Signatures of Solvation Thermodynamics in Spectraof Intermolecular Vibrations

摘要

This study explores the thermodynamic and vibrational properties of water in the three-dimensional environment of solvated ions and small molecules using molecular simulations. The spectrum of intermolecular vibrations in liquid solvents provides detailed information on the shape of the local potential energy surface, which in turn determines local thermodynamic properties such as the entropy. Here, we extract this information using a spatially resolved extension of the two-phase thermodynamics method to estimate hydration water entropies based on the local vibrational density of states (3D-2PT). Combined with an analysis of solute–water and water–water interaction energies, this allows us to resolve local contributions to the solvation enthalpy, entropy, and free energy. We use this approach to study effects of ions on their surrounding water hydrogen bond network, its spectrum of intermolecular vibrations, and resulting thermodynamic properties. In the three-dimensional environment of polar and nonpolar functional groups of molecular solutes, we identify distinct hydrationwater species and classify them by their characteristic vibrationaldensity of states and molecular entropies. In each case, we are ableto assign variations in local hydration water entropies to specificchanges in the spectrum of intermolecular vibrations. This providesan important link for the thermodynamic interpretation of vibrationalspectra that are accessible to far-infrared absorption and Raman spectroscopyexperiments. Our analysis provides unique microscopic details regardingthe hydration of hydrophobic and hydrophilic functional groups, whichenable us to identify interactions and molecular degrees of freedomthat determine relevant contributions to the solvation entropy andconsequently the free energy.