Introduction to Interpretation of Infrared Spectra

IR Spectroscopy is an extremely effective method for determining the presence or absence of a wide variety of functional groups in a molecule. (For a detailed listing, see the table showing important IR absorptions of various functional groups.) One way to begin analyzing an IR spectrum is to start at the high wavenumber end of the spectrum (typically 4000 cm-1) and look for the presence and absence of characteristic absorptions as you move toward lower wavenumbers. Some of the most common, and distinctive, absorptions are organized into several regions below. This is the type of analysis that you should be able to do without consulting notes. If necessary, a more detailed analysis could then be attempted by consulting a text on IR interpretation.

Important Regions of the IR Spectrum

3600-2700 cm-1
X-H stretch region

3600-3300 cm-1 Alcohol O-H
Amine or Amide N-H
Alkyne C-H
The alcohol OH stretch is usually a broad and strong absorption near 3400. The NH stretch is typically not as broad or strong as the OH, and in the case of an NH2 it may appear as two peaks. The terminal alkyne C-H may be confirmed by a weak CC triple bond stretch near 2150 cm-1
3300-2500 cm-1 Acid O-H This is normally a very broad signal centered near 3000 cm-1.
3200-3000 cm-1 Aromatic (sp2) =C-H
Alkene (sp2) =C-H
The aromatic CH's usually appear as a number of weak absorptions, while the alkene C-H is one or a couple stronger absorptions.
3000-2800 cm-1 Alkyl (sp3) C-H Almost all organic compounds have alkyl CH's so this is not usually too informative. However, the intensity of these peaks relative to other peaks gives a hint as to the size of the alkyl group.
2850 and 2750 cm-1 Aldehyde C-H Two medium intensity peaks on the right hand shoulder of the alkyl C-H's. Look for confirming carbonyl C=O peak.

2300-2100 cm-1

2260-2210 cm-1 A sharp, medium intensity peak. Carbon Dioxide in the atmosphere may also result in an absorption in this area if not subtracted out.
2260-2100 cm-1 This peak's intensity varies from medium to nothing. Since the intensity is related to the change in dipole moment, symmetrical alkynes will show little or no absorption here!

1850-1500 cm-1
C=X stretch region

1850-1750 cm-1 Anhydride C=O
3-4 membered ring C=O
Anhydrides have two absorptions, one near 1830-1800 and one near 1775-1740. The absorption frequency increases as the ring size decreases. For example: cyclohexanone=1715, cyclopentanone=1745, cylobutanone=1780, cyclopropanone=1850.
1750-1700 cm-1 Aldehyde C=O
Ketone C=O
Ester C=O
Acid C=O
This is usually the most intense absorption in the spectrum.
1700-1640 cm-1 Amide C=O
Conjugated C=O
Because of the weakening of the C=O due to resonance, amides and conjugated carbonyl's come slightly lower than "normal" C=O. In general, conjugation lowers the absorption by 20-50 cm-1.
1680-1620 cm-1 Alkene C=C This absorption is not as intense as that seen for C=O. It is variable and may be fairly small in symmetrical, or nearly symmetrical cases. Look for confirming alkene C-H peaks above 3000.
1600-1400 cm-1 Aromatic C=C Multiple sharp, medium peaks. The pattern of peaks varies depending upon the substitution pattern. Usually there is one peak around 1600 and several others at lower wavenumbers. Look for confirming aromatic C-H peaks slightly above 3000.

1500-400 cm-1
Fingerprint Region

1300-1000 cm-1 C-O A strong absorption.
1500-400 cm-1 Various Interpretation of peaks in the fingerprint region is complicated by the large number of different vibrations that occur here. These include single bond stretches and a wide variety of bending vibrations. This region gets its name since nearly all molecules(even very similar ones) have a unique pattern of absorptions in this region.

Other Helpful Characteristics to Keep in Mind When Interpreting IR Spectra

Wavenumber of Absorption

The absorption wavenumber for a stretching vibration is related to both the force constant between the two atoms (k) and the mass of the two atoms (m1 and m2) by the Hooke's Law equation:

From this relationship, two important trends in the wavenumber for stretching vibrations can be deduced.
  1. As the bond strength increases, the wavenumber increases. For example:

  2. As the mass of one of the two atoms in the bond increases, the wavenumber decreases. (Assuming the change in bond strength is relatively small.) For example:

Intensity of Absorption

The intensity of an absorption in the IR spectrum is related to the change in dipole that occurs during the vibration. Consequently, vibrations that produce a large change in dipole (e.g. C=O stretch) result in a more intense absorption than those that result in a relatively modest change in dipole (e.g. C=C). Vibrations that do not result in a change in dipole moment (e.g., a symmetrical alkyne C triple bond C stretch) will show little or no absorption for this vibration.

For additional information on IR spectroscopy check out the Organic Chemistry Online IR Page.

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