Vibrational And Theoretical Investigations Of Molecular Conformations And Intramolecular Pi Type Hydrogen Bonding

The molecular conformations, potential energy functions and vibrational spectra of several cyclic molecules have been investigated by ab initio and density functional theory calculations and by infrared and Raman spectroscopy. The ab initio computations of 3-cyclopenten-1-ol predict that its lowest energy conformer has a weak pi-type intramolecular hydrogen bonding. The three other conformers lie 301 to 411 cm^-1 higher in energy. The infrared and Raman spectra of this molecule confirm the presence of the four conformers. The energy difference between the two conformers of lowest energy was also determined from the experimental spectroscopic data and was found to be 435 plus/minus 160 cm^-1, in reasonable agreement with the ab initio computations results. Ab initio calculations for cyclopentane and d1, 1,1-d2, 1,1,2,2,3,3-d6, and d10 isotopomers confirm cyclopentane confirmed that has twist and bent structures and that these differ in energy by less than 10 cm^-1. The bending angle is 41.5 degrees and the twisting angle is 43.2 degrees. A complete vibrational assignment for each of the isotopomers was achieved. Ab initio calculations were also carried out for methylcyclopropane, cyclopropylsilane, cylopropylgermane, cyclopropylamine, cyclopropanethiol and cyclopropanol. The structure and the potential energy function for internal rotation was calculated for each and compared to available experimental results determined from infrared and Raman spectra. The calculated barriers to internal rotation agree very well with the experimental data. The structures, relative energies, and frequencies for the lowest energy vibrations of the twisted, bent, and planar forms of cyclohexene and four of its oxygen analogs were calculated and compared to experimental results. The calculated structural data agree very well with that from the microwave work, but the computed barriers are somewhat lower than those based on far-infrared data. 4-Silaspiro-(3,3)-heptane possesses two four-membered rings, each puckered with and angle of 34 degrees. The molecule possesses a two-dimensional ring-puckering potential energy surface with four equivalent minima. The ab initio calculations predict a barrier to planarity of each ring of 582 cm^-1 while the energy of the structure with both rings planar is 1220 cm^-1 higher. The calculated infrared and Raman spectra were compared to those previously published, and the agreement is excellent.

This book uses examples from experimental studies to illustrate theoretical investigations, allowing greater understanding of hydrogen bonding phenomena. The most important topics in recent studies are covered. This volume is an invaluable resource that will be of particular interest to physical and theoretical chemists, spectroscopists, crystallographers and those involved with chemical physics.

Hydrogen bonded systems play an important role in all aspects of science but particularly chemistry and biology. Notably, the helical structure of DNA is heavily reliant on the hydrogens bonds between the DNA base pairs. Although the area of hydrogen bonding is one that is well established, our understanding has continued to develop as the power of both computational and experimental techniques has improved. Understanding Hydrogen Bonds presents an up-to-date overview of our theoretical and experimental understanding of the hydrogen bond. Well-established and novel approaches are discussed, including quantum theory of ‘atoms in molecules’ (QTAIM); the electron localization function (ELF) method and Car–Parinnello molecular dynamics; the natural bond orbital (NBO) approach; and X-ray and neutron diffraction and spectroscopy. The mechanism of hydrogen bond formation is described and comparisons are made between hydrogen bonds and other types of interaction. The author also takes a look at new types of interaction that may be classified as hydrogen bonds with a focus on those with multicentre proton acceptors or with multicentre proton donors. Understanding Hydrogen Bonds is a valuable reference for experimentalists and theoreticians interested in updating their understanding of the types of hydrogen bonds, their role in chemistry and biology, and how they can be studied.

The structure and hydrogen bonding of water molecules provides this unique solvent with properties essential to many physical, chemical and biological processes. The intermolecular hydrogen bonding between water molecules in the bulk medium is disrupted at the surface, imparting the surface with unique structural and thermodynamic properties. In this article we provide an overview of a range of experimental studies from this laboratory in which the structure, orientation and hydrogen bonding of interfacial water molecules at liquid interfaces are directly probed by resonant vibrational sum frequency spectroscopy. The studies provide insight into the difference in water structure and hydrogen bonding at an air/water interface relative to the interface between two bulk immiscible liquids, namely the CCl4/H2O interface. Also described are studies aimed at understanding how the presence of a charged alkyl surfactant alters the structure of water at these two interfaces. In both cases field induced alignment of water molecules in the double layer region is prevalent. This induced alignment has been examined under a variety of experimental conditions. A series of isotopic dilution studies conducted for the first time at liquid surfaces are also described. In these studies the intermolecular and intramolecular coupling of vibrational modes in the water molecules are diminished. The results of these and above mentioned studies provide valuable information for those interested in developing theoretical descriptions of water at surfaces and interfaces.

