Infrared spectroscopy Intro

Infrared Radiation is absorbed by molecules to create higher energy bonding vibrational states. The vibrational modes affected include stretching and bending modes, and require a change in the dipole moment of the molecule.

At right is a typical Infrared Spectrum, in which the frequency of radiation in units of reciprocal centimeters is plotted against the percentage of transmitted light. Each signal represents a complicated coupling of various vibrational modes resonating in that frequency range. Stretching frequencies for stronger bonds are typically easiest to identify, because they occur at higher frequencies.

Rather than try to identify each signal, the analysis of an Infrared Spectrum involves a systematic inspection of specific regions in the spectrum.

Acquaint yourself with as many examples of Infrared Spectra as you can, beginning with the examples given here:

IR Examples

Click on prominent signals in the spectrum above to reveal their identities

Check out each of the following regions of any IR spectrum:

CLICK ON TABS BELOW

Any OH or NH?

OH signals from alcohols are very strong, broad and appear at around 3400 cm^-1

NH signals from amines are moderate in intensity. 1° amines appear as two signals and 2° amines appear as one signal around 3400^-1

Any sp2 C=C-H?
Any sp3 C-C-H?

C_sp2–H stretch appears at 3150-3000cm^-1
C_sp3–H stretch appears at 3000-2850cm^-1

Any C=O?

Carbonyls are very strong, sharp signals at around 1700cm^-1

Their exact frequency varies according to functional group, but better diagnostics are found in 13C NMR

Any Aryl rings?

In addition to sp2C-H stretches, Aryl rings will show out of plane bending modes at 900-690cm^-1 that reveal the alkyl substitution pattern:
mono-substituted-2 bands around 750 and 700 cm^-1
ortho disubstituted-one band near 750cm^-1
meta-disubstituted-3 bands near 890, 800, and 690 cm^-1
para-disubstituted: 1 band near 830 cm^-1
also look for weak signals near 1600-1475cm^-1

Any C-O?

Usually fairly strong signal in fingerprint region at around 1100cm^-1 (1000-1300cm^-1)
Since this is the fingerprint region, it is the absence of a signal rather than the presence of one that provides information.

Any other outlying signals?

Although less common, any signals at frequency >1700cm-1 are diagnostic.
Look them up if you encounter them (eg nitriles at 2400cm^-1; alkynes at 3300cm^-1)

Which of the following is most consistent with the IR spectrum above?

2-hexanol
acetophenone (methyl phenyl ketone)
3-hexene
Toluene
ethyl acetate
2-methyl butylamine

Click for Answer