Overtone absorptions

The NIR spectra contain less structural information than the corresponding IR spectra, since only the overtone absorptions of X-Fl (X = C, N, O) are detected. Using chemometric approaches has, however, enlarged the applications of this method, particularly for quantitative and classification analyses. 

Thus for the transition n = 0 to n = 2 the flrst overtone absorption is seen at... 

However, in polyatomic molecules, transitions to excited states involving two vibrational modes at once (combination bands) are also weakly allowed, and are also affected by the anharmonicity of the potential. The role of combination bands in the NIR can be significant. As has been noted, the only functional groups likely to contribute to the NIR spectrum directly as overtone absorptions are those containing C-H, N-H, O-H or similar functionalities. However, in combination with these hydride bond overtone vibrations, contributions from other, lower frequency fundamental bands such as C=0 and C=C can be involved as overtone-combination bands. The effect may not be dramatic in the rather broad and overcrowded NIR absorption spectrum, but it can still be evident and useful in quantitative analysis. 

Anharmonicity leads to deviations of two kinds. At higher quantum numbers, AE becomes smaller, and the selection rule is not rigorously followed as a result, transitions of A 2 or 3 are observed. Such transformations are responsible for the appearance of overtone lines at frequencies approximately two or three times that of the fundamental line the intensity of overtone absorption is frequently low, and the peaks may not be observed. Vibrational spectra are further comphcated by the fact that two different vibrations in a molecule can interact to give absorption peaks with frequencies that are approximately the sums or differences of their fundamental frequencies. Again, the intensities of combination and difference peaks are generally low. 

The infrared absorption of 1-butene that occurs at 1830 cm 1 (Figure 10-1) falls in the region where stretching vibrations of alkene bonds usually are not observed. However, this band actually arises from an overtone (harmonic) of the =CH2 out-of-plane bending at 915 cm 1. Such overtone absorptions come at exactly twice the frequency of the fundamental frequency, and whenever an absorption like this is observed that does not seem to fit with the normal fundamental vibrations, the possibility of its being an overtone should be checked. 

However, what unite all applications of NIRS for PAC are the unique features of the NIR spectrum. The NIR is in effect the chemical spectroscopy of the hydrogen atom in its various molecular manifestations. The frequency range of the NIR from about 4000 cm-1 up to 12 500 cm-1 (800-2500 nm) covers mainly overtones and combinations of the lower-energy fundamental molecular vibrations that include at least one X—H bond vibration. These are characteristically significantly weaker in absorption cross-section, compared with the fundamental vibrational bands from which they originate. They are faint echoes of these mid-IR absorptions. Thus, for example, NIR absorption bands formed as combinations of mid-IR fundamental frequencies (for example v + u2), typically have intensities ten times weaker than the weaker of the two original mid-IR bands. For NIR overtone absorptions (for example 2v, 2v2) the decrease in intensity can be 20-100 times that of the original band. 

Figure 6.2-3 Comparison of observed line shapes (solid curves) with those calculated (data points) according to the model by Bouanich et al. (1981, 1983) for the first overtone absorption of pure carbon monoxide at 227 °C and various densities g.

Figure 6.2-4 Density dependence of the wavenumber at the P and R branch first overtone absorption maxima, vpimdx) and i>/f(max), of pure CO at temperatures between 20 and 227 °C Vm is the arithmetic mean of vpimsx) and t>/ (max) -f is quoted from Vu et al. (1963) and -o- from Bouanich et al. (1981).

Diels-Alder reactions have also been studied via the fundamental and the first overtone absorptions of carbonyl stretching modes. Absorbance spectra measured during the cycloaddition of 2 (with R = CCI3) and 3 in CH2CI2 solution at 1000 bar and 95 °C are shown in Fig. 6.7-18. 

Two experiments have dealt with the effect of higher levels of vibrational excitation on a bimolecular reaction. Overtone absorption from v—0 to o = 6 can be induced in HCl using visible light from a dye laser. Due to the small cross-section associated with overtone transitions, the high laser intensities obtainable intracavity are needed. For reaction (17), the activation energy... 

In recent experiments lasers have been used to excite unimolecular reactants. In overtone excitation, states in which an MH or MD bond contains n quanta of vibrational energy are prepared by direct single-photon absorption. Here, M is a massive atom in contrast to H or D. For states with large n, the energy in the bond may exceed the molecule s unimolecular threshold. Overtone excitation of CH and OH bonds has been used to decompose molecules (Crim, 1984, 1990). One limitation of this technique is the very weak oscillator strengths of overtone absorptions. 

From a time-dependent point of view, recurrences in the probability of occupying the initially prepared state give rise to the fine structure in the overtone absorption spectrum. Though rudiments of these recurrences may be present in the short-time trajectory P n,t), chaotic classical motion destroys the longer time recurrences, which occur quantum mechanically. It is these latter recurrences which are needed to evaluate fine details in the absorption spectrum. Thus, the classical trajectory method may be limited to the evaluation of low-resolution absorption spectra. However, it should be pointed out that progress is being made in extracting information from systems with... 

Many weak combination and overtone absorptions appear between 2000 and 1667 cm The relative shapes and number of these peaks can be used to tell whether an aromatic ring is mono-, di-, tri-, tetra-, penta-, or hexasubstituted. Positional isomers can also be distinguished. Since the absorptions are weak, these bands are best observed by using neat liquids or concentrated solutions. If the compound has a high-frequency carbonyl group, this absorption will overlap the weak overtone bands so that no useful information can be obtained from the analysis of the region. 

The interpretation of NIR spectra is not as straightforward as that of IR spectra. The NIR spectra are smoother and the peaks broader. In NIR spectra, bonds with high dipole moments give the strongest overtone absorptions. A basis for quantification in absorption spectroscopy is the Beer-Lambert equation, which relates concentration directly to the absorption... 

Figure I. a) Oscillation models for NIR absorptions. The. solid line represents energy as a function of atomic-bond distances in the anharmonic model. The dashed line shows the corresponding function for the harmonic oscillator b) Graphical representation of the overtone absorptions used in NIR specironielry. The fundamental absorptions form the basts of IR spectroscopy.

With the use of model systems and blends of polycaprolactone and phenoxy resin, it is shown that data obtained from the OH stretch overtone absorption in NIR is consistent with those from the OH stretch fundamental in the mid-IR region. The advantages of NIR analysis of specific interactions are illustrated with spectra obtained from thick films of blends of nylon 6 and the zinc salt of sulphonated PS. 9 refs. 

In principle, serum or blood glucose may be quantified either by using MIR spectroscopy or by exploiting any of three sets of NIR absorptions, namely those corresponding to vibrational combination bands (2000-2500 nm), the first overtone absorptions (1400-1800 nm), or the second overtone bands (950-1250nm). All of these have been explored in attempting to quantitate serum glucose,... 

Fig. 1.9 (a) Water overtone absorption line measured with unmodulated (b) and with phase-modulated single-mode diode laser... 

---