Abstract-Terahertz echoes reveal the inhomogeneity of aqueous salt solutions

• Andrey Shalit,
• Saima Ahmed,
• Janne Savolainen
• & Peter Hamm

• http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2642.html

The structural and dynamical properties of water are known to be affected by ion solvation. However, a consistent molecular picture that describes how and to what extent ions perturb the water structure is still missing. Here we apply 2D Raman–terahertz spectroscopy to investigate the impact of monatomic cations on the relaxation dynamics of the hydrogen-bond network in aqueous salt solutions. The inherent ability of multidimensional spectroscopy to deconvolute heterogeneous relaxation dynamics is used to reveal the correlation between the inhomogeneity of the collective intermolecular hydrogen-bond modes and the viscosity of a salt solution. Specifically, we demonstrate that the relaxation time along the echo direction t1 = t2 correlates with the capability of a given cation to ‘structure’ water. Moreover, we provide evidence that the echo originates from the water–water modes, and not the water–cation modes, which implies that cations can structure the hydrogen-bond network to a certain extent.

Abstract-Two-dimensional Raman-terahertz spectroscopy of water

1. 
   1. 
      
2. Janne Savolainen, 
3. Saima Ahmed, and 
4. Peter Hamm
5. 1. http://www.pnas.org/content/early/2013/11/27/1317459110.short

1. Department of Chemistry, University of Zurich, CH-8057 Zurich, Switzerland

1. Edited by Andrei Tokmakoff, The University of Chicago, Chicago, IL, and accepted by the Editorial Board November 10, 2013 (received for review September 16, 2013)

Significance

The discussion about the structure of water becomes increasingly more controversial, and appears to be stuck because the major techniques used to study it (small-angle X-ray scattering emphasizing the structure, multidimensional infrared spectroscopy emphasizing the dynamics, and molecular dynamics simulations using questionable force fields) do not seem to provide mutually converging results. This calls for a different experimental approach, which we offer here with 2D Raman-terahertz spectroscopy. The work extends multidimensional vibrational spectroscopy into the far-IR regime where thermally excited soft modes are found that are directly responsible for molecular dynamics (in contrast with the high-frequency spectator modes investigated in 2D-IR spectroscopy, which per se are quiet at room temperature and sense the environment only indirectly).

Abstract

Two-dimensional Raman-terahertz (THz) spectroscopy is presented as a multidimensional spectroscopy directly in the far-IR regime. The method is used to explore the dynamics of the collective intermolecular modes of liquid water at ambient temperatures that emerge from the hydrogen-bond networks water forming. Two-dimensional Raman-THz spectroscopy interrogates these modes twice and as such can elucidate couplings and inhomogeneities of the various degrees of freedoms. An echo in the 2D Raman-THz response is indeed identified, indicating that a heterogeneous distribution of hydrogen-bond networks exists, albeit only on a very short 100-fs timescale. This timescale appears to be too short to be compatible with more extended, persistent structures assumed within a two-state model of water.

Footnotes

• 1To whom correspondence should be addressed. E-mail: phamm@pci.uzh.ch.

• Author contributions: J.S. and P.H. designed research; J.S. and S.A. performed research; J.S., S.A., and P.H. analyzed data; and J.S. and P.H. wrote the paper.
• The authors declare no conflict of interest.
• This article is a PNAS Direct Submission. A.T. is a guest editor invited by the Editorial Board.
• This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1317459110/-/DCSupplemental.