Infra-red spectroscopy

INFRA-RED SPECTROSCOPY

Key Notes

Introduction

Energy absorption in the IR region of the electromagnetic spectrum results in the stretching or bending of covalent bonds. More energy is required in a stretching vibration than a bending vibration. More energy is required to stretch multiple bonds compared to single bonds. Bonds to light atoms vibrate faster than bonds to heavy atoms.

The IR spectrum

An IR spectrum is a measure of energy absorption versus the reciprocal wavelength (known as the wavenumber) of the radiation involved. The higher the wavenumber the greater the energy involved.

Detectable absorptions

IR energy is only absorbed if the vibration results in a change in dipole moment.

The fingerprint region

The fingerprint region contains many peaks and it is not possible to identify the majority of peaks present in that region. Some peaks associated with particular functional groups can be identified if they are more intense than their neighbors. The fingerprint region is useful when comparing two com-pounds to see if they are identical.

Identification of functional groups

Functional groups display characteristic absorptions at particular regions of the IR spectrum allowing the identification of such groups in a molecule.

Introduction

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 finger-print region below 1500 cm⁻¹, illustrating the effect of mass on vibrational frequency.

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 absorptions

Not all vibrations can be detected by infra-red spectroscopy. IR energy is only 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 region

For most organic molecules, there are a large number of possible bond vibrations, 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 fingerprintregion. This region is particularly useful when comparing the spectrum of a testcompound 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 functional groups

IR spectra are particularly useful for identifying the presence of specific functionalgroups  in  a  molecule,  since  the  characteristic  vibrations  for  these  groups  areknown 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 thefunctional groups present.