Infrared Spectroscopy: Theory, Method and Uses

The Infrared spectroscopy Is the study of how molecules absorb infrared radiation and finally convert it to heat.

This process can be analyzed in three ways: measuring absorption, emission and reflection. This precision makes infrared spectroscopy one of the most important analytical techniques available to scientists today.

Infrared spectroscopy scheme

One of the great advantages of infrared spectroscopy is that virtually any sample can be studied in almost all states.

Liquids, powders, films, solutions, pastes, fibers, gases and surfaces can be examined with a judicious selection of the sampling technique. As a consequence of the improved instrumentation, a variety of new sensitive techniques have now been developed to examine previously intractable samples.

Infrared spectroscopy, among many other uses and applications, is useful for measuring the degree of polymerization In the manufacture of polymers. Changes in the amount or character of a particular link are evaluated by measuring a specific frequency over time.

Modern research instruments can take infrared measurements across the range of interest as often as 32 times per second.

This can be done while simultaneous measurements are performed using other techniques, making observations of chemical reactions and processes faster and more accurate.

Theory of infrared spectroscopy

An invaluable tool in determining and verifying organic structures involves electromagnetic radiation (REM) with frequencies between 4000 and 400 cm-1 (wave numbers).

The category of EM radiation is called infrared (IR) radiation, and its application to organic chemistry known as IR spectroscopy.

Radiation in this region can be used in the determination of the organic structure making use of the fact that it is absorbed by interatomic bonds in organic compounds.

Chemical bonds in different environments will absorb variable intensities and variable frequencies. Therefore, IR spectroscopy involves collecting the absorption information and analyzing it as a spectrum.

Frequencies where there are IR absorptions (peaks or signals) can be directly correlated with bonds within the compound in question.

Because each interatomic link can vibrate in several different motions (stretching or flexing), individual bonds can absorb on more than one IR frequency.

Stretch absorptions often produce stronger peaks than flexure, however the weaker bending absorptions may be useful for differentiating similar types of bonds (eg, aromatic substitution).

It is also important to note that symmetrical vibrations do not cause absorption of IR radiation. For example, none of the carbon-carbon ethylene Or ethanol absorbs IR radiation.

Instrumental Methods of Structure Determination

Nuclear Magnetic Resonance (NMR)

Excitation of the nucleus of atoms through the radiofrequency irradiation. It provides extensive information on the molecular structure and connectivity of atoms.

Infrared Spectroscopy (IR)

It consists of triggering molecular vibrations through infrared irradiation. It mainly provides information on the presence or absence of certain functional groups.

Mass spectrometry

Sample bombardment with electrons and detection of resultant molecular fragments. Provides information on molecular mass connectivity and atoms.

Ultraviolet (UV) spectroscopy

Promotion of electrons at higher energy levels by irradiating the molecule with ultraviolet light. It provides information on the presence of conjugated π systems and double and triple bonds.

Spectroscopy

It is the study of spectral information. After irradiation with infrared light, certain bonds respond faster by vibration. This response can be detected and translated into a visual representation called the spectrum.

Spectrum interpretation process

  1. Recognize a pattern.
  2. Associate patterns with physical parameters.
  3. Identify possible meanings, that is, propose explanations.

Once a spectrum is obtained, the main challenge is to extract the information it contains in an abstract or hidden form.

This requires the recognition of certain patterns, the association of these patterns with physical parameters, and the interpretation of these patterns in terms of meaningful and logical explanations.

Electromagnetic spectrum

Most organic spectroscopy uses electromagnetic energy, or radiation, as a physical stimulus. Electromagnetic energy (such as visible light) does not have a detectable mass component. In other words, it can be called"pure energy."

Other types of radiation, such as alpha rays, which consist of helium nuclei, have a detectable mass component and therefore can not be classified as electromagnetic energy.

The important parameters associated with electromagnetic radiation are:

• Energy (E): The energy is directly proportional to the frequency, and inversely proportional to the wavelength, as indicated in the equation below.

