The dissolution of alkali species in water is a fundamental process in acid–base chemistry and plays a crucial role in a wide range of applications, from energy storage to pharmaceuticals.
Understanding how water molecules initiate the dissociation of alkali at the molecular level has been a long-standing challenge due to the difficulty in probing hydrogen bonding, proton transfer, and electrostatic interactions in complex solvent environments. Neutral hydrated alkali clusters serve as ideal models for investigating these early solvation processes, yet their spectroscopic study has been hindered by the lack of charge, making them difficult to detect and mass-select.
In a study published in the Journal of the American Chemical Society, a research team led by Prof. JIANG Ling and Prof. LI Gang from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) has experimentally revealed that three water molecules can separate Ba and OH in neutral BaOH(H2O)n (n = 1−5) clusters, uncovering the microscopic mechanism of alkali dissolution exemplified by hydrated BaOH clusters.
Experimental IR-VUV spectra and identified structures of hydrated BaOH clusters (Image by YAN Wenhui)
Researchers developed a neutral cluster infrared spectroscopy station based on infrared excitation and vacuum ultraviolet threshold photoionization (IR-VUV). This setup enables high-sensitivity IR spectral detection, structural characterization, and reactivity studies of mass-selected neutral clusters. Utilizing this platform along with tabletop extreme ultraviolet sources, the team measured the IR spectra of neutral BaOH(H2O)n (n = 1−5) clusters.
By comparing experimental spectra with high-level quantum chemical harmonic calculations and anharmonic molecular dynamics simulations, the researchers found that when n = 1 and 2, water molecules interact directly with BaOH via O–H⋯O hydrogen bonds without dissociation of Ba and OH. When n ≥ 3, Ba and OH dissociate to form a solvent-shared ion pair structure. Electronic structure analysis revealed that as the number of water molecules increases, charge transfer reduces electrostatic attraction, and the formation of a hydrogen-bond network promotes the separation of Ba and OH.
This work provides critical insights into the early solvation in closed-shell systems and establishes a model for understanding electrostatic and inductive interactions between ionic species and water molecules. The findings pave the way for further size-resolved studies of solvation mechanisms in chemical and biological processes.