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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.
Comprehensive spectroscopic view of the state-of the-art in theoretical and experimental hydrogen bonding research Spectroscopy and Computation of Hydrogen-Bonded Systems includes diverse research efforts spanning the frontiers of hydrogen bonding as revealed through state-of-the-art spectroscopic and computational methods, covering a broad range of experimental and theoretical methodologies used to investigate and understand hydrogen bonding. The work explores the key quantitative relationships between fundamental vibrational frequencies and hydrogen-bond length/strength and provides an extensive reference for the advancement of scientific knowledge on hydrogen-bonded systems. Theoretical models of vibrational landscapes in hydrogen-bonded systems, as well as kindred studies designed to interpret intricate spectral features in gaseous complexes, liquids, crystals, ices, polymers, and nanocomposites, serve to elucidate the provenance of spectroscopic findings. Results of experimental and theoretical studies on multidimensional proton transfer are also presented. Edited by two highly qualified researchers in the field, sample topics covered in Spectroscopy and Computation of Hydrogen-Bonded Systems include: Quantum-mechanical treatments of tunneling-mediated pathways in enzyme catalysis and molecular-dynamics simulations of structure and dynamics in hydrogen-bonded systems Mechanisms of multiple proton-transfer pathways in hydrogen-bonded clusters and modern spectroscopic tools with synergistic quantum-chemical analyses Mechanistic investigations of deuterium kinetic isotope effects, ab initio path integral methods, and molecular-dynamics simulations Key relationships that exist between fundamental vibrational frequencies and hydrogen-bond length/strength Analogous spectroscopic and semi-empirical computational techniques examining larger hydrogen-bonded systems Reflecting the polymorphic nature of hydrogen bonding and bringing together the latest experimental and computational work in the field, Spectroscopy and Computation of Hydrogen-Bonded Systems is an essential resource for chemists and other scientists involved in projects or research that intersects with the topics covered within.
The properties and nature of water clusters studied with novel spectroscopic approaches are presented in this thesis. Following a general introduction on the chemistry of water and water clusters, detailed descriptions of the experiments and analyses are given. All the experimental results, including first size-selective spectra of large clusters consisting of 200 water molecules, are presented with corresponding analyses. Hitherto unidentified hydrogen bond network structures, dynamics, and reactivity of various water clusters have been characterized at the molecular level. The main targets of this book are physical chemists and chemical physicists who are interested in water chemistry or cluster chemistry.
The structure and structural dynamics of hydrogen bonded liquids were studied experimentally and theoretically with coherent two-dimensional infrared (2DIR) spectroscopy. The resonant intermolecular interactions within the fully resonant hydrogen bond networks give access to spatial correlations in the dynamics of the liquid structures. New experimental and theoretical tools were developed that significantly reduced the technical challenges of these studies. A nanofluidic flow device was designed and manufactured providing sub-micron thin, actively stabilized liquid sample layers between similarly thin windows. A simulation protocol for nonlinear vibrational response calculations of disordered fluctuating vibrational excitons was developed that allowed for the first treatment of resonant intermolecular interactions in the 2DIR response of liquid water.The 2DIR spectrum of the O-H stretching vibration of pure liquid water was studied experimentally at different temperatures. At ambient conditions the loss of frequency correlations is extremely fast, and is attributed to very efficient modulations of the two-dimensional O-H stretching vibrational potential through librational motions in the hydrogen bond network. At temperatures near freezing, the librational motions are significantly reduced leading to a pronounced slowing down of spectral diffusion dynamics. Comparison with energy transfer time scales revealed the first direct proof of delocalization of the vibrational excitations. This work establishes a fundamentally new view of vibrations in liquid water by providing a spatial length scale of correlated hydrogen-bond motions.The linear and 2DIR response of the amide I mode in neat liquid formamide was found to be dominated by excitonic effects due to largely delocalized vibrational excitations. The spectral response and dynamics are very sensitive to the excitonic mode structure and infrared activity distributions, leading to a pronounced asymmetry of linear and 2DIR line shapes. This was attributed to structurally different species in the liquid characterized by their degree of medium range structural order. The response is dominated by energy transfer effects, sensitive to time-averaged medium range structural order, while being essentially insensitive to structural dynamics. This work is the first to recognize the importance of energy transfer contributions to the 2DIR response in a liquid, and provides additional proof of the well-structured character of liquid formamide.
