Vibrations of methylene group

Changes in a Cr-Al-CM-41 catalyst during exposure to ethene at 373 K were monitored by UV-vis-NIR spectroscopy by Weckhuysen et al. (2000), who used the Harrick equipment. Reduction of chromium (initially Cr6+, Cr5+, and Cr3+) was observed, and C-H vibrations of methylene groups that were detected in the NIR region indicated polymerization. 

In the region 760-670 cm1, there are bands which concern to pendular vibrations of methylene groups (-CH2-) their presence in spectra of polymers in the solid state is caused by the interaction between the next molecules. Location and the form of these bands depend also on connection of various functional groups to a chain. For the sample treated by mechanical stirring, these bands are very intensive and narrow. 

The chemical modification of the clay can be studied with the help of Fourier transform infrared spectroscopy (FTIR). In FTIR spectra of co-treated and mono-treated clays, the transmittance bands at 2922 cm and 2854 cm correspond to asymmetric and symmetric stretching vibration of methylene groups (Figure 9.5). Carbonyl stretching shows a band at 1715 cm. Both the mono-treated and co-treated montmorillonites have the absorbance bands of methylene, which is due to the successful tethering of octodecylammonium and aminoimdecanoic acid to the clay platelets [24]. 

The first overtone stretching vibrations of methylene groups of strained-ring cyclic compounds such as cyclopropane occur near 6135 cm (1630 nm). The effects of various substituents on the ring have been studied by several authors. Gassman and Zalar list the band positions of 37 cyclopropane derivatives. Gassman also published a table of first overtone CH band positions of aliphatic nortricyclene derivatives. These overtones were at sUghtly lower wavenumber maxima than the cyclopropanes — about 6024 cm (1660 nm). 

Estimate the frequencies of the peaks in the IR spectrum of methylene chloride shown in Figure 26F-2. From these frequencies, assign molecular vibrations of methylene chloride to each of the peaks. Notice that some of the group frequencies that you will need are not listed in Table 26-5, so you will have to look elsewhere. 

The stretching vibrational modes of methylene group. The two drawings show stretching vibrations — symmetric (on the left) and asymmetric on the right. 

The bending vibrational modes of methylene group The top two dii rams frrocking modes. The bottom two dii rams show wagging and twisting modes. 

Progression of bands in solid state spectra, probably due to wagging and/or bending mode of vibration of the C—H bonds of methylene groups. The number of bands in the progression is indicative of chain length. 

The temperature dependence of the hyperfine splitting width due to P-protons of alkyl radicals shows much of a mobile character in the case of the polyethylene chain in UPEC than in the case of bulk systems, as shown in Sect. 5 and vibration appears to occur around the chain axis since data analysis suggested vibrational motion of methylene groups. 

One factor, which is of value in identifying a band in the 750—720 cm" re on as being due to this type of methylene-group vibration, is its behaviour when the material is examined in different states. Like the CH2 deformation frequency, this band is always double below the transition point and in the solid state, and single in... 

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