Group frequencies scissoring

Tables V through X are other group frequency assignments for scissors vibrations of CH3 and CH2 groups. These have been abstracted from the recent literature and should be of value to the student in group-frequency assignments.

We can summarize the group frequency assignments for normal alkanes by stating that the CH stretching and scissors vibrations and the CH2 rock near 720 cm are considered good group frequencies. 

CH3 Scissors Vibrations. There appear to be several generalizations that can be made concerning the group frequencies of the CH3... 

CH3—C—C—, the symmetric scissors moves to a lower value near 1360-1355 cm" In Table 5-VI several examples of vibrational assignments of the symmetric scissors in this range can be found, and a sufficient number of ketones have been investigated to make this a fairly certain group frequency assignment (see footnote 1 to Table 5-VII). However, it appears that this group frequency assign-... 

We have listed the position of the symmetric scissors in the 1360-1355 cm range as a characteristic group frequency of methyl groups in ketone structures in Table 5-VII. 

The asymmetric scissors of CH3 groups in CH3—C— structures appears to be at a lower frequency than that found in alkanes (1460 cm" ). While this has not been investigated for a large number of compounds it is listed in Table 5-VII as a characteristic group frequency. 

A number of acetylacetonates have been investigated, and for these the CH3 asymmetric scissors appears in the 1415-1380 cm" range while the symmetric scissors appears in the 1390-1350 cm" range. These too are listed as characteristic group frequencies in Table 5-VII. 

To find characteristic group frequencies for the CH2 group, we can utilize the vibrational analyses reported for molecules containing this group. In Table 5-VIII, the vibrational analyses of a number of these molecules are presented. Only vibrations which may be characteristic group frequencies are listed. A CH2 structural unit can have a symmetric and an asymmetric stretch, a symmetric scissors, and wags, twists, and rocks. These are the general classifications listed in Table 5-VIII. 

In general the CH2 scissors is therefore a good group frequency, only slightly perturbed by its environment. 

A CH2 scissors vibration can also be visualized for CH2 ring systems, and usually one or more bands are found near 1465 cm" which are characteristic group frequencies of the CH2 group. 

The characteristic group frequencies for CH3 and CH2 structural units when these units are attached to aromatic ring systems remain fairly constant in position and generally are not obscured by aromatic ring vibrations. However, an aromatic ring vibration near 1500 cm can sometimes obscure the 1465 cm scissors vibration of the CH3 and CH2 groups for some molecules, and for solid aromatic compounds, where sample preparation is difficult, the bands may be broad and may overlap. 

Compound CH stretching 1465 scissors 1380 scissors Aromatic ring group frequencies ... 

In summary, we can state that in general the stretching and scissors vibrations of CH2 and CH3 groups can be identified, although for some of the aromatic compounds more bands may appear in the 1450-1300 cm region than just the two scissors vibrations. In addition, for some compounds, the CH3 and CH2 group frequencies may be obscured by vibrations from groups such as NO2. 

One of the best series of group frequencies for recognizing the presence of aromatic ring structures occurs in the 1600-1450 cm region. Since this region is near the 1465 cm CH3 scissors vibration, we have discussed some of these bands for aromatic compounds in Section 5.41 (see Figures 5-10, 5-11, and 5-12 and Table 5-XIII). We shall refer to the data presented in that section as well as spectra shown in Figures 5-20 to 5-25. 

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