Absorption band distortions

For a configuration, the ground state in octahedral symmetry is a Eg term and the excited state is a "T2g term. On distortion to >4/, geometry, these terms split, as shown in Figure 11-10. In an octahedral complex, we would expect excitation from the Eg state to the state and a single absorption band. Distortion of the complex to >4 , geometry splits the "Tig level into two levels, the Eg and the B2g. Excitation can now occur from the ground state (now the Big state) to the Ajg, the Eg, or the B2g (the... 

For thin-film samples, abrupt changes in refractive indices at interfrees give rise to several complicated multiple reflection effects. Baselines become distorted into complex, sinusoidal, fringing patterns, and the intensities of absorption bands can be distorted by multiple reflections of the probe beam. These artifacts are difficult to model realistically and at present are probably the greatest limiters for quantitative work in thin films. Note, however, that these interferences are functions of the complex refractive index, thickness, and morphology of the layers. Thus, properly analyzed, useful information beyond that of chemical bonding potentially may be extracted from the FTIR speara. 

From this equation it can be seen that the depth of penetration depends on the angle of incidence of the infrared radiation, the refractive indices of the ATR element and the sample, and the wavelength of the radiation. As a consequence of lower penetration at higher wavenumber (shorter wavelength), bands are relatively weaker compared to a transmission spectrum, but surface specificity is higher. It has to be kept in mind that the refractive index of a medium may change in the vicinity of an absorption band. This is especially the case for strong bands for which this variation (anomalous dispersion) can distort the band shape and shift the peak maxima, but mathematical models can be applied that correct for this effect, and these are made available as software commands by some instrument manufacturers. 

Liquids can be sampled as either the neat liquid (pure) or mixed with a solvent (solution). Neat liquids are tested when the purpose of the experiment is either identification or the determination of purity. Identification is possible because the spectrum is a fingerprint when no solvent or contaminant is present. Impurities are found when extraneous absorption bands or distortions in analyte absorption bands appear. 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

Figure 8.2 presents the fluorescence of pyrene on silica gel. The loading is low so that pyrene is predominantly adsorbed as nonaggregated monomers (Mi). The backward fluorescence spectrum Fb of this sample is very comparable to the spectrum in polar solvents and not distorted by reabsorption. However, the forward spectrum Ft is almost completely suppressed in the region of overlap with the o -transition and hot sidebands of the weak first absorption band Si. The absorption coefficients of the sample vary widely from k" = 0.1 cm 1 (Si-band, Aa = 350-370 nm) to k = 25 cm-1 (S2-band, 1 290-340 nm), and in a first approximation the excitation spectrum of Fh reflects this variation correctly (Figure 8.2, left). The Ff-excitation spectrum, however, has only little in common with the real absorption spectrum of the sample. 

The first studies of dendrimer-encapsulated metal nanoparticles focused on Cu [82]. This is because Cu + complexes with PAMAM and PPI dendrimers are very well behaved and have easily interpretable UV-vis and EPR spectra. For example, Fig. 4a shows absorption spectra for Cu + coordinated to different ligands. In the absence of dendrimer and in aqueous solutions Cu + exists primarily as [Cu(H20)g] +, which gives rise to a broad, weak absorption band centered at 810 nm. This corresponds to the well-known d-d transition for Cu in a tetra-gonally distorted octahedral or square-planar ligand field. 

It is sometimes found that the 0-0 band is not the most intense in the doublet absorption band. In Cr(NH3)83+ the 0-0 band is only the third strongest in the spectrum and in Cr(CN)e3 the 0-0 band is so weak it can be observed only with difficulty.15 Observation of a weak 0-0 band may be taken to indicate that at equilibrium the 2Eg state is significantly distorted relative to the iA2g state. The problem of using the observed structure of absorption and emission spectra to assign vibrational levels in complexes has been discussed by Porter and Schlafer.15... 

The values of both n and k of the metal film have an influence on the line shape of the adsorbate. With the middle spectrum as the reference, Fig. 6 shows that increasing n leads to a more distorted line shape, whereas increasing k leads to a less distorted line shape. Increasing k (use of a more absorbing metal) also leads to a decreased absorbance. Without a metallic film, the absorption band is symmetric (not shown). Dispersive line shapes such as shown in Fig. 6 were also observed experimentally for CO on platinum films (26). 

Chromium(II) double sulfates can be crystallized by the addition of ethanol to concentrated aqueous solutions containing equimolar quantities of the components. The hexahydrates A2S04>CrS04-6H20 (A = NH4, Rb or Cs) are high-spin and have reflectance spectra compatible with the presence of tetragonally distorted [Cr(H20)6]2+ ions (Table 24). 4 The potassium and sodium salts crystallize as pale blue dihydrates with similar properties, and from the splittings of the SO2- absorption bands in their IR spectra it seems that coordinated sulfate anions are present. 

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