  • Frequency (μ)
  • Wavelength (λ)
  • Equation: E = hμ

Vibrational modes

  • The Covalent bonds Can vibrate in various modes, including stretching, balancing, and scissors.
  • The most useful bands in an infrared spectrum, correspond to stretching frequencies.

Transmission vs. Absorption

When a chemical sample is exposed to the action of IR LIGHT (infrared light), it can absorb some frequencies and transmit the rest. Some of the light can also be reflected back to the source.

The detector detects the transmitted frequencies, and in doing so also reveals the values ​​of the frequencies absorbed.

An IR spectrum in absorption mode

The IR spectrum is basically a graph of frequencies transmitted (or absorbed) versus the intensity of the transmission (or absorption). The frequencies appear on the x-axis in units of inverse centimeters (wavenumbers), and the intensities are plotted on the y axis in percentage units. The graph shows a spectrum in absorption mode:

Infrared Spectroscopy: Theory, Method and Uses

An IR spectrum in transmission mode

Infrared Spectroscopy: Theory, Method and Uses 1

The graph shows a spectrum in transmission mode. This is the most commonly used representation and is found in most chemistry and spectroscopy books.

Uses and applications

Because infrared spectroscopy is a reliable and simple technique, it is widely used in organic synthesis, polymer science, petrochemical engineering, pharmaceutical industry and food analysis.

In addition, since FTIR spectrometers can be sanitized to chromatography, the mechanism of chemical reactions and the detection of unstable substances can be investigated with such instruments.

Some applications and applications include:

Quality controls

It is used in quality control, dynamic measurement and monitoring applications such as long-term unattended measurement of CO2 concentrations in greenhouses and growth chambers using infrared gas analyzers.

Forensic analysis

It is used in forensic analysis in criminal and civil cases, for example in the identification of degradation of the polymer. It can be used to determine the blood alcohol content of a driver suspected of being drunk.

Analysis of solid samples without cutting

A useful way to analyze solid samples without the need to cut is to use ATR or attenuated total reflectance spectroscopy. Using this approach, the samples are pressed against the face of a single crystal. Infrared radiation passes through the crystal and only interacts with the sample at the interface between the two materials.

Analysis and identification of pigments

IR spectroscopy has been used successfully in the analysis and identification of pigments in paints and other objects of art, such as illuminated manuscripts.

Use in the food industry

Another important application of Infrared Spectroscopy is in the food industry to measure the concentration of various compounds in different food products.

Precision studies

With increased technology in computer filtering and manipulation of results, samples in solution can now be accurately measured. Some instruments will also automatically tell you what substance is being measured from a store of thousands of stored reference spectra.

Field tests

The instruments are now small, and can be transported, even for use in field trials.

Gas exhausts

Infrared spectroscopy is also used in gas leak detection devices such as the DP-IR and EyeCGAs. These devices detect leaking gas from hydrocarbons in the transport of natural and crude gas.

Use in space

NASA uses a very up-to-date database, based on infrared spectroscopy, for the tracking of polycyclic aromatic hydrocarbons in the universe.

According to the scientists, more than 20% of the carbon in the universe may be associated with polycyclic aromatic hydrocarbons, possible starting materials for life formation.

Polycyclic aromatic hydrocarbons appear to have been formed soon after the Big Bang. They are widespread throughout the universe and are associated with new stars and exoplanets.

References

  1. Nancy Birkner (2015). Mind Touch. How to FTIR Spectrometer Operates. Retrieved from: mindtouch.com.
  2. [Links] Theory and Interpretation of IR spectra. Pearson Prentice Hall. Retrieved from: utdallas.edu.
  3. Barbara Stuart (2004). Infrared Spectroscopy. Wiley. Retrieved from: kinetics.nsc.ru.
  4. Wikipedia (2016). Infrared spectroscopy. Wikipedia, the free encyclopedia. Retrieved from: en.wikipedia.org.


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