Chemical bonds molecular spectroscopy

Boggs, J.E. Nuclear Vibrations and Force Constants. In Theoretical Models of Chemical Bonding Molecular Spectroscopy, Electronic Structure and Intramolecular Interactions, Maksic,Z.B., Ed. Springer-Verlag Berlin, 1991, pp. 1-24. 

Principles of symmetry and group theory find applications in several areas of quantum chemistry like chemical bonding, molecular spectroscopy, ligand field theory, crystal field theory etc. The procedure in all these cases involves—... 

ARRANGEMENT OF TOPICS The topics have been arranged in what I find to be a convenient and logical sequence, but some instructors may well decide to follow a different order. For example, the first three chapters, which cover quantum mechanics, chemical bonding, molecular spectroscopy, and structure in biological systems, can be dealt with at a later stage. The last two chapters on equilibrium and transport in molecular systems and isotopes in biology stand somewhat apart and can be omitted if time does not permit their inclusion. 

Dynamic SIMS is used to measure elemental impurities in a wide variety of materials, but is almost new used to provide chemical bonding and molecular information because of the destructive nature of the technique. Molecular identihcation or measurement of the chemical bonds present in the sample is better performed using analytical techniques, such as X-Ray Photoelectron Spectrometry (XPS), Infrared (IR) Spectroscopy, or Static SIMS. 

The interactions of photons with molecules are described by molecular cross-sections. For IR spectroscopy the cross-section is some two orders of magnitude smaller with respect to UV or fluorescence spectroscopy but about 10 orders of magnitude bigger than for Raman scattering. The peaks in IR spectra represent the excitation of vibrational modes of the molecules in the sample and thus are associated with the various chemical bonds and functional groups present in the molecules. The frequencies of the characteristic absorption bands lie within a relatively narrow range, almost independent of the composition of the rest of the molecule. The relative constancy of these group frequencies allows determination of the characteristic... 

Infrared reflectance spectroscopy provides information on the vibrational states in the interphase. It can be interpreted in terms of molecular symmetry, force constants and chemical bond lengths. The intensity of the spectral peaks of the adsorbed molecules is determined both by standard... 

Chemical bonds can have covalent character, and EPR spectroscopy is an excellent tool to study covalency An unpaired electron can be delocalized over several atoms of a molecular structure, and so its spin S can interact with the nuclear spins /, of all these atoms. These interactions are independent and thus afford additive hyperfine patterns. An unpaired electron on a Cu2+ ion (S = 1/2) experiences an / = 3/2 from the copper nucleus resulting in a fourfold split of the EPR resonance. If the Cu is coordinated by a... 

The applications of polarized x-ray absorption spectroscopy (PXAS) for structure determination in inorganic and bioinorganic systems are discussed. PXAS studies of oriented samples add angular detail to the information obtained from x-ray absorption edges and from EXAFS. In some cases, PXAS can be used to determine molecular orientation. In other cases, PXAS can be used to infer the details of electronic structure or of chemical bonding. Some of the potential future applications of PXAS are discussed. 

And third, energy is possessed by virtue of the potential energy, and the translational, vibrational, rotational energy states of the atoms and bonds within the substance, be it atomic, molecular or ionic. The energy within each of these states is quantized, and will be discussed in greater detail in Chapter 9 within the subject of spectroscopy. These energies are normally much smaller than the energies of chemical bonds. 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

Infrared (IR) spectroscopy has been widely used in polymer studies for the assignment of molecular structure and for monitoring changes in the arrangement of chemical bonds (1.8-11). 

Quotation from Robert Mulliken, "Spectroscopy, Molecular Orbitals, and Chemical Bonding," Nobel Lectures. Chemistry. 19631970 (Amsterdam Elsevier, 1972) 131160, on 137. See Friedrich Hund, "Zur Deutung der Molekulspektren. IV." ZP 40 (1927) 742764, 42 (1927) 93120, 43 (1927) 805826 and Oyvind Burrau, "Berechnung des Energiewertes des Wasserstoff-Molekel-Ions (H2+) im Normalzustand," Danske Videnskabernes Selskab. Mat.-fys. Meddelser 7 (1927) 14. 

Robert Mulliken, "Spectroscopy, Molecular Orbitals, and Chemical Bonding," 133138, 141142. 

Nuclear magnetic resonance (NMR) spectroscopy is a powerful and versatile analytical technique that can provide site-specific information about chemical bonding, structure and dynamics in molecular systems. NMR applications have made a major impact in a variety of disciplines ranging from materials science to molecular biology and bioinorganic... 

It is essential to have tools that allow studies of the electronic structure of adsorbates in a molecular orbital picture. In the following, we will demonstrate how we can use X-ray and electron spectroscopies together with Density Functional Theory (DFT) calculations to obtain an understanding of the local electronic structure and chemical bonding of adsorbates on metal surfaces. The goal is to use molecular orbital theory and relate the chemical bond formation to perturbations of the orbital structure of the free molecule. This chapter is complementary to Chapter 4, which... 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

A basic understanding of the quantum theory is essential in many areas of chemistry, especially in connection with spectroscopy and with theories of atomic and molecular structure. The present book gives an introduction to the theory, and its application to elementary atomic structure, but chemical bonding is not discussed. I have tried to put the essential ideas in their historical context, but without retaining the historical introduction which has been traditional with this topic. With the crucial and difficult concepts of wave-particle duality, it seemed to me more important to give modem illustrations to show that they have current applications in chemistry. 

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