Hydrogen bonding is crucial in many chemical and biochemical reactions, as well as in determining material properties. A good insight into the theoretical methods of treating hydrogen bonding is essential for those wishing to model its effects computationally in a wide range of fields involving hydrogen bonding, as well as those wishing to extract the maximal amount of information from experimental data. Theoretical Treatments of Hydrogen Bonding presents the reader with the state of the art of the key theoretical approaches to hydrogen bonding and considers the hydrogen bond from the various aspects. The first five chapters are devoted to the methods used for treating the electronic basis of hydrogen bonding, including a consideration of the electrostatic model, density functional theory and molecular orbital methods. Later chapters consider the dynamics of hydrogen bonds with particular attention to the treatment of proton transfer; manifestations of dynamics as reflected in infrared spectra and nuclear magnetic relaxation are also considered. Hydrogen bonding in liquids and solids such as ferroelectrics are included. The book concludes with an epilogue which discusses the likely development of hydrogen bond computations in very large chemical systems. Theoretical Treatments of Hydrogen Bonding offers the reader a comprehensive view of the current theoretical approaches to hydrogen bonding. It is a valuable presentation of theoretical tools useful to those looking for the most appropriate approach for treating a particular problem involving hydrogen bonding.

This book gives an extensive description of the state-of-the-art in research on excited-state hydrogen bonding and hydrogen transfer in recent years. Initial chapters present both the experimental and theoretical investigations on the excited-state hydrogen bonding structures and dynamics of many organic and biological chromophores. Following this, several chapters describe the influences of the excited-state hydrogen bonding on various photophysical processes and photochemical reactions, for example: hydrogen bonding effects on fluorescence emission behaviors and photoisomerization; the role of hydrogen bonding in photosynthetic water splitting; photoinduced electron transfer and solvation dynamics in room temperature ionic liquids; and hydrogen bonding barrier crossing dynamics at bio-mimicking surfaces. Finally, the book examines experimental and theoretical studies on the nature and control of excited-state hydrogen transfer in various systems. Hydrogen Bonding and Transfer in the Excited State is an essential overview of this increasingly important field of study, surveying the entire field over 2 volumes, 40 chapters and 1200 pages. It will find a place on the bookshelves of researchers in photochemistry, photobiology, photophysics, physical chemistry and chemical physics.

Reflecting a rich technical and interdisciplinary exchange of ideas, Water and Life: The Unique Properties of H20 focuses on the properties of water and its interaction with life. The book develops a variety of approaches that help to illuminate ways in which to address deeper questions with respect to the nature of the universe and our place withi

Because of the importance of the hydrogen bond, there have been scores of insights gained about its fundamental nature by quantum chemical computations over the years. Such methods can probe subtle characteristics of the electronic structure and examine regions of the potential energy surface that are simply not accessible by experimental means. The maturation of the techniques, codes, and computer hardware have permitted calculations of unprecedented reliability and rivaling the accuracy of experimental data. This book strives first toward an appreciation of the power of quantum chemistry to analyze the deepest roots of the hydrogen bond phenomenon. It offers a systematic and understandable account of decades of such calculations, focusing on the most important findings. This book provides readers with the tools to understand the original literature, and to perhaps carry out some calculations of their very own on systems of interest.

Dieser Band beschäftigt sich mit der schwachen Wechselwirkung zwischen zwei Bindungen, die wesentlich zur Gestalt und Reaktivität eines Moleküls beiträgt. Experimentelle Basis, theoretischer Hintergrund, Eigenschaften und Auswirkungen der CH-Pi-Wechselwirkung werden diskutiert und mit den Merkmalen anderer H-Bindungen verglichen. Ein systematisch angelegter Text zu einem hochaktuellen Thema! (07/98)

The renowned theoretical physicist Victor F. Weisskopf rightly pointed out that a real understanding of natural phenomena implies a clear distinction between the essential and the peripheral. Only when we reach such an understanding - that is to say when we are able to separate the relevant from the irrelevant, will the phenomena no longer appear complex, but intelectually transparent. This statement, which is generally valid, reflects the very essence ofmodelling in the quantum theory of matter, on the molecular level in particular. Indeed, without theoretical models one would be swamped by too many details embodied in intricate accurate molecular wavefunctions. Further, physically justified simplificqtions enable studies of the otherwise intractable systems and/or phenomena. Finally, a lack of appropriate models would leave myriads of raw experimental data totally unrelated and incomprehensible. The present series ofbooks dwells on the most important models of chemical bonding and on the variety of its manifestations. In this volume the electronic structure and properties of molecules are considered in depth. Particular attention is focused on the nature of intramolecular interactions which in turn are revealed by various types ofmolecular spectroscopy. Emphasis is put on the conceptual and interpretive aspects of the theory in line with the general philosophy adopted in the series.