Vibrational dynamics of aqueous hydroxide solutions studied using ultrafast infrared spectroscopy
Massachusetts Institute of Technology. Department of Chemistry.
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Liquid water possesses an extended network of hydrogen bonds that is responsible for many of its interesting properties. Mobility of hydroxide ions in aqueous solutions is much higher compared to the ions of similar size and charge density. A proton can efficiently move from a neighboring water molecule to the hydroxide ion due to the presence of hydrogen bonds, resulting fast structural diffusion of the ion. On the other hand, this hydrogen bonding network of water undergoes fluctuations on femtoseconds to picoseconds timescale, influencing the intertwined hydroxide transport process. Studying the influence of water's hydrogen bonding network on the proton transport process in aqueous hydroxide solutions is experimentally challenging, largely due to the lack of a suitable technique that is sensitive to the changes in the system on few tens of femtoseconds timescale. Vibrations in aqueous hydroxide solutions are sensitive to the strength of hydrogen bonding and hence vibrational frequencies, intensities and line shapes are closely associated with the structure and dynamics of the hydroxide ions. In this thesis, we have employed ultrafast infrared spectroscopy in conjunction with theoretical modeling to understand the nature of the vibrations and their dynamics in aqueous hydroxide solutions. The infrared spectra of aqueous solutions of NaOH and other strong bases exhibit a broad continuum absorption for frequencies between 800 and 3500 cm-¹, which is attributed to the strong interactions of the hydroxide ion with its solvating water molecules. This continuum absorption has limited distinguishable features whose molecular origin holds the key in explaining the vibrational dynamics. We have performed ultrafast transient absorption and 2DIR experiments on aqueous NaOH solutions, by exciting the O-H stretch vibrations and probing the response from 1350-3800 cm-¹, using a newly developed sub-70 fs broadband infrared source. By probing the entire mid-infrared continuum absorption of aqueous hydroxide solutions with ultrafast pulses, the broadband infrared source allows us to monitor time-dependent changes in this broad spectral window. These experiments, in conjunction with harmonic vibrational analysis of OH-(H₂O)₁₇ clusters, reveal that O-H stretch vibrations of aqueous hydroxides arise from coupled vibrations of multiple water molecules solvating the ion. These delocalized vibrations cannot be distinguished based on the local structure of the hydroxide ion. However, they can be classified according to the symmetry defined by the relative phase of vibrations of the O-H bonds hydrogen bonded to the ion. In general, we find the asymmetric O-H stretch vibrations to be more intense and shifted to lower frequencies compared to the symmetric ones. Analysis of transient absorption and waiting time dependent 2DIR spectroscopy shows that the vibrations in aqueous hydroxide solutions relax on 100-300 fs timescale. Alongside, the O-H stretch vibrations originating from the bulk-like water molecules as well as the asymmetric O-H stretch vibrations of the water molecules solvating the hydroxide ion lose their frequency memory within 160-180 fs. Such loss in frequency memory on similar timescales is likely to happen through migration of vibrational excitation between two types of O-H stretch vibrations. Spectral features in strongly hydrogen bonded systems like water and aqueous hydroxide solutions are very broad, particularly the induced absorption features in the transient absorption and 2DIR spectra. With the development of broadband mid-infrared pulses, we are able to detect nonlinear response of these systems in the frequency window of 1350-3800 cm-¹, observing >1700 cm-¹ broad induced absorption features. Qualitatively, strong coupling between intra- and intermolecular vibrations lead to such broadening. In order to explain the experimental results, we have developed a self-consistent phenomenological model that consists of an intramolecular and an intermolecular vibration, with strong nonlinear coupling between them. We find that the experimental results are reproduced when the coupling between the vibrations is strong enough to yield eigenstates with mixed intra- and intermolecular vibrational character. In such scenarios, the identities of individual intra- and intermolecular vibrational modes are lost.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2015.Cataloged from PDF version of thesis. Vita.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Department of Chemistry
Massachusetts Institute of Technology