Absorption peaks, overlapping

As with electronic spectra, the use of infrared spectra for quantitative determinations depends upon the measurement of the intensity of either the transmission or absorption of the infrared radiation at a specific wavelength, usually the maximum of a strong, sharp, narrow, well-resolved absorption band. Most organic compounds will possess several peaks in their spectra which satisfy these criteria and which can be used so long as there is no substantial overlap with the absorption peaks from other substances in the sample matrix. 

Because of peak overlappings in the first- and second-derivative spectra, conventional spectrophotometry cannot be applied satisfactorily for quantitative analysis, and the interpretation cannot be resolved by the zero-crossing technique. A chemometric approach improves precision and predictability, e.g., by the application of classical least sqnares (CLS), principal component regression (PCR), partial least squares (PLS), and iterative target transformation factor analysis (ITTFA), appropriate interpretations were found from the direct and first- and second-derivative absorption spectra. When five colorant combinations of sixteen mixtures of colorants from commercial food products were evaluated, the results were compared by the application of different chemometric approaches. The ITTFA analysis offered better precision than CLS, PCR, and PLS, and calibrations based on first-derivative data provided some advantages for all four methods. ... 

Excitation-wavelength-dependent emission polarization studies indicate the presence of an overlapping xy polarized transition in the bluer part of the 290-315-nm range, as indicated in Fig. 5. The combination of static absorption, time-resolved emission, and emission quantum yield measurements suggests that the emitting state has the same polarization (z axis, linear), but is not the same state as that giving rise to the 362-nm absorption peak. These assignments for the 3.5-nm particles are summarized in Fig. 5. 

Blandamer et al. (4) have recently made a half-hearted effort to resurrect the e2 2 species. They state that if a species such as M2 or M contains two ammoniated electrons with sufficient overlap of the electronic wave functions to cause the species to exist in a singlet state, then only by a coincidence could the absorption spectrum be similar to that of the far-separated ammoniated electrons. They suggest that a comparable coincidence could just as well occur in the case of the e2 2 species. Some clarification of the ionic aggregate model is therefore needed. It should be recognized that the optical absorption peak does shift slightly... 

For N K-edge of S2 (Figure 7.2(a)) two absorption peaks were formed at the transition energy of 387.0 and 389.0 eV. They were related to the anti-bonding interaction of Hf 5d and N 2p. Considering the bond overlap population of Hf 5d and O 2p, the peak to the anti-bonding interaction of Hf 5d and O 2p is placed at... 

The physical and optical properties of the NPs used in this investigation are described in Table 6.1. The optical absorption properties of the ruthenium dye complex are also detailed in the table. It can be seen that there is good overlap between 7 of the pure silver and alloy NPs and the absorption band of the complex, while the gold NPs lie outside the absorption peak and are used as a negative control. The dependence of the excitation spectra of the dye complex on NP-dye distance is shown in Figure 6.14 for the case of the pure silver NPs. Also included in the figure is the excitation spectrum for the complex coated on the PEL layer in the absence of NPs. From Table 6.1, it can be seen that there is very good overlap between 7 of the silver NPs and the dye absorption band which constitutes the optimum plasmonic enhancement condition for the case of excitation enhancement. 

Even with CD, spectra are not always easily interpreted because there may be overlapping bands of different signs. Interpretation requires determination of the overall symmetry around the metal ion and assignment of absorption spectra to specific transitions between energy levels (discussed in Chapter 11) in order to assign specific CD peaks to the appropriate transitions. Even then, there are cases in which the CD peaks do not match the absorption peaks and interpretation becomes much more difficult. 

The electronic spectra of the triphenylphosphonium, triphenylarsonium and triphenyl-stibonium tetraphenylcyclopentadienylides confirmed the impression derived from their stabilities, basicities and reactivities in Wittig-type reactions, that the polarity of the ylidic bonding increased in the order P, As, Sb and concomitantly, the double-bond character decreased. Thus the longest-wavelength absorption peaks were at 288 nm (P), 291 nm (As), 349 nm (Sb) this was attributed to the less efficient overlap between the 2p-orbitals of the ylidic carbon atom and the d-orbitals of antimony, because of the greater size and diffuseness of the d-orbitals on going down the Periodic TableNone of these compounds was solvatochromic . 

---