Terahertz Spectra of Microsolvated Ions: Do They Reveal Bulk Solvation Properties?

Complementing mid-infrared (mid-IR) spectroscopy mainly in the OH stretching region, liquid-state far-IR spectroscopy is successful in elucidating the properties of aqueous solutions by providing direct access to the hallmark of H-bonding at terahertz (THz) frequencies, namely, the H–bond network peak of water at roughly 200 cm^–1 and its modifications in the hydration shells around solutes. Here, the idea is scrutinized whether ion hydration can be understood by studying the THz regime of “small” ion–water clusters in the gas phase as a function of size with subsequent extrapolation to the bulk limit. Our ab initio simulations of Na⁺(H₂O)_n clusters followed by rigorous decomposition of their THz response demonstrate that the 200 cm^–1 network peak is suppressed even at n = 20 in the gas phase, yet it emerges when transferring ion–water complexes as small as n = 7 out of the liquid into vacuum. The underlying physical reason is not missing electronic polarization or charge-transfer effects in the gas phase, but rather the distinctly different structural dynamics of finite ion–water clusters in the gas phase compared to ion–water complexes of the same size in the liquid phase.