Concentrated aqueous lithium chloride solution dynamics: The role of chemical exchange on anisotropy and vibrational population relaxations.

Ultrafast polarization-selective pump-probe experiments, conducted on the OD stretch of dilute HOD, are reported for LiCl/H2O solutions ranging from 1-24 to 1-128 (ion pairs-water molecules), 2.3-0.4 m. The results are compared to prior and revised experiments on 1-4 to 1-16 concentrations, 13.9-3.5 m. Vibrational population relaxation and anisotropy decays were measured for hydroxyls hydrogen-bonded to chlorides (HBCs). In contrast to higher salt concentrations, at ≤∼1-32 (1.7 m salt), the HBC population relaxation times and anisotropy decays are concentration independent. 1-32 marks a transition from high concentrations of ion pairs, clusters, and ion networks to concentrations of ion pairs low enough not to affect observable molecular level dynamics. At a concentration of approximately 1-32 and lower salt concentrations, chemical exchange is responsible for HBC anisotropy decay and plays a role in population relaxation. Wavelength-dependent population relaxation was used to obtain lifetime amplitude spectra (LAS), which show distinct species that are not observable with FT-IR. At very high salt concentrations, e.g., 1-6, there are no "pure" water regions, and the LAS has two bands: HBCs and hydroxyls of water oxygens solvating Li+. At lower salt concentrations, there is also a "pure" water band in the LAS. The HBC band shape is concentration independent from 1-4 to 1-128.