It is no overstatement to claim that hydrogen bonding is the most important intermolecular interaction. On a day-to-day basis, we encounter the peculiar effects of hydrogen bonding in liquid water; however, it is well appreciated that hydrogen bonding is immensely important in many contexts and, in particular, in biological ones. Despite this apparent significance, a general molecular picture of the dynamics of hydrogen-bonding systems is lacking. Over the last two decades, ultrafast multidimensional infrared spectroscopy has emerged as powerful technique for studying molecular dynamics in the condensed phase. By taking advantage of the complex relationship between a molecular oscillator's frequency and its environmental structure, we may understand molecular dynamics from an experimental perspective. However, the study of hydrogen bonding poses a significant technical challenge in that the interaction gives rise to broad resonances in the mid-infrared absorption spectrum. Traditional methods for generating short pulses of mid-infrared light are fundamentally limited in the bandwidth they can produce. Oftentimes, the width of a hydrogen-bonded oscillator's absorption resonance exceeds the broadest bandwidth mid-infrared laser pulse. In this thesis, I describe our development and use of a novel source of short, broadband mid-infrared light pulses that span the entire region of high-frequency molecular vibrations. We use this source as a probe in two-dimensional infrared spectroscopy experiments to study a wide variety hydrogen-bonding systems, including hydrogen-bonded dimers and protein films, with a particular emphasis on liquid water. Across these systems, we observe fascinating trends in the changes in molecular dynamics with increasing complexity of hydrogen bonding. In particular, we find experimental evidence for large deformations of the nuclear potential energy surface, giving rise to extremely anharmonic and collective dynamics. The effect is most dramatic in liquid water, where the rapidly fluctuating hydrogen-bond network results in vibrational excitons wherein O-H stretching motion is delocalized over multiple molecules. In this case, the nuclear potential energy surface is so complex that even simple changes in the mass of the oscillators result in qualitatively different dynamics.

Hydrogen bonds represent type of molecular interaction that determines the structure and function of a large variety of molecular systems. The elementary dynamics of hydrogen bonds and related proton transfer reactions, both occurring in the ultra fast time domain between 10-14 and 10-11s, form a research topic of high current interest. In this book addressing scientists and graduate students in physics, chemistry and biology, the ultra fast dynamics of hydrogen bonds and proton transfer in the condensed phase are reviewed by leading scientists, documenting the state of the art in this exciting field from the viewpoint of theory and experiment. The nonequilibrium behavior of hydrogen-bonded liquids and intramolecular hydrogen bonds as well as photo induced hydrogen and proton transfer are covered in 7 chapters, making reference to the most recent literature.

