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Abstract

The aqueous proton is one of the fundamental species of aqueous chemistry, but a clear picture of its structure and transport dynamics have eluded chemists for over a century. This arises from the complex interactions with the rapidly fluctuating hydrogen bond (H-bond) network in water, generating a distribution of structures that can interconvert on femtosecond-picosecond timescale. As a result, there is currently an intense debate on the structure of the aqueous proton, typically described in terms of the limiting gas-phase structures, the Eigen cation H3O+(H2O)3 and the Zundel complex H5O2+. Additionally, acidic solutions display anomalously high conductivity, described in terms of Grotthuss diffusion where a series of small hydrogen displacements result in the rapid flow of net charge along a H-bonded “water wire.” Despite the advances of molecular dynamics simulations, there have been few experiments that can directly probe the structural fluctuations and ultrafast dynamics involved in proton diffusion. Additionally, the vibrational spectrum lacks distinct vibrational peaks, instead displaying a continuum of absorption spanning 1000-3200 cm-1. The work in this thesis harnesses recent technological developments in nonlinear IR spectroscopy to assign the vibrational features in the continuum, correlate them to instantaneous structures, and track their evolution during proton transfer. This thesis presents the most comprehensive 2D IR dataset to date of the aqueous proton. All of the vibrations of the aqueous proton complex show off-diagonal cross peaks with each other, which reveals that these modes all belong to the same molecular complex versus distinct Eigen and Zundel species in solution. Along with high-level anharmonic calculations, the polarization-sensitive 2D IR spectrum provides reliable assignments for the vibrations of the aqueous proton complex. The broad feature at 1200 cm-1 shows a distinctive anharmonicity pattern due to the stretching motion of the shared proton in Zundel-like instantaneous configurations. The nuclear potential for this vibration is sensitive to the confinement by two solvating waters and asymmetry from the solvation environment. This vibration is strongly coupled to the concerted flanking bending vibrations at 1750 cm-1 and the O–H stretches of the flanking waters at 3100 cm-1. The continuous absorption from 2000-3000 cm-1 arises from hydronium O–H stretches in instantaneous Eigen-like configurations which are broadened by a distribution of H-bond strengths to the solvation environment. There is also a large distribution of complexes in between the Eigen and Zundel extremes which appear spectrally similar to Zundel-like complexes, but show breadth like the Eigen vibrational response. Cross peaks between Eigen-like and Zundel-like vibrations at the earliest waiting times support the theory that the aqueous proton is a “fluxional complex” that rapidly fluctuates between instantaneous Zundel-like and Eigen-like configurations. The polarization anisotropy decay of flanking bending modes reveals a 2.5 ps timescale due to proton transfer that rotates the aqueous proton complex. Arrhenius analysis reveals that the barrier for this process is 2.4 kcal/mol, consistent with temperature-dependent measurements of proton transfer measured with other techniques. The anisotropy barrier and timescale are both sensitive to the concentration of the acid and the identity of the counterion. Additionally, this timescale grows monotonically, yet nonlinearly with the viscosity of the solution. These all point to a proton transfer mechanism that depends on the collective rearrangement of the H-bond network. In situations where the barrier decreases, we find that the proton transfer rate also slows down. With Eyring analysis, we conclude that there are entropic penalties associated with proton transfer at high concentrations, arising from obstructed proton transfer pathways and hindered H-bond reorganization. These measurements reveal the complex behavior of proton transport in water and provide necessary data to benchmark molecular dynamics simulations of this process.

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