Ultraviolet-visible spectroscopy principles

Optical Spectroscopy General principles and overview, 246, 13 absorption and circular dichroism spectroscopy of nucleic acid duplexes and triplexes, 246, 19 circular dichroism, 246, 34 bioinorganic spectroscopy, 246, 71 magnetic circular dichroism, 246, 110 low-temperature spectroscopy, 246, 131 rapid-scanning ultraviolet/visible spectroscopy applied in stopped-flow studies, 246, 168 transient absorption spectroscopy in the study of processes and dynamics in biology, 246, 201 hole burning spectroscopy and physics of proteins, 246, 226 ultraviolet/visible spectroelectrochemistry of redox proteins, 246, 701 diode array detection in liquid chromatography, 246, 749. 

Fourier transform methods have come into their own as a means of studying the optical spectra of gas-phase radicals. Both infrared (FTIR) and ultraviolet/visible spectroscopy (FTUV/VIS) are now used to scrutinize these reactive molecules. We discuss the underlying principles of Fourier transform spectroscopy (FTS) with particular emphasis on the advantages and drawbacks of FTIR and FTUV/VIS measurements. Extensive tables are presented of metastable molecules that have been studied by Fourier transform methods. 

The molecular orbital theory of polyatomic molecules follows the same principles as those outlined for diatomic molecules, but the molecular orbitals spread over all the atoms in the molecule. An electron pair in a bonding orbital helps to bind together the whole molecule, not just an individual pair of atoms. The energies of molecular orbitals in polyatomic molecules can be studied experimentally by using ultraviolet and visible spectroscopy (see Major Technique 2, following this chapter). 

The fundamental principles of spectroscopy which are applied for visible spectroscopy are also applicable to ultraviolet region. 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

Identification of the intermediates in a multi-step reaction is the major objective of studies of reaction mechanisms. It is most useful to study intermediates present in low concentrations without chemical interference with the reacting system, i.e. by rapid spectroscopic methods. The most common methods in organic chemistry include ultraviolet-visible (UV-VIS), IR, and EPR spectroscopy. In principle, all other spectroscopic methods for the detection of reaction intermediates are also applicable provided that they are fast enough to monitor the intermediate and able to provide sufficient structural information to assist in the identification of the transient species. 

To describe the principles, methodology, instmmentation and applications of ultraviolet (UV)/visible spectroscopy. 

Ultraviolet/Visible Microspectrophotometry. Infrared spectroscopy and ultraviolet/visible microspectrophotometry are both based on the principle of the interaction of radiation with a sample. However, ultraviolet/visible microspectrophotometry is typically used to compare the dye or pigment composition of samples. The technique is used to determine the color of a sample and identify subtle differences in color that cannot be seen with the naked eye. 

The present data collection is intended to serve as an aid in the interpretation of molecular spectra for the elucidation and confirmation of the structure of organic compounds. It consists of reference data, spectra, and empirical correlations from and nuclear magnetic resonance (NMR), infrared (IR), mass, and ultraviolet-visible (UV/vis) spectroscopy. It is to be viewed as a supplement to textbooks and specific reference works dealing with these spectroscopic techniques. The use of this book to interpret spectra only requires the knowledge of basic principles of the techniques, but its content is structured in a way that it will serve as a reference book also to specialists. 

Four basic types of elution are used in HPLC, namely, the isocratic system, the basic gradient system, the inert system and the advanced gradient system (see Figure 1.1). The most commonly used detectors are those based on spectroscopy in the region 185-400 nm, visible-ultraviolet (UV) spectroscopy in the region 185-900 nm, post-column derivatisation with fluorescence detection (see next), conductivity [7] and multiple wavelength UV detectors using a diode array system detector (see next). Other types of detectors available are those based on electrochemical principles, refractive index, differential viscosity, and mass detection [8]. 

Previous authors have taught the principles of solving organic structures from spectra by using a combination of methods NMR, infrared spectroscopy (IR), ultraviolet spectroscopy (UV) and mass spectrometry (MS). However, the information available from UV and MS is limited in its predictive capability, and IR is useful mainly for determining the presence of functional groups, many of which are also visible in carbon-13 NMR spectra. Additional information such as elemental analysis values or molecular weights is also often presented. 

There are various kinds of spectroscopy visible and ultraviolet (UV) absorption spectroscopy, Raman and infrared spectroscopy, nuclear magnetic resonance spectroscopy, and electron-spin resonance (ESR) spectroscopy. A brief description of the principles of these techniques and their application to the study of ions in solution follows (see also Section 2.11). 

The instruments used for the fluoremetry, fluoremeters, are very similar to the ones used for Raman spectroscopy. However, the light spectral region used in fluoremetry is usually the ultraviolet or the visible spectral area. The principle of the scheme of a fluoremeter is shown in Figure 2.53. 

When dealing with low-energy infrared radiation, the interaction with matter is limited to the absorption of light by the outer shell electrons, i.e. those used in forming compounds. Hence, particular bonds will absorb particular wavelengths. This is the principle used for infrared spectroscopy. There are equivalent techniques for ultraviolet radiation and visible radiation, but they are mostly used to provide information about concentration of a given compound, rather than for identification purposes such as XRF or IR techniques. 

Most kinetic studies have been carried in solution, and the main analytical methods are listed in Table 1. with a choice of references. Electronic absorption spectroscopy is the most common method, as in other fields of chemical kinetics. Frequently, unless it has been shown by preparative techniques that more than one product is present, analyses by visible-ultraviolet, as well as by infrared spectroscopy, are effected with wavelengths at which absorption by one of the reactants prevails. Gas-volumetric methods are advantageous for analysis of products in special systems where adducts quickly decompose producing small molecules (see also Section I.). Gas chromatography could, in principle, be used to give analysis of both reactants and product(s) however it has been mainly used to determine one of them, as the other methods. 

This article provides some general remarks on detection requirements for FIA and related techniques and outlines the basic features of the most commonly used detection principles, including optical methods (namely, ultraviolet (UV)-visible spectrophotometry, spectrofluorimetry, chemiluminescence (CL), infrared (IR) spectroscopy, and atomic absorption/emission spectrometry) and electrochemical techniques such as potentiometry, amperometry, voltammetry, and stripping analysis methods. Very few flowing stream applications involve other detection techniques. In this respect, measurement of physical properties such as the refractive index, surface tension, and optical rotation, as well as the a-, //-, or y-emission of radionuclides, should be underlined. Piezoelectric quartz crystal detectors, thermal lens spectroscopy, photoacoustic spectroscopy, surface-enhanced Raman spectroscopy, and conductometric detection have also been coupled to flow systems, with notable advantages in terms of automation, precision, and sampling rate in comparison with the manual counterparts. 

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