IR SPECTROSCOPY: IR Light and Matter
Recall that the IR region of the electromagnetic spectrum contains less energy (lower frequency, higher wavelength) than the UV/visible region. Each photon of IR radiation contains less energy than a UV/visible photon. Recall that when photons of UV/ visible light are absorbed by a substance, electrons are promoted to higher energy levels. In the case of photons of IR radiation, there is not enough energy to promote electrons. Instead, each bond between all atoms that make up a given molecule will vibrate much like two weights connected by a spring. The particular wavelengths of light that are absorbed by chemical bonds depend upon the atoms on each side of the bond and the strength of the bond that holds them together. This is analogous to the weights and spring model of bonds. The frequency of vibration of two weights and a spring depends upon the mass of each weight and the strength of the spring. The weight and spring model is shown in Figure 5.10. The chemical bond is the spring and the atoms on either side of the bond are the weights. When the weights attached to a spring are pulled apart the spring will vibrate back and forth at a frequency that depends upon the strength of the spring and the amount of weight on either side. When IR radiation of the proper energy strikes a molecule, one of its bonds may absorb it and cause a vibration to take place. Like UV/visible interactions with matter, IR absorptions are also quantized. The energy of a photon must exactly match the proper energy of vibration of one of the bonds in the molecule. Each different chemical bond in the molecule has its own characteristic vibrations and each bond can undergo a number of different kinds of vibrations. Some of the most common vibrations of molecules are shown in Figure 5.11.
FIGURE 5.10 A molecule can be visualized as a set of weights connected by springs. The springs are the chemical bonds made up of shared electrons. This diagram shows a molecule of water (H2O) and a model using weights and springs. When the springs are pulled or bent, they contract and expand or wag back and forth. When the bonds between two atoms absorb energy, they vibrate back and forth at characteristic frequencies.
FIGURE 5. Each chemical bond in a molecule can undergo several types of vibrations as shown in the figure. These vibrations absorb energy and give rise to the infrared (IR) spectrum. More complex molecules contain more bonds, which results in a more complex IR spectrum. Each molecule has a unique set of vibrational absorptions.
The result is that, unlike UV/visible absorptions, there are many IR absorptions in each type of molecule. Even the slightest change in the composition of a molecule will result in a different IR spectrum. Thus, the IR spectrum of substance is unique and can be used to unequivocally identify that substance. IR spectrophotometry is one of the two analytical techniques, along with gas chromatography/mass spectrometry, that can be used for identification of pure substances, such as drugs. So, thus far we have seen two important differences between UV and IR radiation; IR is of lower energy and causes bond vibrations rather than electron promotion and all substances absorb IR radiation whereas only some substances absorb UV radiation.
