Vibrational overtones and combination

Although we have been able to see on inspection which vibrational fundamentals of water and acetylene are infrared active, in general this is not the case. It is also not the case for vibrational overtone and combination tone transitions. To be able to obtain selection mles for all infrared vibrational transitions in any polyatomic molecule we must resort to symmetry arguments. 

The vibrational overtones and combinations of hydroxyl groups and thek associated molecular water occurring in the spectra of various gel siUca materials are summarized in Table 2 and discussed in References 3, 5, and 22. These peaks and bands found in the preparation of alkoxide-derived siUca gel monoliths are identical to those described for siUca gel powders (41). 

The functional groups almost exclusively involved in NIRS are those involving the hydrogen atom C-H, N-H, O-H (see Figure 5.1). These groups are the overtones and combinations of their fundamental frequencies in the mid-infrared and produce absorption bands of useful intensity in the NIR. Because the absorptivi-ties of vibrational overtone and combination bands are so much weaker, in NIRS the spectra of condensed phase, physically thick samples, can be measured without sample dilution or the need to resort to difficult short-path length sampling techniques. Thus conventional sample preparation is redundant, and fortunately so, because most PAT applications require direct measurement of the sample " either in situ, or after extraction of the sample from the process in a fast loop or bypass. 

NIR Vibrational overtones and combination bands Very high S/N Relatively low sensitivity facilitates minimum sample preparation Most popular on-line technique, but not as sensitive or diagnostic as IR and Raman. Sensitivity to physical as well as chemical status can be a problem or an opportunity, depending on application... 

A rule of thumb for hydride stretches [56, 57] is that the intensities of the vibrational overtone and combination transitions decrease, approximately, as IQ-Ay jjjg drop-off in intensity for the first few quanta of excitation may be even steeper, by another factor of 10. This implies that, in a specific spectral interval, the strongest vibrational transitions from the vibrationless ground state level correspond to the transition with the smallest Av and the greatest anharmonicity. However, as shown later, even these small absorption cross sections of vibrational overtone transitions can be sufficient for overtone preexcitation. 

The vapor spectra of the M(thd)3 compounds with M = Pr, Nd, Sm, Eu, Dy, Ho, Er, and Tm are shown in Figures 5 and 6. The arrows indicate absorption owing to vibrational overtone and combination bands of the organic chelate moiety. The remaining absorption bands arise from f f transitions of the rare-earth constituents. The energies and molar absorptivities of the f f absorption maxima are shown in Table VII. 

Absorption bands that are attributed to overtone and combination vibrations are also observed in the IR spectrum of polyatomic molecules. Overtone vibrations occur at frequencies of approximately integral multiples of the fundamental frequencies. Combination vibrations appear at frequencies that are the sum or the difference of the frequencies of two or more frmdamental vibrations. Overtone and combination bands are much less intense than fundamental bands. 

The near infrared (NIR) spans the range from 12500—4000 cm (800 2500 nm) and is dominated by overtones and combinations of O—H, N H, C—H and C=0 vibrations. Overtone and combination bands are rather weak. Band intensities... 

A property of organic samples that can also affect the results of FT-Raman measurements is the reabsorption of the Raman light by the sample itself. This arises from the overlap between the Raman emission and the direct absorption by vibrational overtones and combinations that occur between 1.064 tm (the excitation) and 1.786 tm (the end of the Raman spectrum at the 3800-cm shift from the laser). These absorptions will affect relative intensities in the Raman spectrum, especially in large samples where the laser is penetrating deep below the surface. The effects of self-absorption will be especially... 

Mg(ll) complexes with DBF show the wC-O mode at 1688 cm, whereas TlCl -DBP complexes exhibit a vQMi doublet at 1650 and 1584 cm, The i/OO band Is relatively sharp with no significant shoulder at ca. 1650 cm, due to Tl-DBF complexes and minimal absorption at 1900-1750 cm, where FDC exhibits the t/C>0 mode and various vibrational overtones and combination modesfree FDC shows strong maxima at 1865,... 

Aromatic C=C Stretching Vibrations Overtone and Combination Bands Aromatic Ring Deformation Below 700 cm Polynuclear Aromatic Compounds... 

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