Electronic Absorption Spectrometry

Spectrometry. Electronic absorption spectra were measured by Hitachi EPS-3T type spectrometer using 1 cm optical path length cell. For So->T- absorption spectra in C2H5I, 10 cm optical path length cells were used. In the latter case, the corrections were carried out for the deviation of the spectra arised from the high concentra-tlon(30wt ) of the samples. Phosphorescence spectra were measured by Hitachi MPF-2A type spectrometer at 77°K and corrected by method published elsewhere.(20) IR-spectra were measured by Hitachi EPI-G31ype spectrometer in KBr tablets. 

Both absorption and emission may be observed in each region of the spectrum, but in practice only absorption spectra are studied extensively. Three techniques are important for analytical purposes visible and ultraviolet spectrometry (electronic), infrared spectrometry (vibrational) and nuclear magnetic resonance spectrometry (nuclear spin). The characteristic spectra associated with each of these techniques differ appreciably in their complexity and intensity. Changes in electronic energy are accompanied by simultaneous transitions between vibrational and rotational levels and result in broadband spectra. Vibrational spectra have somewhat broadened bands because of simultaneous changes in rotational energy, whilst nuclear magnetic resonance spectra are characterized by narrow bands. 

The peaks were collected and freed from adjacent peaks and shoulders by further HPLC, before examination by mass spectrometry and electronic absorption spectroscopy. Compounds of the following molecular masses were progressively eluted on reverse-phase HPLC 114, 345, 178, 194, 194, 194, 192, 272, 272,... 

In addition to characterization of molecular and macroscopic electro-optic activity, it is important to define optical loss. Optical loss can be influenced both by absorption and by scattering effects. In order to minimize overall loss, it is important to understand the independent contributions made by scattering and absorption. To separate these effects, we need to determine the contributions made by both chromophore and polymer host to the optical absorption at device operating wavelengths. Chromophore interband electronic absorption can be measured on resonance by traditional UV-Visible spectrometry however, we will typically be concerned with optical absorption at telecommunication wavelengths of 1.3 and 1.55 microns where such techniques do not provide accurate information. Total optical absorption at 1.3 microns is occasionally determined by both the interband electronic absorption of the chromophore and by C-H vi-... 

Degraded Carotenoids Physical Methods Separation and Assay N.M.R. Spectroscopy Mass Spectrometry Chiroptical Methods Electronic Absorption Spectroscopy Infrared and Resonance Raman Spectroscopy Other Spectroscopic Techniques Miscellaneous Physical Chemistry Photoreceptor Pigments Biosynthesis and Metabolism Stereochemistry Enzyme Systems Inhibition and Regulation... 

The majority of ultrafast instruments have employed electronic absorption spectrometry for determining the ultrafast dynamic properties of the systems of interest. However, in recent years some investigators have successfully turned their attention... 

In electronic absorption spectrometry, the spectral bands are broad and often featureless, thus providing minimal information as to the structure of the absorber. 

The simplicity of the structure of phosgene has precipitated many structural and spectroscopic studies. This Chapter summarizes the results of the studies by electron diffraction, microwave and vibrational spectroscopy, nuclear magnetic resonance and nuclear quadrupole resonance spectroscopy, and mass spectrometry. Studies by electronic absorption and emission spectroscopy, and photoelectron spectroscopy, are discussed in Chapter 17. 

The paramagnetic d ion VO + is an excellent probe for electron paramagnetic resonance (EPR) spectroscopy. In combination with proton potentiometry and/or electron absorption spectrometry (UV-Vis), species distribution schemes for VO + in the presence of various ligands have been obtained, comparable to those discussed in Section 2.2.1 for vanadate(V) systems derived on the basis of NMR plus potentiometry. EPR also allows, via the anisotropic hyperfine coupling constant in field direction (A or An,... 