As one of the typical intermolecular interactions, hydrogen-bonding plays a significant role in molecular structure and function. When the hydrogen bond research system is connected with the photon, the hydrogen-bonding effect turns to an excited-state one influencing photochemistry, photobiology, and photophysics. Thus, the hydrogen bond in an excited state is a key topic for understanding the excited-state properties, especially for optoelectronic or luminescent materials.The approaches presented in this book include quantum chemical calculation, molecular dynamics simulation and ultrafast spectroscopy, which are strong tools to investigate the hydrogen bond. Unlike other existing titles, this book combines theoretical calculations and experiments to explore the nature of excited-state hydrogen bonds. By using these methods, more details and faster processes involved in excited-state dynamics of hydrogen bond are explored.This highly interdisciplinary book provides an overview of leading hydrogen bond research. It is essential reading for faculties and students in researching photochemistry, photobiology and photophysics, as well as novel optoelectronic materials, fluorescence probes and photocatalysts. It will also guide research beginners to getting a quick start within this field.
The advent of laser-based sources of ultrafast infrared pulses has extended the study of very fast molecular dynamics to the observation of processes manifested through their effects on the vibrations of molecules. In addition, non-linear infrared spectroscopic techniques make it possible to examine intra- and intermolecular interactions and how such interactions evolve on very fast time scales, but also in some instances on very slow time scales. Ultrafast Infrared Vibrational Spectroscopy is an advanced overview of the field of ultrafast infrared vibrational spectroscopy based on the scientific research of the leading figures in the field. The book discusses experimental and theoretical topics reflecting the latest accomplishments and understanding of ultrafast infrared vibrational spectroscopy. Each chapter provides background, details of methods, and explication of a topic of current research interest. Experimental and theoretical studies cover topics as diverse as the dynamics of water and the dynamics and structure of biological molecules. Methods covered include vibrational echo chemical exchange spectroscopy, IR-Raman spectroscopy, time resolved sum frequency generation, and 2D IR spectroscopy. Edited by a recognized leader in the field and with contributions from top researchers, including experimentalists and theoreticians, this book presents the latest research methods and results. It will serve as an excellent resource for those new to the field, experts in the field, and individuals who want to gain an understanding of particular methods and research topics.
The almost universal presence of water in our everyday lives and the very `common' nature of its presence and properties possibly deflects attention from the fact that it has a number of very unusual characteristics which, furthermore, are found to be extremely sensitive to physical parameters, chemical environment and other influences. Hydrogen-bonding effects, too, are not restricted to water, so it is necessary to investigate other systems as well, in order to understand the characteristics in a wider context. Hydrogen Bond Networks reflects the diversity and relevance of water in subjects ranging from the fundamentals of condensed matter physics, through aspects of chemical reactivity to structure and function in biological systems.
(Cont.) At short times, vibrational dephasing reflects an underdamped 180 fs hydrogen bond oscillation, but the long time behavior indicates collective structural reorganization of the hydrogen bond network a 1.4 ps timescale. The anisotropy decays on timescales of 50 fs and 3 ps, which are attributed to librations and rotational diffusion, respectively. Lastly, I used two-dimensional infrared spectroscopy to investigate the frequency dependence of the dynamics. Preliminary results reflect a distribution of timescales for both local motion and collective reorganization.
Both an introductory course to broadband dielectric spectroscopy and a monograph describing recent dielectric contributions to current topics, this book is the first to cover the topic and has been hotly awaited by the scientific community.