Raman Spectroscopic Study of Concentrated Aqueous LiBr Solutions

Abstract

Abstract 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 range of 30—1000 cm−1. A polarized peak intensity centered at 160—190 cm−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 higher frequency sides. The corresponding peak in the isotropic spectrum for D2O solutions is shifted to the lower frequency by ca. 10 cm−1. The isotropic spectra for highly concentrated H2O solutions, such as 20 and 25 mol% LiBr, have another component at 340 cm−1, which is also shifted 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 frequency by ca. 13 cm−1 when isotopically substituting 6Li (95.5% 6Li) for natLi (92.5% 7Li), 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 reported previously.