Date of Award

Spring 2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Johnson, Mark

Abstract

Water is arguably the most important solvent and reactant on earth. It has many special properties compare to other liquids thanks to its hydrogen bonding. Understanding the interaction between water molecules as well as with other solutes in aqueous solutions is of great importance for studying reactions that happens in aqueous environments. Water in its many forms is heavily investigated by many experimental and theoretical methods including rotational, vibrational and electronic spectroscopy, ultrafast spectroscopy, interface specific spectroscopy, neutron scattering, X-ray diffraction, atomic force microscopy, mass spectrometry, nuclear magnetic resonance spectroscopy and so on. With improvements in instrumentation and computation power, our knowledge of water’s special properties keeps advancing toward a more accurate molecular level understanding. Among the experimental techniques, vibrational spectroscopy is widely used in several variations including linear absorption spectroscopy, ultrafast multidimensional spectroscopy, and interface specific spectroscopy, among others. The advantage of using vibrational spectroscopy to study water is its sensitivity towards structural information as well as relatively high time resolution. Linear absorption spectroscopy is the most common ways of studying isolated water clusters in the gas phase and provides highly resolved fundamental frequencies that are useful for theoretical calibration. The non-linear methods are mostly used in the condense phase to provide structural and dynamical information about water and aqueous systems. However, these approaches each have their own limitations. For the traditional cluster study, the cluster size are usually small and the hydrogen bond environments are less complex compared to those in the condense phase. Several studies of very large water cluster run into another problem, where the large number of overlapping bands masks detailed structural information about the OH oscillators. The condense phase study suffers from a similar problem, where even with isotopic dilution, many thousands of oscillators are sampled every time. Hence, it is impossible with the current experimental sensitivity in the condense phase to isolate a single water molecule’s spectral feature and information about its specific hydrogen bond environment. In this thesis, I report new experiments that tracks single OH oscillators or single intact water molecules imbedded in an extended hydrogen bond network made of D2O molecules. Such experiment allowed unprecedented molecular level insight into the OH spectral mechanics including hydrogen bond environment’s effect on the OH frequency as well as linewidth up to the second hydration shell, the intra- and intermolecular couplings, and Fermi resonances. After establishing the correlation between the OH frequency and the structural information, a new type of experiment was developed to observe spectral diffusion inside a water cluster via water reorientation. Various pathways were activated at different temperatures, which display different rate constants, and hence allowing the determination of activation energy of each pathway. Together with the OH frequency of interest, detailed reorientation pathway can be inferred to reveal what type of water molecules are involved in the H-bond rearrangement. It was observed that the water molecule at the surface of the cluster starts to move first at lower temperature which resembles the surface melting phenomena.

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