The Low Frequency Modes of Water: Effects of Ionic Concentration, Surface Confinement, Surface Charge Density, and Surface-Driven Perturbations

The vibrational spectroscopy of water is a powerful means of probing and better understanding the fundamental dynamics of water molecules. The low-frequency spectrum (i.e., 0-1000 1/cm) of water includes hydrogen bond bending and stretching modes, as well as molecular librational modes. Hydrogen bonding is responsible for many of the properties that make water uniquely essential for countless biological and chemical processes, and librational modes have been shown to have an ultrafast relaxation. However, the low-frequency spectrum of water has not been studied with as much detail as the higher frequency modes. In this work, using Molecular Dynamics (MD), the low-frequency spectrum is decomposed into translational and rotational components along each of the basis vectors of the local molecular reference frame of each molecule, providing a more detailed understanding of the composition of each of the modes. Furthermore, the effects of ionic concentration, surface confinement, surface charge density, and surface-driven perturbations are systematically studied. Finally, the cross-correlation between each pair of modes is computed, increasing the fundamental understanding of these highly important dynamical modes.