Low-Frequency Modes of Aqueous Alkali Halide Solutions: An Ultrafast Optical Kerr Effect Study

Abstract

A detailed picture of aqueous solvation of ions is central to the understanding of diverse phenomena in chemistry and biology. In this work, we report polarization resolved THz time domain measurements of the Raman spectral density of a wide range of aqueous salt solutions. In particular, the isotropic Raman spectral density reveals the frequency of the hydrogen bond formed between the halide ion and water. The frequency of this mode is measured for the series Cl(-), Br(-), and I(-) as a function of concentration, cation size, and charge. The frequencies extrapolated to zero concentration permit an estimation of the force constant of the mode, which is found to decrease with increasing halide mass and to be similar to the force constant associated with the water-water hydrogen bond. This result is consistent with recent calculations. The extrapolation of the frequency of the chloride hydrogen bond to zero concentration reveals a dependence of the frequency on the nature of the cation. This is ascribed to an interaction between the solvated anion and cation even at the lowest concentration studied here (<0.15 M). It is suggested that this behavior reflects the influence of the electric field of the cation on the hydrogen bond of an adjacent anion. Such interactions should be taken into account when modeling experimental data recorded at concentrations of ions in excess of 0.1 M. These measurements of the isotropic Raman spectral density are compared with those for the anisotropic response, which reflects the frequencies of the full range of hydrogen bonds in aqueous salt solutions. The anisotropic spectral density recovered can be modeled in terms of a concentration-dependent population of water-water H-bonds with a frequency unaffected by the ions, the halide-water hydrogen bonds, and a low-frequency collision-induced contribution.

Water Dynamics at Protein Interfaces: Ultrafast Optical Kerr Effect Study

“…For example, they play a 24 crucial role in the folding and function of proteins and 25 enzymes, whereas the structure and conformation of DNA 26 depends on the hydration water. 1,2 Therefore, the understand- 27 ing of water at these interfaces is of great importance in 28 chemistry and biology. 3À7 29 Water molecules in aqueous solutions of proteins can be 30 grouped into three broad categories: (1) the internal waters that 31 occupy specific sites in the protein and can be identified crystal- 32 lographically, (2) the hydration water immediately surrounding 33 the protein, and (3) bulklike water.…”

Raman vibrational dynamics of hydrated ions in the low-frequency spectral region

“…[15] Indeed early on, we were able to identify clear spectral signatures for doubly and triply charged cations such as calcium, magnesium and aluminium. [1] Nevertheless, singly charged cations did not seem to produce well-defined low frequency modes, the exception being lithium (Li + ) which, due to its strong interaction with water oxygen, presented a clear isotropic vibrational mode in the region around 220 cm -1 . [16] In this work we present a series of measurements intended to identify low frequency vibrational features related to the hydrated cation and also to investigate the effect on those vibrational modes of changing the solute concentration varying the counter-ion.…”

“…[1][2][3][4] Characteristic low frequency modes can reveal information about ion hydration, ionic dissociation and ion-pair formation. [5] The understanding of the structure and dynamics of ion hydration is important, especially in the fields of chemistry and biology.…”

“…[1,5,6,10,11] The latter techniques such as ND and X-ray are able to provide detailed structural information whereas DRS enables determination of hydration numbers and the association constants for ion pairs in differing conditions, such as solvent shared or separated or contact ion pairs. [5] Recently it was shown that by measuring the isotropic optical Kerr effect (OKE) response of aqueous alkali halide and hydroxide solutions, the hydrogen bond (H-bond) vibration between water and anions could be observed in the time domain.…”

“…[5] Recently it was shown that by measuring the isotropic optical Kerr effect (OKE) response of aqueous alkali halide and hydroxide solutions, the hydrogen bond (H-bond) vibration between water and anions could be observed in the time domain. [1,2] Through a Fourier Transform procedure, it proved possible to recover the low frequency Raman spectrum which showed well-defined modes related to the anion-water H-bond vibration in those solutions. This in-turn permitted the unambiguous determination of some important water-anion H-bond parameters, such as frequency, force constant and dephasing…”