Absorption band Correlation analysis

Once the spectrum is obtained, the question remains as to how to interpret it. We stated previously (Section 8.6) that IR spectra are characterized by rather sharp absorption bands (peaks) and each such peak is characteristic of a particular covalent bond in a molecule in the path of the light. Thus for qualitative analysis (identification and purity characterization), the analyst correlates the location (the wavenumber), the shape, and the relative intensity of a given peak observed in a spectrum with a particular type of bond. 

Analysis of the data of Table 1 verifies the same finding that the shift of the near-UV absorption band of singlet arylnitrenes correlates with the shift of the intense near-UV absorption band of triplet nitrenes. Furthermore, the ort/io-substituents influence the absorption spectra of singlet and triplet phenylnitrenes more significantly than do para-substituents. 

The possibility of obtaining precise absorbance measurements and the storage of numerical data have given a new dimension to quantitative analysis by IR. This is especially true in situations where correlations can be made between the spectra and the composition or physico-chemical properties of the sample. This method is widely used because it is easier in the mid IR than UV/VIS to find absorption bands specific to compounds within a mixture. 

Structural analysis from electronic spectra yields little information because of their relative simplicity. In the 1940s, however, before the advent of more powerful identification techniques, UV/VIS visible spectroscopy was used for structural identification. The study of a great number of spectra of various molecules has revealed correlations between structures and the positions of absorption maxima. The most widely known empirical rules, due to Woodward, Fieser and Scott, involve unsaturated carbonyls, dienes and steroids. Using incremental tables based on various factors and structural features, it is possible to predict the position of the n —> n absorption bands in these conjugated systems (Table 11.3). Agreement between the calculated values and the experimentally determined position of absorption bands is usually good, as can been seen by the following four examples ... 

Tables 6.3-6.5 record data developed to undertake structural analysis in systems possessing chromophores that are conjugated or otherwise interact with each other. Chromophores within a molecule interact when linked directly to each other or when they are forced into proximity owing to structural constraints. Certain combinations of functional groups comprise chromophoric systems that exhibit characteristic absorption bands. In the era when UV-VIS was one of the principal spectral methods available to the organic chemist, sets of empirical rules were developed to extract as much information as possible from the spectra. The correlations referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 6.3) and enones and dienones (Table 6.4) to be predicted. When this method is applied, wavelength increments correlated to structural features are added to the respective base values (absorption wavelength of parent compound). The data refer to spectra determined in methanol or ethanol. When other solvents are used, a numerical correction must be applied. These corrections are recorded in Table 6.5.

Absorption bands in the visible and near-infrared spectra of Moon and Mars — correlate well with a narrow choice of minerals. — they provide a perhaps unique means of remote analysis of some of the abundant mineral phases on the surfaces of the bodies. 

Using fiber optics, Groothaert et al. (2003a) monitored the decomposition of NO and of N2O on CuZSM-5. A bis(/i-oxo)dicopper species, [Cu2(iu-0)2]2+, which had been previously identified by XAFS and UV-vis analysis (Groothaert et al., 2003b), was proposed to be a key intermediate and 02-releasing species. Although no quantitative correlation was established between decomposition rate and intensity of the absorption band of the copper species, the performance appeared to be related to the intensity of this band. 

The experimental results showed that, during the first 20 h of irradiation, the photochemical behavior of the blend may be related to that of PVME. To understand better the results of the surface analysis and to try to explain the phase separation phenomenon, a correlation should be made with PVME results. The IR analysis of the photooxidation of PVME homopolymer showed that during the first 5-6 h of irradiation an important increase in a band at 3290 cm-1 was observed [4]. This band has been attributed to tertiary hydroperoxides. For prolonged irradiations, the hydroperoxide absorption band decreased. The decomposition of hydroperoxides yields acetates ... 

UV data collected from substituted ferrocene systems have been summarized by Rosenblum (68) and some analysis of the electronic transitions involved has been provided. Until recently, little use had been made of the extensive tabulation of UV data of various substituted ferrocenes found in the literature. Several Russian workers have now attempted a qualitative correlation of the absorption bands of a series of mono- (20) and 1,1 -disubstituted (19) ferrocenes with the nature of the substituent. For each of the four different absorption bands observed in the spectra of these substituted ferrocenes, both bathochromic and hypsochromic shifts were exhibited for those ferrocenes containing electron-donating or -withdrawing... 

Ultraviolet spectrophotometry is considered a valuable tool as an aid for confirming the identification of pesticide residues. A correlation between the UV spectrum and the structure of several pesticides is discussed. Knowledge of such correlation may provide clues about the general type of chromophore present and may help the analyst to design analytical procedures. The transparency of many groups in the near UV imposes a limitation on interpretations of the absorption bands in this region. However, when taken in conjunction with the information obtained by IR, NMR, and mass spectroscopy, UV spectra may lead to structural proposals of value to the pesticide analyst. A discussion of the methods that have been utilized for the analysis of pesticides on the submicrogram level is also presented. 

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