RAMAN-SPECTROSCOPIC STUDY OF CONCENTRATED AQUEOUS LIBR SOLUTIONS

The Raman spectra for concentrated aqueous LiBr solutions (LiBr)x(H2O) 1-x and (LiBr)x(D2O)1-x, x=0.05-0.25, were measured in the frequency r ange of 30-1000 cm-1. A polarized peak intensity centered at 160-190 c m-1 in the isotropic spectrum for H2O solutions exhibits a systematic increase with increasing LiBr content. At LiBr concentrations below 15 mol%, this peak contains several shoulders at both the lower and high er frequency sides. The corresponding peak in the isotropic spectrum f or D2O solutions is shifted to the lower frequency by ca. 10 cm-1. The isotropic spectra for highly concentrated H2O solutions, such as 20 a nd 25 mol% LiBr, have another component at 340 cm-1, which is also shi fted to the lower frequency by ca. 20 cm-1 in D2O solUtiOns' This 340 cm-1 peak in a 25 mol% LiBr solution is also shifted to the higher fre quency by ca. 13 cm-1 when isotopically substituting Li-6 (95.5% 6 Li) for (nat)Li (92.5% Li-7), while the position of 190 cm-1 peak remains unchanged when substituting. Consequently, the peaks centered at 160- 190 cm-1 and at 340 cm-1 can, respectively, be assigned to the totally symmetric stretching vibration of tetrahedrally hydrated lithium ion, Li+(H2O)4, and that of the solvent-shared ion pair, Li+(H2O)4 ... Br- , by taking into consideration the results of diffraction studies repo rted previously.