The Interplay of Structure and Dynamics in the Raman Spectrum of Liquid Water over the Full Frequency and Temperature Range.

While many vibrational Raman spectroscopy studies of liquid water have investigated the temperature dependence of the high-frequency O-H stretching region, few have analyzed the changes in the Raman spectrum as a function of temperature over the entire spectral range. Here, we obtain the Raman spectra of water from its melting to boiling point, both experimentally and from simulations using an ab initio-trained machine learning potential. We use these to assign the Raman bands and show that the entire spectrum can be well described as a combination of two temperature-independent spectra. We then assess which spectral regions exhibit strong dependence on the local tetrahedral order in the liquid. Further, this work demonstrates that changes in this structural parameter can be used to elucidate the temperature dependence of the Raman spectrum of liquid water and provides a guide to the Raman features that signal water ordering in more complex aqueous systems.

Decomposition of the Experimental Raman and Infrared Spectra of Acidic Water into Proton, Special Pair, and Counterion Contributions.

Textbooks describe excess protons in liquid water as hydronium (H3O+) ions, although their true structure remains lively debated. To address this question, we have combined Raman and infrared (IR) multivariate curve resolution spectroscopy with ab initio molecular dynamics and anharmonic vibrational spectroscopic calculations. Our results are used to resolve, for the first time, the vibrational spectra of hydrated protons and counterions and reveal that there is little ion-pairing below 2 M. Moreover, we find that isolated excess protons are strongly IR active and nearly Raman inactive (with vibrational frequencies of ∼1500 ± 500 cm-1), while flanking water OH vibrations are both IR and Raman active (with higher frequencies of ∼2500 ± 500 cm-1). The emerging picture is consistent with Georg Zundel's seminal work, as well as recent ultrafast dynamics studies, leading to the conclusion that protons in liquid water are primarily hydrated by two flanking water molecules, with a broad range of proton hydrogen bond lengths and asymmetries.

Water-mediated aggregation of 2-butoxyethanol.

Water plays an important role in mediating hydrophobic interactions, and yet open questions remain regarding the magnitude, and even the sign, of water-mediated contributions to the potential of mean force between a pair of oily molecules dissolved in water. Here, the water-mediated interaction between 2-butoxyethanol (BE) molecules dissolved in water is quantified using Raman multivariate curve resolution (Raman-MCR), molecular dynamics (MD) simulations, and random mixing (RM) predictions. Our results indicate that the number of contacts between BE molecules at concentrations between 0.2 M and 1 M exceeds RM predictions, but is less than some MD predictions. Moreover, the potential of mean force between BE molecules in water has a well depth that is shallower than the direct interaction between 1-ethoxybutane chains in the gas phase, and thus the water-mediated contribution to BE aggregation is repulsive, as it pulls BE molecules apart rather than pushing them together.

Influence of Cononsolvency on the Aggregation of Tertiary Butyl Alcohol in Methanol-Water Mixtures.

The term cononsolvency has been used to describe a situation in which a polymer is less soluble (and so is more likely to collapse and aggregate) in a mixture of two cosolvents than it is in either one of the pure solvents. Thus, cononsolvency is closely related to the suppression of protein denaturation by stabilizing osmolytes. Here, we show that cononsolvency behavior can also influence the aggregation of tertiary butyl alcohol in mixtures of water and methanol, as demonstrated using both Raman multivariate curve resolution spectroscopy and molecular dynamics simulations. Our results imply that cononsolvency results from the cosolvent-mediated enhancement of the attractive (solvophobic) mean force between nonpolar groups, driven by preferential solvation of the aggregates, in keeping with Wyman-Tanford theory.