Water is one of the most important substances in the world. It is used in a wide range of technologies and is an essential ingredient in all living cells we know today. The structure of water molecule is simple, yet it can form extended and versatile hydrogen bond (HB) network. This ability gives water extraordinary properties, such as high boiling and melting point. At the same time, the hydrogen bond network is not static. The constant breaking and re-forming of hydrogen bond occurs on the picosecond timescale. This dynamic network facilitates many functions of water, including ions solvation, protein folding and electricity conduction. Understanding the structure and dynamics of these processes is therefore of great importance. Ultrafast infrared (IR) spectroscopies offer a great method for accessing the sub-picosecond to picoseconds dynamics while a system in an electronic ground state. During the past two decades, hydrogen bond dynamics has been investigated extensively using ultrafast IR spectroscopies. But many questions still exist such as the effect of ions and confinement on the hydrogen bonding dynamics and the relation between the anomalous proton diffusion in dilute solution and hydrogen bonding. In Chapter 3, we examined the nature of molecular anion hydrogen bonding. The CN stretch of selenocyanate anions (SeCN-) was used as the vibrational probe in heavy water D2O. We observed the non-Condon effect on the CN stretch whose transition dipole changes with the strength of hydrogen bonding with water. In addition, HB rearrangement dynamics reported by SeCN- is almost the same as was that of the OH stretch of HOD molecules. This result shows that this anion does not perturb the surrounding HB network significantly in the low salt concentration solution. This ionic perspective is important and complements the results using OD or OH stretch of HOD molecules, which can only probe the effect of ions in a high salt concentration condition. In Chapter 4, we used SeCN- as the probe to examine water dynamics in confinement, and I focused on the nano waterpool formed in reverse micelles. The water pool is surrounded by surfactants which are further solvated by organic hydrophobic solvents. For large reverse micelle whose diameter is larger than 4 nm, the water pool is usually divided into two regions: the core region where water dynamics is like that in pure water and the interface region where water dynamics is slowed significant due to the confinement. Here we used ultrafast IR spectroscopies to measure the orientational relaxation of SeCN-, which reflects its interaction with water molecules and how "rigid" the HB network is. Based on the comparison between linear IR decomposition and ultrafast anisotropy dynamics, we proposed a three-component model of water in large reverse micelles. The interface component should be further separated into two layers. One layer corresponds to water in contact with the surfactant head group and has very slow reorientation. The other layer corresponds to water molecules whose coordinating structure still resembles that of bulk but the dynamics is slowed down due to the perturbation from confinement. In Chapter 5 and 6, hydrogen bonding dynamics in concentrated salt and acid solutions were investigated. Through electrochemical method, it was found decades ago that proton has extraordinary ion mobility, about 6 times larger than that of cations of similar sizse, such as sodium, ammonium or lithium. The great difference between them results from the cation transport mechanism. In dilute solution, the main transport mechanism of proton is through relay mechanism where the identity of proton transfers from one water molecule to another. This minimizes the physical diffusion of the atoms and greatly increases the proton mobility. The mechanism is generally called Grotthuss mechanism, which was came up with by Grotthuss in 1806 though not on the molecular level. However, the step time of a single proton transfer event between two water molecules is difficult to observe experimentally. Here we used the CN stretch of methyl thiocyanate (MeSCN) as the vibrational probe. In concentrated hydrochloric solutions, it has two frequency resolved states. One state refers to water hydrogen bonded to the nitrogen lone pair while the other state corresponds to hydronium ion hydrogen bonded to the CN. Chemical exchange phenomenon was observed between these two states. Ab initio simulation done by our collaborator shows that the proton hopping is the dominate mechanism for chemical exchange. The comparison experiment done in lithium chloride solution provides further contrast between hydronium and other metal ions. Therefore, we were able to track proton hopping in a time-resolved manner for the first time. Extrapolation to the dilute limit demonstrates that the HB rearrangement in pure water is the driving force of proton hopping in dilute solution.

Hydrogen-bonded systems are ubiquitous in nature, where they provide structure and pathways for energy dissipation. Cyclic, hydrogen-bonded interfaces are capable of mediating proton transfer, but these structures have broad and complex vibrational spectra. To study these vibrational features, an ultrafast continuum midinfrared (CIR) laser pulse has been incorporated as the probe pulse in several vibrational spectroscopies used to study the vibrational dynamics and proton transfer of cyclic, hydrogen-bonded dimers. Unlike traditional ultrafast vibrational spectroscopy, which is limited to a few hundred cm-1 of bandwidth in a single experiment, ultrafast mid-infrared continuum spectroscopy allows vibrational dynamics and coupling to be observed across the full vibrational spectrum. The vibrational dynamics of the 7-azaindole- acetic acid heterodimer were studied with mid-infrared pump-CIR probe and two dimensional infrared (2D IR) spectroscopy, which revealed strong coupling across the spectrum and very fast energy transfer across the bridging hydrogen bonds. Additionally, photoinduced proton transfer was studied in the 7-azaindole homodimer with preliminary UV pump-CIR probe experiments, which showed the formation of the doubly proton-transferred tautomer and spectral signatures of proton transfer. Further development of ultrafast mid-IR spectroscopy was explored with the generation of high energy continuum mid-IR pulses in bulk chalcogenide glass.