Coordination structure of Li+ in concentrated aqueous lithium alaninate solutions

Time-of-Flight (TOF) neutron diffraction measurements with the 6Li/7Li isotopic substitution technique were carried out for aqueous 18 mol% lithium alaninate solutions in D2O in order to obtain structural information on the interaction between Li+ and the amino acid molecule in the concentrated aqueous solution. The first-order difference function, ΔLi(Q), was obtained from the difference between scattering cross sections observed for solutions involving dl-CH3CH(ND2)COO6Li and dl-CH3CH(ND2)COO7Li. The distribution function around Li+, GLi(r), exhibits well resolved first and second peaks that are tentatively attributable to the nearest neighbor Li+ ⋯ O and Li+ ⋯ D interactions. However, the coordination number, nLiD, estimated from the area under the second peak is found to be much larger than the value, 2nLiO, evaluated from the first peak of the present GLi(r). This implies that the nearest neighbor Li+ ⋯ alaninate ion interaction may contribute to the second peak. Structural parameters concerning the first coordination shell of the Li+ has been determined through the least squares fitting analysis of the observed ΔLi(Q). It has been revealed that the Li+ is surrounded by 2.4(1) D2O molecules and 2.3(2) alaninate ions with interatomic distances of r(Li+ ⋯ OD2O) = 1.97(1) Å, r(Li+ ⋯ DD2O) = 2.62(1) Å and r(Li+ ⋯ Oalaninate) = 2.38(1) Å, respectively.