Alkanes group frequencies

Let us first consider the vibrational spectra (infrared) of IPP in the melt (Figure 3-9a) [74], In this physical state, polymer chains possess a conformationally irregular structure like any liquid branched n-alkane. We expect the infrared spectrum to consist of relatively few bands easily identified as the group frequencies of CH3 and CH2 and CH groups. Other modes may be identified with caution, but their location is irrelevant to the present discussion. The experimental infrared spectrum of Figure 3-9a is in full agreement with the expectations. 

Alkanes. The C—H vibrational modes of the alkanes (or mixed compounds containing alkyl groups) that are characteristic and reliable group frequencies are summarized in Table 8.1 (also see Chapter 8W, IR section. Part 11 A). 

In vinylidenes, both groups shift the CH2 wag frequency additively. The CH2 wag frequency has been correlated with the ortho-para directing ability of the group, and with the total electron density on the =CH2 carbon as calculated from molecular orbital theory. The CH2 wag wavenumber can be approximately by the following relationship for vinyls and vinylidenes, for which a selection of A values are given for the X and Y substituents R is an alkane group). 

These spectra were plotted from runs on a Jarrell-Ash 25-300 Raman spectrophotometer with a 4880 A argon ion laser. In some spectra the region from 4000 to 2000 cm" has been plotted so that the intensity is 0.5 times its true value compared to the rest of the spectrum. These are marked xO.5. Like the infrared spectra, these Raman spectra illustrate a group frequencies which are labeled directly on the spectra. Groups illustrated include alkanes in spectra 1-6, cyclohexanes 7-8, aromatics 9-12,15,17,18,20,21,25, 32-34, double bonds 13,14,24, isocyanate 15, triple bond 16, nitrile 17,18, carbonyls 19-26, alcohols 27-29, ether 30, amines 31, 32, nitro 33, C—Cl 34, C Br 35, and mercaptan 36. A molecular formula index of the Raman spectra follows. 

Although the group frequency vibrations are given in Table 5-IV for solution spectra, similar bands are found in liquid and solid state spectra of normal alkanes. For example, the spectrum of w-hexane... 

Solid state spectra of normal alkanes have similar vibrations to those found in the liquid state, and group frequency assignments are similar to those discussed above. In some instances, bands are split because of the symmetry factors present in the solid state. Also, there is the possibility that some of the motions of extended CH2 chains will give distinct bands. These facts have been used in structural assignments but are of limited applicability and will not be discussed here. 

The CH3 rock and CH2 wags and twists are usually not considered good group frequencies since, although there generally is a band near 1305 cm in normal alkanes that is a CH2 wagging vibration, they are weak and difficult to assign. 

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. 

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. 

In a previous section we have seen that a CH2 structural group in saturated alkanes has characteristic group frequencies. In this section, we shall consider CH2 groups in structures where an atom other than a saturated carbon is adjacent to the CH2 as, for example, in compounds such as NH2CH2CH2NH2 and CH2CI2. For the present we shall not discuss structures like CH2=CRiR2, where the CH2 is a terminal group. 

Thus, hydrocarbon group frequencies are the only recognizable absorption bands in the Nujol spectrum. These group frequencies, of course, also are found in the n-hexane spectrum. The remaining bands in the hexane spectrum become merged into background absorption in the mixture of alkanes in the mineral oil and a very simple spectrum emerges. 

Most of the methyl and methylene C—H stretching modes that are good group frequencies in the linear alkanes transfer directly to branched alkanes with little change in wavenumber value. The relative intensity of the C H stretch methyl doublet compared to the methylene doublet, however, often will be quite different from the linear chain isomer. 

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