Anisotropic and Finite Effects on Intermolecular Vibration and Relaxation Dynamics: Low-Frequency Raman Spectroscopy of Water Film and Droplet on Graphene by Molecular Dynamics Simulations

T Inagaki and M Hatanaka and S Saito, JOURNAL OF PHYSICAL CHEMISTRY B, 127, 5869-5880 (2023).

DOI: 10.1021/acs.jpcb.3c02109

Thestructural and dynamical properties of water can be greatlyaltered by the anisotropic interfacial environment. Here, we studythe intermolecular vibration and relaxation dynamics of a water filmand a water droplet on a graphene surface based on low-frequency Ramanspectra calculated from molecular dynamics simulations. The calculatedRaman spectra of the interfacial water systems show a weakened librationpeak and an enhanced intermolecular hydrogen bond (HB) stretchingpeak compared to the spectrum of bulk water, which are attributedto softened orientation motion. We also find that the collective polarizabilityrelaxation in the droplet is much slower than that in the film andbulk, which is completely different from the collective dipole relaxation.The slow relaxation is due to a positive correlation between the inducedpolarizabilities of distinct molecules caused by the global and anisotropicstructural fluctuations of the water droplet. Furthermore, we findthat the two-dimensional HB network by the orientation-ordered interfacialwater molecules leads to different intermolecular vibration dynamicsbetween the parallel and perpendicular components. The present theoreticalstudy demonstrates that low-frequency Raman spectroscopy can revealthe anisotropic and finite effects on the intermolecular dynamicsof the water film and droplet.

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