Question

Summarize IR Spectroscopy and what are real-life analogies to IR spectroscopy?

Answer 1

Infrared spectroscopy: Also called as IR spectroscopy or Vibrational Spectroscopy and this is a branch of spectroscopy that deals with the infrared region of the electromagnetic spectrum. This region of light has a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals. For a given sample which may be solid, liquid, or gaseous, the method or technique of infrared spectroscopy uses an instrument called an infrared spectrometer to produce an infrared spectrum.

Theory: Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are resonant frequencies, i.e. the frequency of the absorbed radiation matches the transition energy of the bond or group that vibrates. Further, resonant frequencies are also related to the strength of the bond and the mass of the atoms.

Selection rule for IR: In order for a vibrational mode in a molecule to be "IR active" it must be associated with changes in the dipole of the bond by any means. A permanent dipole is not necessary, as the rule requires only a change in dipole moment. A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules with N number of atoms in them, linear molecules have 3N – 5 degrees of vibrational modes, whereas nonlinear molecules have 3N – 6 degrees of vibrational modes.

Sample preparation: Gaseous samples require a sample cell with a long pathlength to compensate for the diluteness. Whereas Liquid samples can be sandwiched between two plates of a salt (commonly NaCl, although a number of other salts such as KBr or CaF₂ are also used). The plates are transparent to the infrared light and do not introduce any lines onto the spectra. The region of permeability for IR frequency may vary with different plates.

Solid samples can be prepared by either crushing the sample with an oily mulling agent (usually mineral oil Nujol) or to grind a quantity of the sample with a specially purified and fully dried inorganic salt (KBr). This powder mixture is then pressed in a mechanical press to form a translucent pellet through which the beam of the spectrometer can pass. Other way of sample preparation for polymeric materials is called as: "cast film" technique.

FTIR: Fourier transform infrared (FTIR) spectroscopy is a measurement technique that allows one to record infrared spectra. Infrared light is guided through an interferometer and then through the sample (or vice versa). A moving mirror inside the apparatus alters the distribution of infrared light that passes through the interferometer. The signal directly recorded, called an "interferogram", represents light output as a function of mirror position. A data-processing technique called Fourier transform turns this raw data into the desired result (the sample's spectrum): Light output as a function of infrared wavelength (or equivalently, wave number).

Absorption bands: IR spectroscopy is often used to identify structures because functional groups give rise to characteristic bands both in terms of intensity and position (frequency).

Uses and applications: Infrared spectroscopy is a simple and reliable technique widely used in both organic and inorganic chemistry, in research and industry. It is used in quality control, dynamic measurement, and monitoring applications such as the long-term unattended measurement of CO₂ concentrations in greenhouses and growth chambers by infrared gas analyzers. For simple example: It is also used in forensic analysis in both criminal and civil cases, for example in identifying polymer degradation. It can be used in determining the blood alcohol content of a suspected drunk driver. Further, IR-spectroscopy has been successfully used in analysis and identification of pigments in paintings and other art objects such as illuminated manuscripts.