A THz/FTIR fingerprint of the solvated proton: evidence for Eigen structure and Zundel dynamics

Abstract

In continuation of earlier work on La(III), Ni(II) and Mn(II) halides, we present low frequency (30–400 cm⁻¹) spectra of solvated HCl and HBr as a function of solute concentration. This frequency range provides direct access to water network modes and changes induced by solvated solutes. We were able to dissect the spectra into components associated to solvated ions and ion pairs using a chemical equilibrium model in combination with principal component analysis. While the Cl⁻ rattling mode at 190 cm⁻¹ is found to be unchanged, the Br⁻ resonance around 90 cm⁻¹ is decreased in intensity below the detection threshold when replacing the divalent or trivalent metal ions by a proton. The solvated proton shows two resonances: a solvation water mode around 140 cm⁻¹ and a high frequency resonance at 325 cm⁻¹ that we assign to the rattling motion of an Eigen structure H₃O⁺ in its solvation cage. This assignment is corroborated by isotopic substitution measurements which show a redshift of the high frequency peak when HCl/H₂O is replaced by DCl/D₂O. The linewidth of the H₃O⁺ rattling mode corresponds to a relaxation time of the oscillatory process of τ ≈ 60 fs, considerably faster than the relaxation time of τ ≈ 160 fs for Cl⁻. In addition, we find a broad background that we attribute to fast non-oscillatory motions of a proton in a Zundel-like complex. Our results are in agreement with an Eigen–Zundel–Eigen (EZE) model of proton transport. Upon ion pairing the broad background is strongly reduced indicating a reduction of fast proton transfer processes. The Cl⁻ resonance blueshifts by 20 cm⁻¹ which indicates a transition from free ions to a solvent shared ion pair. Surprisingly, the center frequency of the Eigen complex does not change upon ion pairing. This can be rationalized in terms of an unchanged local solvation structure.