Molecules can absorb energy in the infra-red region of the electromagnetic
spectrum resulting in the increased vibration of covalent bonds. There are two
types of vibration resulting either in the stretching or the bending of bonds. These
vibrations occur at specific frequencies (or energies) depending on the bond
involved. It is useful to think of the bonds as springs and the atoms as weights in
order to rationalize the energy required for such vibrations. There are two factors
affecting the frequency of vibration – the masses of the atoms and the ‘stiffness’ of
the bond. Multiple bonds such as double or triple bonds are stronger and stiffer
than single bonds and so their stretching vibrations occur at higher frequency (or
energy). The stretching vibration of bonds also depends on the mass of the atoms.
The vibration is faster when the bond involves a light atom rather than a heavy
atom. Stretching vibrations require more energy than bending vibrations.
The IR spectrum
An IR spectrum is a graph of the absorbed energy versus the wavenumber (υ).
The wavenumber is the reciprocal of the wavelength (i.e. 1/λ) and is measured in
units of cm−1. It is proportional to the frequency or energy of the radiation and so
the higher the wavenumber, the higher the energy. For example, the absorption
peak due to the stretching of an alkyne triple bond comes in the region
2100–2600 cm−1. This corresponds to a higher energy than the stretching
absorption of an alkene double bond that is in the range 1620–1680 cm−1.
The stretching vibration for a C-H bond occurs in the region 2853–2962 cm−1,
compared to the stretching vibration of a C–O bond which occurs in the fingerprint
region below 1500 cm−1, illustrating the effect of mass on vibrational
Most stretching vibrations occur in the region 3600–1000 cm−1, whereas bending
vibrations are restricted to the region below 1600 cm−1. The normal range for IR
spectra is 4000–600 cm−1.
Detectable Not all vibrations can be detected by infra-red spectroscopy. IR energy is only
absorptions absorbed if the vibration leads to a change in the molecule’s dipole moment. Thus,
the symmetrical C=C stretching vibration of ethene does not result in the
absorption of IR energy, and no absorption peak is observed.
The fingerprint For most organic molecules, there are a large number of possible bond vibrations,
region and this number increases as the molecule becomes more complex. As a result,
there are usually a large number of peaks observed such that the IR spectrum of
one molecule is almost certain to be different from that of another. The region
where most peaks occur is generally below 1500 cm−1 and is called the fingerprint
region. This region is particularly useful when comparing the spectrum of a test
compound against the spectrum of a known compound. If the spectra are
identical this is good evidence that both compounds are identical.
Since the fingerprint region is usually complex with many peaks present,
it is not possible to assign the type of vibration associated with each peak unless
a particular peak shows greater intensity over its neighbors or ‘stands alone’.
Absorptions for some functional groups such as esters, nitro or sulfonate groups
do occur in the fingerprint region and can be identified because of their position
and intensity. Identification of IR spectra are particularly useful for identifying the presence of specific functional
functional groups groups in a molecule, since the characteristic vibrations for these groups are
known to occur in specific regions of the IR spectrum. For example, absorptions
due to the carbonyl stretching of an aldehyde occur in the region 1690–1740 cm−1
whereas the corresponding absorptions for an ester occur in the region
1735–1750 cm−1. IR tables can be used to assign the various peaks and hence the
functional groups present.