Stretching overtone absorption

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. 

Molecular vibration absorptions are typically observed at infrared wavelengths, which correspond to the resonance frequencies for fundamental molecular vibrations. Because of the molecular potential anharmonicity, however, overtone and combination absorption bands also appear at visible and near-infrared wavelengths. The predominant factor for attenuation in POFs has been the stretching overtone absorptions of C-H bonds. 

As mentioned in the previous subsection, absorption losses in POFs have been attributed mainly to the stretching overtone absorption of the C-H bonds, resulting in much higher attenuation in POFs than in GOFs. Therefore, the... 

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. 

Figure 6.2-7 Molar absorption coefficient e of pure CO (density 0.15 g cm 127 °C) in the fundamental as well as the first and second stretching overtone regions.

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. 

Raman spectrum of I2 is observed. A solution of the Br2 cation also gives a resonance Raman spectrum with a fundamental of 360 cm and strong overtones 14). Edwards and Jones (17) reported that solid Br2" Sb3Pie has a Raman band at 368 cm which they attributed to the Brg cation. Table I shows the stretching frequencies, absorption maxima, and bond lengths of the halogens and the diatomic halogen... 

To illustrate the occurrence of free 0-H first overtone absorption using a fundamental absorption occurring at approximately 3625 cm, the first overtone (in wavenumbers) of free 0-H stretching should occur at... 

The C-H stretch first overtone of terminal methylene groups of vinyl and vinylidene structures is isolated enough that it can be used in traditional quantitative analysis. Figure 3.1 provides one example, and Table 3.1 provides some typical peak locations. Goddu provides tables of absorp-tivities for the first overtone absorption of the terminal methylene group in a variety of compounds and solvents. Molar absorptivities are about 0.2-0.5 1/mol-cm. Put another way, a 100-ppm amount of methylene gives an absorbance of 0.01 in a 10-cm cell. Analyses using this peak to measure the vinyl content of acrylate monomers, butadienes, and edible oils" have been reported. 

As in the C-H stretch overtones, trans double bonds do not have distinctive absorptions in the combination region, but cis double bonds do. There is a strong cis peak near 4673 cm (2140 nm) that can be used for quantification, and a second peak near 4587 cm (2180 nm). These peaks are pointed out in Figure 3.2. 

C-H, N-H, and O-H Stretch Absorption Bands for Specific Long-Wavelength NIR (1100-2500 nm) Functional Croups (1st (2v) through 4th (2v) C-H-Stretching Overtones)... 

Iv) Fundamental stretching vibrational absorption band (2v) First overtone of fund ental stretching band (3v) Second overtone of fundamental stretching band (4v) Third overtone of fundamental stretching band (5v) Fourth overtone of fundamental stretching band (6v) Fifth overtone of fundamental stretching band... 

The intensities of overtone absorption bands depend on the anharmonicity, and it has been shown [23] that vibrations with low anharmonicity constants also have low overtone intensities. X—H stretching vibrations, for example, have the largest anharmonicity constants and therefore dominate the spectra in the NIR region. Table 2.1 summarizes the anharmonicity constants of the vibrations of some characteristic functionalities. 

The copoljnneric fiber is prepared by the rod-m-tube method, and the core composition is 65/35 = MMA/PFPMA (mol.%). The emission wavelength (670-680 nm) of VCSEL is located at the shoulder of the fifth overtone of the C-H stretching vibrational absorption. Whereas the attenuation at the wavelengths of 670-680 nm for the PMMA-based GI POF is as high as 295-323 dB km" on account of the strong C-H absorption losses, the copolymeric GI POF has much smaller peaks and the attenuation is 172-185 dB km" this satisfies the required limit on attenuation for optical home network systems. 

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