Dynamics of water and aqueous protons studied using ultrafast multi-dimensional infrared spectroscopy

Abstract

Liquid water consists of a highly dynamic network of hydrogen bonds, which evolves on timescales ranging from tens of femtoseconds to a few picoseconds. The fast structural evolution of water's hydrogen bond network is at the heart of numerous fundamental aqueous processes, such as proton transport, solvation, the hydrophobic effect and protein folding. In this thesis, I present our efforts in understanding the dynamics governing hydrogen bond switching and vibrational energy dissipation in water, and the transport of excess protons in strong acid solutions. We use ultrafast nonlinear infrared spectroscopy to study hydrogen bond and proton transfer dynamics in water and acids since vibrational frequencies, intensities and line shapes are closely associated with chemical structure and dynamics. We employed and characterized a new source of ultrafast broadband infrared pulses that span the entire mid-infrared region from 4000 cm-1 down to hundreds of cm-I, with <70 fs pulse duration. We have demonstrated the use of these pulses in studying ultrafast vibrational dynamics in water and aqueous proton transfer dynamics in acids, where broad and feature-less vibrational transitions are present across the mid-infrared. Rearrangements of the hydrogen bond network in liquid water involve rapid switching of hydrogen bonds, which is believed to be a concerted process where a water molecule undergoes large angle molecular reorientation as it exchanges hydrogen-bonding partners. To test this picture of hydrogen bond dynamics, we performed ultrafast 2D IR spectral anisotropy on the OH stretching vibration of HOD in D₂0 to directly track the reorientation of water molecules as they change hydrogen bonding environments. Interpretation of the experimental data is assisted by modeling drawn from molecular dynamics simulations, and we quantify the degree of molecular rotation on changing local hydrogen bonding environment within the framework of restricted rotation models. Our results show evidence for concerted motions involving large angular deviations in the molecular dipole when a water molecule evolves from a strained configuration to a stable hydrogen bonded geometry. In addition to its rapidly evolving hydrogen bonding network, the ability of liquid water to efficiently dissipate energy through ultrafast vibrational relaxation processes plays a key role in the stabilization of reactive intermediates and the outcome of aqueous chemical reactions. The ability of liquid water to efficiently dissipate energy through ultrafast vibrational relaxation plays a key role in the stabilization of reactive intermediates and the outcome of aqueous chemical reactions. The vibrational couplings that govern energy relaxation in liquid H₂0 remain difficult to characterize due to the complex interplay of inter- and intramolecular forces present in the liquid and the limitations of current methods to visualize these ultrafast motions simultaneously. Using ultrafast broadband infrared pulses, we performed 2D IR spectroscopy, pump-probe spectroscopy, and polarization anisotropy of H₂0 by exciting the OH stretching transition in water and characterizing the response of the liquid from 1350-4000 cm-¹ with <70 femtosecond time resolution. These spectra reveal vibrational transitions at all frequencies simultaneous to the excitation, including cross peaks to the bend vibration and a continuum of induced absorptions to previously unobserved combination bands that are not present in linear spectra. These observations provide evidence for strong mixing of inter- and intra-molecular character and delocalization of the vibrations in liquid H20. Unlike traditional weak coupling models, it appears that excitation of OH stretch motion simultaneously drives bending and intermolecular motions as a result of strong anharmonic mixing. These delocalized vibrations, or excitons, have mixed stretch and bend character and evolve over several hundred femtoseconds, giving rise to a complex network of vibrational energy relaxation processes. Protons are known to exhibit very high mobility in water compared to other ions due to the Grotthuss proton hopping mechanism, which describes proton transfer as a process where displacement of charge takes place through breaking and forming of O-H covalent bonds in water. Infrared spectra of strong acids are marked by broad and featureless transitions that span the entire mid-infrared due to a large distribution of rapidly exchanging solvated proton configurations. While previous research efforts have assigned different regions of the mid-infrared spectrum of acids to vibrations of the Eigen (H₉O₄+) and the Zundel (H₅O₂+) limiting structures of the solvated proton, experimental evidence for these assignments in solution and the dynamics that underlie the broad transitions have been largely absent. Using ultrafast broadband infrared pulses in pump-probe and 2D IR spectroscopy, we studied the evolution of frequency correlations across the entire mid-infrared spectrum of concentrated HCl in H₂O. Our results provide experimental evidence for the existence of Eigen and Zundel configurations of the solvated proton in HCl/H₂O with persistence times of at least 0.8 to 1 ps and 250 fs, respectively, and show that fleeting excursions of the proton from the Eigen configuration towards shared configurations are stabilized in less than 100 fs by electric field fluctuations of the solvating water molecules.