29.6 Infrared (IR) Spectroscopy

Vibrating Molecules

Photon energies associated with the infrared (from 1 to 15 kcal/mole) (Figure 29.6a.) are not large enough to excite electrons but may induce vibrational excitation of covalently bonded atoms and groups.

The covalent bonds in molecules are not rigid sticks or rods, such as found in molecular model kits, but are more like stiff springs that can be stretched and bent. We must now recognize that, in addition to the rotation of groups about single bonds, molecules experience a wide variety of vibrational motions, characteristic of their component atoms. Consequently, virtually all organic compounds will absorb infrared radiation that corresponds in energy to these vibrations. Infrared spectrometers permit chemists to obtain absorption spectra of compounds that are a unique reflection of their molecular structure.

At room temperature, organic molecules are always in motion, as their bonds stretch, bend, and twist. These complex vibrations can be broken down mathematically into individual vibrational modes. Vibrational modes of bonds within molecules are often given descriptive names, such as stretching, bending, scissoring, rocking and twisting (Figure 29.6b.).

The energy of molecular vibration is quantized rather than continuous, meaning that a molecule can only stretch and bend at certain ‘allowed’ frequencies. If a molecule is exposed to electromagnetic radiation that matches the frequency of one of its vibrational modes, it will in most cases absorb energy from the radiation and jump to a higher vibrational energy state – what this means is that the amplitude of the vibration will increase, but the vibrational frequency will remain the same. The difference in energy between the two vibrational states is equal to the energy associated with the wavelength of radiation that was absorbed. It turns out that it is the infrared region of the electromagnetic spectrum which contains frequencies corresponding to the vibrational frequencies of organic bonds.

Infrared (IR) Spectrum

We will use a ketone sample to illustrate this process. The sample is irradiated with infrared light and the carbonyl bond will specifically absorb light with this same frequency. When the carbonyl bond absorbs this energy, it jumps up to an excited vibrational state (Figure 29.6c.). The molecule does not remain in its excited vibrational state for very long, but quickly releases energy to the surrounding environment in form of heat and returns to the ground state.

With an instrument called an infrared spectrophotometer, we can ‘see’ this vibrational transition. In the spectrophotometer, infrared light is passed through the sample. Most frequencies pass right through the sample and are recorded by a detector on the other side.  Our carbonyl stretching frequency, however, is absorbed by the sample, and so the detector records that the intensity of this frequency, after having passed through the sample, is something less than 100% of its initial intensity. The vibrations of a 2-hexanone molecule are not, of course, limited to the simple stretching of the carbonyl bond. The various carbon-carbon bonds also stretch and bend, as do the carbon-hydrogen bonds, and all of these vibrational modes also absorb different frequencies of infrared light. The IR spectrum for 2-hexanone is shown in Figure 29.6d.  The technique is therefore very useful as a means of identifying which functional groups are present in a molecule of interest. If we pass infrared light through an unknown sample and find that it absorbs in the carbonyl frequency range but not in the alkyne range, we can infer that the molecule contains a carbonyl group but not an alkyne.

Some General Trends:

1. Stretching frequencies are higher than corresponding bending frequencies. (It is easier to bend a bond than to stretch or compress it.)
2. Bonds to hydrogen have higher stretching frequencies than those to heavier atoms.
3. Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn have higher frequencies than single bonds. (Except for bonds to hydrogen).
4. Some bonds do not absorb infrared light at all and will not show in a spectrum.

Infrared spectra may be obtained from samples in all phases (liquid, solid and gaseous). Liquids are usually examined as a thin film sandwiched between two polished salt plates (note that glass absorbs infrared radiation, whereas NaCl is transparent). If solvents are used to dissolve solids, care must be taken to avoid obscuring important spectral regions by solvent absorption. Perchlorinated solvents such as carbon tetrachloride, chloroform and tetrachloroethene are commonly used. Alternatively, solids may either be incorporated in a thin KBr disk, prepared under high pressure, or mixed with a little non-volatile liquid and ground to a paste (or mull) that is smeared between salt plates.

The general regions of the infrared spectrum in which various kinds of vibrational bands are observed are outlined in Figure 26.9e. and Figure 26.9f. Note that the blue coloured sections above the dashed line refer to stretching vibrations, and the green coloured band below the line encompasses bending vibrations. The complexity of infrared spectra in the 1450 to 600 cm^-1 region makes it difficult to assign all the absorption bands, and because of the unique patterns found there, it is often called the fingerprint region. Absorption bands in the 4000 to 1450 cm^-1 region are usually due to stretching vibrations of diatomic units, and this is sometimes called the group frequency region.

Attribution & References