Ultraviolet-visible spectroscopy structural properties

Quantitative infrared spectroscopy suffers certain disadvantages when compared with other analytical techniques and thus it tends to be confined to specialist applications. However, there are certain applications where it is used because it is cheaper or faster. The technique is often used for the analysis of one component of a mixture, particularly when the compounds in the mixture are alike chemically or haye very similar physical properties, e.g. structural isomers. In these cases, analysis by using ultraviolet/visible spectroscopy is difficult because the spectra of the components will be almost identical Chromatographic analysis may be of limited use because the separation of isomers, for example, is difficult to achieve. The infrared spectra of isomers are usually quite different in the fingerprint region. Another advantage of the infrared technique is that it is non-destructive and requires only a relatively small amount of sample. 

Molecular dynamics (MD) simulations fill a significant niche in the study of chemical structure. While nuclear magnetic resonance (NMR) yields the structure of a molecule in atomic detail, this structure is the time-averaged composite of several conformations. Electronic and vibrational circular dichroism spectroscopy and more general ultraviolet/visible and infrared (IR) spectroscopy yield the secondary structure of the molecule, but at low resolution. MD simulations, on the other hand, yield a large set of individual structures in high detail and can describe the dynamic properties of these structures in solution. Movement and energy details of individual atoms can then be easily obtained from these studies. 

Volumes 50 and 51 of the Advances, published in 2006 and 2007, respectively, were the first of a set of three focused on the physical characterization of solid catalysts in the functioning state. This volume completes the set. The six chapters presented here are largely focused on the determination of structures and electronic properties of components and surfaces of solid catalysts. The first chapter is devoted to photoluminescense spectroscopy it is followed by chapters on Raman spectroscopy ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy X-ray photoelectron spectroscopy X-ray diffraction and X-ray absorption spectroscopy. 

Electromagnetic radiation is a universal probe for studying the chemical and structural properties of matter. This is quite evident when it is considered the amount of information brought by the different spectroscopies in the infrared, visible and ultraviolet wavelength regions. This is perhaps even more obvious in the field of X-ray diffraction, nowadays an unvaluable analytical tool for the Chemist, the Physicist or the Biologist. 

Despite the extensive studies of the anodic layers on Pt with various ultraviolet-visible optical methods, they have not provided a clear indication of the electronic or structural properties of the layers. Rather these optical methods have been more than just another form of readout to complement the electrochemical measurements of charge and current response of the layer to potential and time. Vibrational spectroscopic data from infrared and Raman measurements would be more helpful in establishing the nature of the layers but it is difficult to use these techniques to study metal-electrolyte and similar interfaces because of solvent interference and sensitivity problems. A noteworthy exception is the quite successful in situ use of Raman spectroscopy to study the electrochemically formed oxide layers on silver by Kotz and Yeager. In the instance of silver electrodes, there is a large surface enhanced Raman effect and the signal-to-noise ratio is not a problem. Unfortunately this is not the situation with other metal surfaces such as Pt. Even so, with improved instrumentation there is hope that in situ Raman studies of the anodic layers on Pt will become practical. 

Elaborate synthetic approaches have been developed that enable significant control over the size and shape of palladium nanostructures. In order to understand the properties of the materials formed based on the preparation method, several characterization techniques have been used. These include electron microscopy, scanning probe microscopy (SPM), nuclear magnetic resonance (NMR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, infrared (IR) spectroscopy, electrochemistry, X-ray diffraction (XRD), thermogravimetric analysis (TGA), electron diffraction, photoelectron spectroscopy, dynamic light scattering (DLS), extended X-ray absorption fine structure (EXAFS), BET surface area analysis andX-ray reflectivity (XRR). In the following section we will describe the information provided by each of these characterization techniques. 

Information about the structure of gas molecules haB been obtained by several methods. Spectroscopic studies in the infrared, visible, and ultraviolet regions have provided much information about the simplest molecules, especially diatomic molecules, and a few polyatomic molecules. Microwave spectroscopy and molecular-beam studies have yielded very accurate interatomic distances and other structural information about many molecules, including some of moderate complexity. Molecular properties determined by spectroscopic methods are given in the two books by G. Herzberg, Spectra of Diatomic Molecules, 1950. and Infrared and Raman Spectra, 1945, Van Nostrand Co., New York. The information obtained about molecules by microwave spectroscopy is summarised by C. H. Townes and A. L. Schawlow in their book Microwave Spectroscopy of Gases, McGraw-Hill Book Co., New York, 1955. 

The selection rules help to predict the probability of a transition but are not always strictly followed. If the transition obeys the rules it is allowed, otherwise it is forbidden. A molecule can become excited in a variety of ways, corresponding to absorption in different regions of the spectrum. Thus certain properties of the radiation that emerges from the sample are measured. The fraction of the incident radiation absorbed or dissipated by the sample is measured in optical (ultraviolet and visible) absorption spectroscopy and some modes of nuclear magnetic resonance spectrometry (NMR). Because the relative positions of the energy levels depend characteristically on the molecular structure, absorption spectra provide subtle tools for structural investigation. 

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