Infrared Spectroscopy

Background

The infrared region of the electromagnetic spectrum extends from 14,000 cm-1 to 10 cm-1. The region of most interest for chemical analysis is the mid-infrared region (4,000 cm-1 to 400 cm-1) which corresponds to changes in vibrational energies within molecules. As with all spectroscopic techniques, infrared spectroscopy can be used to identify compounds or investigate sample composition.

Use of the Technique

The principal strengths of using infrared Spectroscopy are:

  • it is a quick and relatively cheap spectroscopic technique
  • it is useful for identifying certain functional groups in molecules and
  • an IR spectrum of a given compound is unique and can therefore serve as a fingerprint for this compound.

Molecular Vibrations

All molecules vibrate, even at a temperature of absolute zero. In general, a polyatomic molecule with N atoms has 3N-6 distinct vibrations. Each of these vibrations has an associated set of quantum states and in IR spectroscopy the IR radiation induces a jump from the ground (lowest) to the first excited quantum state.

The Fingerprint Region

The fact that there are many different vibrations even within relatively simple molecules means that the infrared spectrum of a compound usually contains a large number or peaks, many of which will be impossible to confidently assign to vibration of a particular group. Particularly notable is the complex pattern of peaks below 1500cm-1 which are very difficult to assign. However, this complexity has an important advantage in that it can serve as a fingerprint for a given compound. Consequently, by referring to known spectra, the region can be used to identify a compound.

Interpretation of Spectra

When assigning peaks to specific groups in the infrared region it is usually the stretching vibrations which are most useful. Broadly speaking, these can be divided into four regions:

  • 3700 – 2500 cm-1 Single bonds to hydrogen
  • 2300 – 2000 cm-1 Triple bonds
  • 1900 - 1500 cm-1 Double bonds
  • 1400 - 650 cm-1 Single bonds (other than hydrogen)

It should also be noted that the region 1650 - 650cm-1 contains peaks due to bending vibrations but it is rarely possible to assign a specific peak to a specific group.

Single Bonds to Hydrogen

Bond Wavenumber (cm-1) Notes
C-H 3000-2850 Saturated alkanes, limited value as most organic compounds contain C-H
=C-H 3100 - 3000 Unsaturated alkene or aromatic
ºC-H 3300 Terminal Alkyne
O=C-H 2800 and 2700 Aldehyde, two weak peaks
O-H
O-H (free)		 3400 - 3000
~3600		 Alcohols and Phenols. If hydrogen bonding
present peak will be broad 3000-2500 (e.g.
carboxylic acids)
N-H 3450 - 3100 Amines: Primary - several peaks, Secondary -
one peak, tertiary - no peaks

Double Bonds

Bond Wavenumber (cm-1) Notes
C=O 1840 - 1800 &
1780 - 1740 		Anhydrides
C=O 1815 – 1760 Acyl halides
C=O 1750 – 1715 Esters
C=O 1740 – 1680 Aldehydes
C=O 1725 – 1665 Ketones
C=O 1720 – 1670 Carboxylic acids
C=O 1690 – 1630 Amides
C=C 1675 – 1600 Often weak
C=N 1690 - 1630 Often difficult to assign
N=O 1560 - 1510 &
1370 - 1330 		Nitro compounds

Triple Bonds

Bond Wavenumber (cm-1) Notes
CºC 2260 – 2120 Alkynes, bands are weak
CºN 2260 - 2220 Nitriles

Single Bonds (not to Hydrogen)

Bond Wavenumber (cm-1) Notes
C-C Variable No diagnostic value
C-O, C-N 1400 – 1000 Difficult to assign
C-Cl 800-700 Difficult to interpret
C-Br, C-I Below 650 Often out of range of instrumentation

Bending Vibrations

Bond Wavenumber (cm-1) Notes
R-N-H 1650 - 1500 Take care not to confuse N-H bend with the C=O stretch in amides
R-C-H 1480 – 1350 Saturated alkanes and alkyl groups
R-C-H 1000 - 680 Unsaturated alkenes and aromatics

Fourier Transform Spectroscopy

Fourier transform infrared (FTIR) spectroscopy is a measurement technique for collecting infrared spectra. Instead of recording the amount of energy absorbed when the frequency of the infra-red light is varied, the IR light is guided through an interferometer. After passing through the sample, the measured signal is the interferogram. Performing a Fourier transform on this signal data results in a spectrum identical to that from conventional (dispersive) infrared spectroscopy.

Measurement of a single spectrum is faster for FTIR than conventional techniques because the information at all frequencies is collected simultaneously. This allows multiple samples to be collected and averaged together resulting in an improvement in sensitivity. By measuring at a specific frequency over time, changes in the character or quantity of a particular bond can be measured.