Instant Notes: Analytical Chemistry

Section E – Spectrometric techniques

E11 INFRARED AND RAMAN

SPECTROMETRY:

APPLICATIONS

Comparison of the infrared (or Raman) spectra of related compounds illustrates

the importance of the group frequency concept. The following examples will

illustrate this.

Example 1.The spectrum of compounds containing aliphatic chains exhibit

bands due to CH 3 – , –CH 2 – and =CH–groups, both in their stretching and

bending modes. The IR spectrum of aliquid paraffin, or Nujol, used as a

mulling agent is shown in Figure 1.

The –CH stretching vibrations all absorb just below 3000 cm-^1 , and provide at

least four peaks. The bending vibrations of –CH 3 – and –CH 2 – absorb around

1450 cm-^1 whereas the characteristic band at 1380 cm-^1 is due to the methyl

Structural
identification

Key Notes

The group frequencies observed in an infrared or Raman spectrum of

organic compounds are useful indicators of molecular structure.

Inorganic materials may also give specific infrared bands characteristic of

their structure and bonding.

The intensity of infrared absorbances obeys the Beer–Lambert law and

may be used for quantitative analysis, especially for time-resolved

measurements and kinetic studies. Near infrared spectrometry is used

extensively for quantitative work.

Related topic Infrared and Raman spectrometry: principles and instrumentation (E10)

Structural

identification

Quantitative

measurements

4000 3000 2000

cm–1

1500 1000 500

% T

Fig. 1. Liquid paraffin: (Nujol).