2006 Volume 79 Issue 2 Pages 228-236
Neutron diffraction measurements have been carried out at 25 °C for aqueous 18 mol % lithium alaninate heavy water solutions, [C*H3C*H(*ND2)COO*Li]0.18(D2O)0.82, in which the isotopic compositions 14N/15N, HM/DM (HM: methyl-hydrogen atom), and HM′/DM′ (HM′: methine-hydrogen atom) within the alaninate ion, and 6Li/7Li for the lithium ion were changed. The hydration structures of both alaninate and lithium ions were derived from the least squares fitting analysis of observed first-order difference functions, ΔN(Q), ΔHM(Q), ΔHM′(Q), and ΔLi(Q). It was revealed that the amino group of the alaninate ion forms a hydrogen bond of the N···Dw1–OwDw2 type (Ow and Dw denote water–oxygen and water–deuterium atoms, respectively) with ca. one D2O molecule (rNDw1 = 2.00(1) Å, ∠N···Dw1–Ow = 165(10)°), and simultaneously forms hydrogen bonds of the N···OD2 type with 1.4(1) D2O molecules (rNO = 2.95(5) Å, rND = 3.35(5) Å). The numbers of water molecules neighboring hydrogen atoms within the methyl- and methine-group of the alaninate ion were determined to be 0.62(1) and 0.86(9), respectively. The first coordination shell of Li+ was found to consist of 2.42(5) D2O molecules and 2.3(2) alaninate ions. The bond angle ∠Li+···Oc–C (Oc: carboxyl-oxygen atom) and the dihedral angle between the plane involving atoms Li+···Oc–C and the plane of the carboxyl group of the alaninate ion were determined to be 101(1) and 74(1)°, respectively.
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