9.4 Infrared Spectroscopy
Some functional groups, for example, C=O or C=C, can be seen in the NMR spectrum because they contain carbon atoms, while the presence of others like OH can be inferred from the chemical shifts of the carbon atoms they are joined to. Others cannot be seen at all. These might include NH2 and NO2, as well as variations around a carbonyl group such as COCl, CO2H, and CONH2. Infrared (IR) spectroscopy provides a way of finding these functional groups because it detects the stretching and bending of bonds rather than any property of the atoms themselves. It is particularly good at detecting the stretching of unsymmetrical bonds of the kind found in functional groups such as OH, C=O, NH2 and NO2 .
- The amount of energy needed for stretching and bending individual bonds, while still very small, corresponds to rather shorter wavelengths. These wavelengths lie in the infrared, that is, heat radiation just to the long wavelength side of visible light. When the carbon skeleton of a molecule vibrates, all the bonds stretch and relax in combination and these absorptions are unhelpful. However some bonds stretch essentially independently of the rest of the molecule.
This occurs if the bond is either:
- much stronger or weaker than others nearby, or
- between atoms that are much heavier or lighter than their neighbours.
- The relationship between the frequency of the bond vibration, the mass of the atoms, and the strength of the bond is essentially the same as Hooke's law for a simple harmonic oscillator. The equation shows that the frequency of the vibration n is proportional to the (root of) a force constant f —more or less the bond strength—and inversely proportional to the (root of) a reduced mass m, that is, the product of the masses of the two atoms forming the bond divided by their sum.
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Infrared spectra are simple absorption spectra. The sample is exposed to infrared radiation and the wavelength scanned across the spectrum. Whenever energy corresponding to a specific wavelength is absorbed, the intensity of the radiation reaching a detector momentarily decreases, and this is recorded in the spectrum. Infrared spectra are usually recorded using a frequency measurement called wavenumber (cm-1) which is the inverse of the true wavelength l in centimetres to give convenient numbers (500–4000 cm-1). Higher numbers are to the left of the spectrum because it is really wavelength that is being scanned.
The energy, E , required to excite a bond vibration can be expressed as the inverse of a wavelength l or as a frequency ν. More energy is needed to stretch a strong bond. Larger E means higher wavenumbers (cm-1) or smaller wavelength (cm).