Near-infrared spectra group frequencies

Figure 4 illustrates the infrared spectrum for a sample of PPE. The absorptions of the peaks at 3.4, 6.9 and 7.3 pm were assigned to C-H stretch and C-H bending frequencies in CH2 and CH3 (33). These absorptions are proportional to the surface density of deposited ethane (16). However, the absorptions at photons near 10 pm are attributable to OH deformations and CO stretchings of alcoholic groups and vibrations of alkyl ketones (22). They also indicate the existence of branches in unsaturated chain (33). 

Although the infrared spectrum is characteristic of the entire molecule, it is true that certain groups of atoms give rise to bands at or near the same frequency regardless of the structure of the rest of the molecule. 

We calculate an index of hydrogen deficiency of three. A quick glance at the infrared spectrum reveals the source of unsaturation implied by an index of three a nitrile group at 2260 cm (index = two) and a carbonyl group at 1747 cm (index = one). The frequency of the carbonyl absorption indicates an unconjugated ester. The appearance of several strong C—O bands near 1200 cm confirms the presence of an ester functional group. We can rule out a CM2 bond because they usually absorb at a lower value (2150 cm ) and have a weaker intensity than compounds that contain C=N. 

Once an infrared spectrum has been recorded, the next stage of this experimental technique is interpretation. Fortunately, spectrum interpretation is simplified by the fact that the bands that appear can usually be assigned to particular parts of a molecule, producing what are known as group frequencies. The characteristic group frequencies observed in the mid-infrared region are discussed in this chapter. The types of molecular motions responsible for infrared bands in the near-infrared and far-infrared regions are also introduced. 

Infared Analysis. The molecule is 4-chlorobenzaldehyde.The infrared spectrum of this aromatic aldehyde (Fig. 6.18, page 182) is rich and interesting.The aromatic aldehyde macro group frequency train (see Strategies for Interpreting Infrared Spectra) consists of peaks near 3090, 3070, 2830 and 2750, 1706, 1602, 1589, and 1396 cm ... 

This is one of the most common analytical techniques used with plastics. The easy operation and availability of this type of equipment have contributed to its popularity. Although the infrared spectrum characterizes the entire molecule, certain groups of atoms give rise to absorption bands at or near the same frequency, regardless of the rest of the molecule s structure. The persistence of these characteristic absorption bands permits identification of specific atomic groupings within the molecular structure of a sample. 

One may well wonder why the in-phase cw-CH wag near 690 cm is so much lower in frequency than the in-phase trans-CYi wag near 970 cm" in the infrared spectrum. In Fig. 4.16, the lone CH wag of the C2C=CHC group has a frequency near 825 cm in hydrocarbons. It can be seen that this vibration twists the C C bond. Also in Fig. 4.16 are seen the in-phase and out-of-phase CH wag vibrations in trans- and cis-C—CH==CH—C groups. Only the in-phase wag is infrared active. In the trans isomer this in-phase vibration gives the C=C bond a double twist, and in the cis isomer the... 

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