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. The far infrared region (400 cm-1 to 10 cm-1) is useful for molecules containing heavy atomssuch as inorganic compounds but requires rather specialised experimental techniques.
Use of the Technique
It is rarely, if ever, possible to identify an unknown compound by using IR spectroscopy alone. Its principal strengths are: (i) it is a quick and relatively cheap spectroscopic technique, (ii) it is useful for identifying certain functional groups in molecules and (iii) an IR spectrum of agiven compound is unique and can therefore serve as a fingerprint for this compound.
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 thefirst excited quantum state. Although approximate, each vibration in a molecule can be associated with motion in a particular group. A simple example is methanal, whose 6 vibrations involve the following motions:
H C H
Symmetric C-H stretch
H O H
Asymmetric C-H stretch
H C O H
H C H
H O H
H C O H
Out-of-plane bend ( and here signifydirections of motion not atomic charges)
Clearly, even for this simple molecule, a description of the vibrations is quite complicated and this complexity rapidly increases as the size of the molecule increases. Fortunately, as we will see, it is almost never necessary to be able to picture all the molecular vibrations for IR spectroscopy to be useful.
Notall possible vibrations within a molecule will result in an absorption band in the infrared region. To be infrared active the vibration must result in a change of dipole moment during the vibration. This means that for homonuclear diatomic molecules such as Hydrogen (H 2), Nitrogen (N2) and Oxygen (O2) no infrared absorption is observed, as these molecules have zero dipole moment and stretching ofthe bonds will not produce one. For heteronuclear diatomic molecules such Carbon Monoxide (CO) and Hydrogen Chloride (HCl), which do possess a permanent dipole moment, infrared activity occurs because stretching of this bond leads to a change in dipole moment (since Dipole Moment = Charge × Distance). It is important to remember that it is not necessary for a compound to have a permanent dipolemoment to be infrared active. In the case of Carbon Dioxide (CO2) the molecule is linear and centrosymmetric and therefore does not have a permanent dipole moment. This means that the symmetric stretch will not be infrared active. However in the case of the asymmetric stretch a dipole moment will be periodically produced and destroyed resulting in a changing dipole moment and therefore infraredactivity.
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 1500 cm-1 which are very difficult toassign. 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
To obtain a more detailed interpretation of an IR spectrum it is necessary to refer to correlation charts and tables of infrared data. There are many...