What principle underlies infrared (IR) spectroscopy?

Infrared spectroscopy is based on the principle that molecules absorb specific frequencies of infrared light, causing molecular vibrations.

Infrared (IR) spectroscopy is a powerful analytical technique used to identify and study chemicals by their infrared absorption. It operates on the principle that molecules absorb specific frequencies of infrared light that are characteristic of their structure. These frequencies correspond to the energy required to excite the molecule from one vibrational level to another.

When infrared radiation is passed through a sample, certain frequencies are absorbed by the sample, causing the molecules to vibrate. This vibration can occur in different modes such as stretching or bending, depending on the type of bond and the molecular structure. The remaining light then passes through the sample and is detected on the other side. By analysing the frequencies of light that are absorbed, we can gain information about the types of bonds present in the sample, and therefore, its molecular structure.

The principle of IR spectroscopy is rooted in quantum mechanics. According to the quantum theory, molecules can only absorb energy in discrete amounts, known as quanta. The energy of a quantum of light, or a photon, is directly proportional to its frequency. Therefore, when a molecule absorbs a photon of infrared light, it gains a specific amount of energy, which causes it to vibrate. The frequency of the absorbed light corresponds to the energy difference between two vibrational levels of the molecule.

In practice, an IR spectrometer records an absorption spectrum, which is a plot of the intensity of absorbed light as a function of frequency. Each peak in the spectrum corresponds to a specific vibrational mode of the molecule. By comparing the spectrum with reference spectra of known substances, we can identify the chemical compounds present in the sample.

In summary, the principle underlying IR spectroscopy is that molecules absorb specific frequencies of infrared light, which correspond to their vibrational energy levels. This absorption causes the molecules to vibrate, and by analysing the absorbed frequencies, we can identify the types of bonds and the molecular structure of the sample.