Hiding in the Crowd: Spectral Signatures of Overcoordinated Hydrogen-Bond Environments.

Molecules with an excess number of hydrogen-bonding partners play a crucial role in fundamental chemical processes, ranging from anomalous diffusion in supercooled water to transport of aqueous proton defects and ordering of water around hydrophobic solutes. Here we show that overcoordinated hydrogen-bond environments can be identified in both the ambient and supercooled regimes of liquid water by combining experimental Raman multivariate curve resolution measurements and machine learning accelerated quantum simulations. In particular, we find that OH groups appearing in spectral regions usually associated with non-hydrogen-bonded species actually correspond to hydrogen bonds formed in overcoordinated environments. We further show that only these species exhibit a turnover in population as a function of temperature, which is robust and persists under both constant pressure and density conditions. This work thus provides a new tool to identify, interpret, and elucidate the spectral signatures of crowded hydrogen-bond networks.

Methane Hydration-Shell Structure and Fragility.

The influence of oily molecules on the structure of liquid water is a question of importance to biology and geology and many other fields. Previous experimental, theoretical, and simulation studies of methane in liquid water have reached widely conflicting conclusions regarding the structure of hydrophobic hydration-shells. Herein we address this question by performing Raman hydration-shell vibrational spectroscopic measurements of methane in liquid water from -10 °C to 300 °C (at 30 MPa, along a path that parallels the liquid-vapor coexistence curve). We show that, near ambient temperatures, methane's hydration-shell is slightly more tetrahedral than pure water. Moreover, the hydration-shell undergoes a crossover to a more disordered structure above ca. 85 °C. Comparisons with the crossover temperature of aqueous methanol (and other alcohols) reveal the stabilizing influence of an alcohol OH head-group on hydrophobic hydration-shell fragility.

Temperature-Dependent Hydrophobic Crossover Length Scale and Water Tetrahedral Order.

Experimental Raman multivariate curve resolution and molecular dynamics simulations are performed to demonstrate that the vibrational frequency and tetrahedrality of water molecules in the hydration-shells of short-chain alcohols differ from those of pure water and undergo a crossover above 100 °C (at 30 MPa) to a structure that is less tetrahedral than pure water. Our results demonstrate that the associated crossover length scale decreases with increasing temperature, suggesting that there is a fundamental connection between the spectroscopically observed crossover and that predicted to take place around idealized purely repulsive solutes dissolved in water, although the water structure changes in the hydration-shells of alcohols are far smaller than those associated with an idealized "dewetting" transition.