The photochemical destruction of ortho, meta and para-nitrophenols induced by ultra-violet light illumination of aqueous slurries of titanium dioxide has been monitored by electronic absorption spectroscopy the products are said to have been identified as dihydroxynitrobenzene isomers by coupled gas chromatography-mass spectrometry although no details are supplied. Nitrophenols have also been identified in the fog shrouding the University of Bayreuth. They are presumed to be the products of photochemical nitration and the possible precursors (phenol, cresol and nitrate) were also detected. 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

Zinc having a filled d shell and two 4 s electrons reacts like a Lewis acid of considerable polarizing power. Electrons are withdrawn from substrates with the consequence that a nucleophilic attack is facilitated. The elements with partially filled d shells and multiple oxidation states include Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Cu. Hence, they are good candidates to actively particii te in redox reactions. Attributable to d-d transitions most of the complexes of these species including many metalloproteins are coloured. This facilitates investigations of their properties by electron absorption spectrometry. 

Fig. 2.3. Absorbance as a function of optical density for selected shock tube investigations employing OH electronic absorption spectrometry. The unmarked curve represents the semi-empirical relationship derived in Reference 37, evaluated at a pressure (5 1 atm) and temperature (1520 K) typical of recombination experiments in an argon diluent. Tlie curves labelled 6 1, 3 1 and 1 3 were empirically determined over a selected range of recombination pressures and temperatures for mixtures dilute in argon with those particular initial H2/O2 ratios (Reference 32). The curve identified by HJ (Reference 24) was empirically determined in a 1 % Hg-l % 02-98 % Ar mixture at 1300 K for a selected range of pressures. The cross-hatched area represents the approximate range of absorbances and optical densities observed with an atomic bismuth line source (Reference 41). Also shown are the line HH derived from photographic spectroscopy using instrumental definition of absorption line centres on a continuum (Reference 48), and a solid circle (beyond the range of the abscissa) denoting the photoelectric absorbance reported in Reference 47 for a continuum source at an optical density of 750 x 10" moles liter cm.

The method of kinetic electronic absorption spectrometry is well known as the principal technique for following radicals, reactants and products following flash photolysis. However, this method has been extensively used much in discharge-flow systems only recently. The main problem is the low optical density encountered because of the... 

Usually, titration methods are used to calibrate spectrometrically determined radical concentrations in absolute terms. Most kinetic studies of radicals have used either e.p.r. spectrometry or electronic absorption spectrophotometry. A survey of these methods has been given in section 4.2 of this article selected examples will be discussed later in this section. 

IR measurements were recorded as KBr pellets on a Unicam-Mattson 1000 FT-IR spectrometer. Electronic absorption spectra were measured on a Unicam UV2-300 UV-vis spectrophotometer. H-NMR measurements were performed on a Varian-Mercury 300 MHz spectrometer. Samples were dissolved in (CD SO with TMS as internal reference. The complexes were also characterized by elemental analysis (Perkin-Elmer 2400 CHN elemental analyzer) and mass spectroscopy (Finnigan MAT SSQ7000). Table 1 gives the elemental analyses and mass spectrometry data for the complexes. 

UV-vis, and IR spectrometry all operate on much faster timescales because they interrogate electronic absorption (emission) and nuclear vibrational motion processes that are much faster than the complexation or decomplexa-tion of a host-guest system. In other words, all binding events are slow on the timescale of these techniques. Unfortunately, although all host-guest exchange processes are slow, experiments with these techniques are not normally as structurally informative as NMR. 

This reflects the fact that the intensity ratio of the atom and ion lines of an element changes considerably with the electron pressure in the plasma, particularly for elements with low ionization energy, such as Na. This is analytically very important as it is the cause of so-called ionization interferences in classical d.c. arc emission spectrometry, atomic absorption, and plasma optical emission, as well as mass spectrometry. 

One advantage of the flowtube technique is that it can be used in conjunction with a wide variety of detection methods electron spin resonance, laser magnetic resonance, mass spectrometry, optical absorption and fluorescence spectroscopies. The method has been applied extensively to the reactions of a number of small radicals, especially atoms e.g., H, O, N, F, Cl, OH, CIO and HOj. Usually... 

Ultraviolet/visible spectrometry Electronic molecular absorption in solution Quantitative determination of unsaturated organic compounds... 

Keywords circular dichroism electronic absorption electrospray ionization mass spectrometry magnetic circular dichroism metal-thiolate cluster structures metallothionein perturbed angular correlation of y-rays spectroscopy X-ray... 

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