THE COORDINATION STRUCTURE OF LI+ IN HIGHLY CONCENTRATED METHANOLIC LIBR AND LII SOLUTIONS

Time-of-flight neutron diffraction measurements have been carried out on 25 mol% LiBr and 33 mol% LiI solutions in CD3OD, using the HIT-II s pectrometer installed at KENS. The Li-6/Li-7 isotopic substitution tec hnique was applied to lithium ions in both solutions in order to deter mine the coordination structure around the lithium ion in highly conce ntrated non-aqueous solutions. The partial distribution function aroun d Li+, G(Li)(r), has been derived from the Fourier transform of the fi rst-order difference function, Delta(Li)(Q), obtained from a numerical difference in observed scattering cross-sections between two solution s with different Li-6/Li-7 composition. The Li+ ... methanol molecule configuration is characterized by the intermolecular distance and coor dination number, r(LiO) = 1.97 +/- 0.05 Angstrom and n(LiO) = 3.0 +/- 0.5 for the LiBr solution and r(LiO) = 1.93 +/- 0.05 Angstrom and n(Li O) = 1.8 +/- 0.5 for the LiI solution, respectively. These values indi cate that Li+ has a well-defined first coordination shell. The possibi lity of the formation of a contact ion pair, Li+ ... X(-) (X = Br or I ), in such a highly concentrated solution is discussed using the prese nt data.