipa
Apr 1, 2001
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Zeitschrift für Kristallographie - New Crystal Structures
Abstract
CioH26lN206Rb, triclinic, P\ (No. 2), a = 5.6871(9) Ä, b = 5.894(1) Ä, c = 14.192(1) Ä, α = 83.24(1)°, β = 82.72(1)°, γ = 73.56(2)°, V = 450.9 Ä, Ζ = 1, Rgt(F) = 0.027, wRTef(F) = 0.073, T= 293 K. Source of material The title compound was synthesized from a saturated water solution of ionic rubidium iodide and betaine. A large single crystal was grown from a seed selected amongst the small grains of the polycrystalline precipitate obtained. A crystal of good quality and appropriate dimensions for X-ray diffraction was cut from the larger one. Experimental details The structure was solved by direct methods. Hydrogen atoms participating in C—Η bonds were placed at calculated positions and refined as riding [1], Positional parameters for the hydrogen atoms of the water molecules were obtained from the results of a difference Fourier synthesis and refined as riding. Discussion Betaine rubidium iodide dihydrate (C5HnN02)2RbI · 2H2O is a member of a new family of hydrates whose general formula is (C5H1 iN02)n · AB · 2H20 (m = 1,2; A = K, Rb; Β = Br, I) [2], The study of dielectric properties of this compound revealed the existence of two phase transitions, one at approximately 100 K, similar to that observed in betaine potassium iodide dihydrate (BKI) and a new one at about 190 K. Examination of the crystal structure shows that it is isomorphic with that of BKI [3]. Rubidium atoms occupy special positions, each one being octahedrally coordinated by four oxygen atoms: two of these belong to betaine molecules lying in a plane with orientation close to (010) and the remaining are from two water molecules. The iodine atom, also at a special position, is bonded to the carboxylic group of the aminoacid by Coulomb interactions. Hydrogen bonds provide links between water and betaine molecules which form zigzag chains running throughout the structure. Features concerning the interactions in which carboxylic atoms Ol and 0 2 are involved are similar to those observed in the structure of BKI [3]. The same applies to the hydrogen bonds between neighbouring water molecules; an abnormally low angle (H31-0-H32) was found in both cases; this may be a sign of some degree of rotational disorder of this molecule. There are no solvent accessible voids in the crystal lattice [4], Table 1. Data collection and handling. Crystal: colourless transparent prism. size 0.1 χ 0 .2x0 .25 mm Wavelength: Mo Ka radiation (0.71073 Ä) μ : 44.79 cm" 1 Diffractometer, scan mode: Enraf-Nonius CAD4, ω/2θ