Scissor bending vibration

The medium-intensity absorption in nonanal (Fig. 2.36) at 1460 cm is due to the scissoring (bending) vibration of the CH2 group next to the carbonyl group. Methylene groups often absorb more strongly when they are attached directly to a carbonyl group. 

The absorption band near 1375 cm-1, arising from the symmetrical bending of the methyl C—H bonds, is very stable in position when the methyl group is attached to another carbon atom. The intensity of this band is greater for each methyl group in the compound than that for the asymmetrical methyl bending vibration or the methylene scissoring vibration. 

C—H Bending Vibrations Cyclization decreases the frequency of the CH2 scissoring vibration. Cyclohexane absorbs at 1452 cm-1, whereas n-hexane absorbs at 1468 cm-1. Cyclopentane absorbs at 1455 cm-1, cyclopropane absorbs at 1442 cm-1. This shift frequently makes it possible to observe distinct bands for methylene and methyl absorption in this region. Spectra of other saturated hydrocarbons appear in Appendix B hexane (No. 1), Nujol (No. 2), and cyclohexane (No. 3). 

Assignments have been made for a few of the more prominent and reliable in-plane bending vibrations. The vinyl group absorbs near 1416 cm-1 because of a scissoring vibration of the terminal methylene. The C—H rocking vibration of a cis-disubstituted alkene occurs in the same general region. 

An in-plane bending An out-of-plane bending vibration (scissoring) vibration (twisting)... 

The remaining two vibrational modes studied imder bending vibrations involves scissoring. This is due to the bending motion in all possible planes around the bond axis. Since turning the plane of vibrations can not alter the energy it leads to deformation vibration to produce a peak in the region 667 cm f This is why CO, shows two absorption peaks, one at 2330 cm and the other at 667... 

A net adsorption value (corrected for any IRS prism contamination) which could be related to calibration curves for surface concentration was obtained from the ratio of the amide I (C=0 stretching) band at 1640 cm to a standard band for each substrate polymer. The standard bands used were the CH bending vibration at 1400 cm for poly (dimethyl siloxane) and the CF2 scissoring deformation at 1450 cm for fluorinated ethylene/propylene copolymer. Because of the amide I band in polyurethanes, a 4X expanded abscissa was used for the other two polymers, the IX scale was sufflcient. 

The vibrations of a molecule containing three or more atoms are more complex (Figure 13.12). Such molecules can experience symmetric and asymmetric stretches and bends, and their bending vibrations can be either in-plane or out-of-plane. Bending vibrations are often referred to by the descriptive terms rock, scissor, wag, and twist. 

Because the central atom is not in tine with the other two, the symmetric stretching vibration produces a change in dipole momeni and is thus IR active. For example, stretching peaks at 3657 and 3766 cm (2.74 and 2.66 pm) appear in the IR spectrum for the symmetric and asymmet ric stretching vibrations of the water molecule. I here is only one component to the scissoring vibration for this nonlinear molecule because motion in the plane of the molecule constitutes a rotational degree of freedom. For water, the bending vibration causes absorption at 1595 cm (6.27 pm). The difference in behavior oflinear and nonlinear triaiomic molecules with two and three absorption bands, respectively. illustrates how IR absorption spectroscopy can sometimes be used to deduce molecular shapes. 

Figure 4.3 Principal modes of vibration between carbon and hydrogen in an alkane (a) symmetrical stretching, (b) asymmetrical stretching and the bending vibrations, (c) scissoring,...

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

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