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

Abstract

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.