Overtone and combination bands

The large number of modes in orthorhombic Ss results in a manifold of overtones and combination bands in the vibrational spectra [133]. As an ex-... 

The latter applies to NIR spectroscopy used for the non-invasive determination of blood glucose by means of a fibre-optical measuring-head (Jagemann et al. [1995] Muller et al. [1997] Danzer et al. [1998]). In addition to the weak overtone and combination bands resulting from glucose, strongly disturbing absorption of water, that is the main component... 

The near-IR technique has been used very successfully for moisture determination, whole tablet assay, and blending validation [23]. These methods are typically easy to develop and validate, and far easier to run than more traditional assay methods. Using the overtone and combination bands of water, it was possible to develop near-IR methods whose accuracy was equivalent to that obtained using Karl-Fischer titration. The distinction among tablets of differing potencies could be performed very easily and, unlike HPLC methods, did not require destruction of the analyte materials to obtain a result. 

Another interesting facet of the vibrational IETS is the weakness of overtone and combination bands. There are sound theoretical reasons to expect that overtone bands should be very weak in IETS [46, 47]. To our knowledge, there has been no theoretical investigation of the intensities of combination bands in tunneling spectra. To be sure, there are experimental papers that contain tunneling band assignments that include assignments as combination and overtone bands. Most... 

Based on empirical observation, a general statement about overtones and combination bands might be Overtones do occur, but they are very weak. Combination bands are seldom observed. Kirtley, for example, says that overtones are about a factor of 200 weaker than fundamentals in the case of the benzoate ion [47, 53]. Ramsier, Henriksen, and Gent identify a single clear overtone in the tunneling spectrum of the phosphite ion (HPO3 2) [54], The fundamental associated with... 

Figure 8.9 Diffuse reflectance infrared spectrum of a silica support, showing silica vibrations at frequencies below 1300 cm1, overtones and combination bands between 1700 and 2050 cm-1, and various hydroxyl groups at frequencies above 3000 cm 1. The sharp peak at 3740 cm"1 is due to isolated OH groups, the band around 3550 cm 1 to paired, H-bonded OH groups, and the band around 3660 cm 1 to hydroxyls inside the silica (courtesy of R.M. van Hardeveld, Eindhoven).

Better determination of overtone and combination bands of familiar molecules and the spectroscopic characterization of new species [radicals (Shida, 1991 Bemath, 1990), ions (Miller and Bondybey, 1983 Leach, 1980), and van der Waals molecules in particular (Nesbitt, 1988 Saykally, 1989 Hutson, 1990 Heaven, 1992)] continue to receive wide attention (Figure 0.3). 

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. 

The challenges in this work concerned the fundamental limits of NIR spectroscopy. Eirst, would NIR, with its typically broad and highly overlapped bands, have enough spectral resolution to distinguish the ortho, meta, and para isomers from each other and from the extractant Second, would NIR, with its typically weak overtone and combination bands, have enough sensitivity to quantify the minor components of the stream, especially the ortho isomer, typically present at only 1% ... 

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... 

Weak combination and overtone bands appear in the 2000-1650 cm-1 region. The pattern of the overtone bands is not a reliable guide to the substitution pattern of the ring. Because they are weak, the overtone and combination bands are most readily observed in spectra obtained from thick samples. The spectrum of Figure 3.13 is that of a typical aromatic (benzenoid) compound. 

Because of the many normal modes and the presence of overtone and combination bands, hot bands, and impurity bands, which may overlap one another, the IR spectra of medium-sized and large molecules are complex and may be difficult to assign. Incorrect IR vibrational assignments have, unfortunately, been quite common. 

FIGURE 5. Schematic diagram of low resolution (0.8 nm) Raman scattering spectrum of O3 excited at 266 nm. The spectrum consists of overtones and combination bands in v-] (antisymmetric stretch) and v3 (antisymmetric stretch up to v" = 7 and V3 = 6 No bands with >2 (bending) are evident, suggesting that the bond angle remains the same during the transition. Reproduced from reference (80) with permission from the American Chemical Society. 

Another complication arises in the interpretation of absorption spectra. If a molecule vibrates with pure harmonic motion and the dipole moment is a linear function of the displacement, then the absorption spectrum will consist of fundamental transitions only. If either of these conditions is not met, as is usually the case, the spectrum will contain overtones (multiples of the fundamental) and combination bands (sums and differences). Most of these overtones and combination bands occur in the near-infrared (0.8-2.0/un). 

Figure 3. Overtone and combination band spectrum oj ethylene and water adsorbed on Mn"A zeolite. The ethylene bands lie close to the gaseous (V -f- vn), (vt + vs), 2vlu and 2vs vibrational modes, indicating that the ethylene molecule has retained its chemical composition and structural integrity (-, 1) MnA 4- ethylene (-------------------,2) MnA hydrated CtHt (g) bands.

DRS has been applied both to the analysis of vibrational spectra of surface species in the fundamental, overtone, and combination band regions,and to the determination of time correlation motion of adsorbed molecules by Fourier inversion of the spectra onto... 

Near-infrared reflectance analysis is particularly well suited to chemical sensing because it operates on "as is" samples and yet has chemical specificity. The absorptions observed originate from vibrations of a relatively few chemical groups whose overtones and combination bands appear in the near infrared region. 

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