Application of IR (Infra-Red) Spectroscopy

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Applications of IR Spectroscopy 1

Applications of Infrared Spectroscopy

Infrared spectroscopy (IR spectroscopy) is the spectroscopy which deals with the infrared

region of the electromagnetic spectrum. It covers a range of techniques, mostly based onabsorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and

study chemicals.

The infrared portion of the electromagnetic spectrum is usually divided into three regions and

named for their relation to the visible spectrum;

1. Near infrared: The higher-energy near-IR, approximately 140004000 cm1 (0.82.5 m wavelength) can excite overtone or harmonic vibrations.

2. Mid infrared: The mid-infrared, approximately 4000400 cm1 (2.525 m) may beused to study the fundamental vibrations and associated rotational-vibrational

structure.

3. Far infrared: The far-infrared, approximately 40010 cm1 (251000 m), lyingadjacent to the microwave region, has low energy and may be used for rotational

spectroscopy.

Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are

characteristic of their structure. These absorptions are resonant frequencies, that is, the

frequency of the absorbed radiation matches the frequency of the bond or group that vibrates.

It is also known as requirement of frequency matching. The energies are determined by the

shape of the molecular potential energy surfaces, the masses of the atoms, and the associated

vibronic coupling.

Moreover, in order for a vibrational mode in a molecule to be "IR active," it must be

associated with changes in the dipole. A permanent dipole is not necessary, as the rule

requires only a change in dipole moment. A molecule can vibrate in six ways, and each way

is called a vibrational mode. The ways are-

1. Symmetric stretching2.Asymmetric stretching

3. Scissoring,4. Rocking,

5. Wagging6. Twisting


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An IR spectrum shows the energy absorptions as one 'scans' the IR region of the EM

spectrum. As an example, the IR spectrum of butanal is shown below.

In general terms it is convenient to split an IR spectrum into two approximate regions:

4000-1000 cm-1 known as the functional group region, and < 1000 cm-1 known as the fingerprint region

Figure01: An infrared spectrum

From this spectrum we can understand that:

Most of the information that is used to interpret an IR spectrum is obtained from thefunctional group region.

In practice, it is the polar covalent bonds than are IR "active" and whose excitationcan be observed in an IR spectrum.

In organic molecules these polar covalent bonds represent the functional groups. Hence, the most useful information obtained from an IR spectrum is what functional

groups are present within the molecule.

In the fingerprint region, the spectra tend to be more complex and much harder toassign.

Remember that some functional groups can be "viewed" as combinations of differentbond types. For example, an ester, CO2R contains both C=O and C-O bonds and bothare typically seen in an IR spectrum of an ester.


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Infrared spectroscopy is widely used in industry as well as in research. It is a simple and

reliable technique for measurement, quality control and dynamic measurement. It is also

employed in forensic analysis in civil and criminal analysis.

Figure 02: A schematic diagram of a dispersive infrared spectroscopy

Applications of Infrared Spectroscopy:

Some of the major applications of IR spectroscopy are as follows:

1. Identification of functional group and structure elucidation

Entire IR region is divided into group frequency region and fingerprint region. Range of

group frequency is 4000-1500 cm-1

while that of finger print region is 1500-400 cm-1

.

Figure 03: Group frequency and fingerprint regions of mid infrared spectrum


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In group frequency region, the peaks corresponding to different functional groups can be

observed. According to corresponding peaks, functional group can be determined.

Each atom of the molecule is connected by bond and each bond requires different IR region

so characteristic peaks are observed. This region of IR spectrum is called as finger print

region of the molecule. It can be determined by characteristic peaks.

Figure 04: Absorption frequencies of some common bonds

2. Identification of substances

IR spectroscopy is used to establish whether a given sample of an organic substance is

identical with another or not. This is because large number of absorption bands is observed in

the IR spectra of organic molecules and the probability that any two compounds will produce

identical spectra is almost zero. So if two compounds have identical IR spectra then both of

them must be samples of the same substances.

IR spectra of two enatiomeric compound are identical. So IR spectroscopy fails to distinguishbetween enantiomers.


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For example, an IR spectrum of benzaldehyde is observed as follows.

Figure 04: IR spectrum of benzaldehyde

C-H stretching of aromatic ring- 3080 cm-1

C-H stretching of aldehyde- 2860 cm-1

and 2775 cm-1

C=O stretching of an aromatic aldehyde- 1700 cm-1

C=C stretching of an aromatic ring- 1595 cm-1

C-H bending- 745 cm-1

and 685 cm-1

3. Studying the progress of the reaction

Progress of chemical reaction can be determined by examining the small portion of the

reaction mixtures withdrawn from time to time. The rate of disappearance of a characteristic

absorption band of the reactant group and/or the rate of appearance of the characteristic

absorption band of the product group due to formation of product is observed.

4. Detection of impurities

IR spectrum of the test sample to be determined is compared with the standard compound. If

any additional peaks are observed in the IR spectrum, then it is due to impurities present in

the compound.

5. Quantitative analysis

The quantity of the substance can be determined either in pure form or as a mixure of two or

more compounds. In this, characteristic peak corresponding to the drug substance is chosen


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and log I0/It of peaks for standard and test sample is compared. This is called base line

technique to determine the quantity of the substance.

6. Semiconductor microelectronics field

IR spectroscopy has also been successfully utilized in the field of semiconductor

microelectronics: for example, this technique can be applied to semiconductors like silicon,

gallium arsenide, gallium nitride, zinc selenide, amorphous silicon, and silicon nitride

7. Polymer manufacture

IR spectroscopy has been highly successful for applications in both organic and inorganic

chemistry. By measuring at a specific frequency over time, changes in the character or

quantity of a particular bond can be measured. This is especially useful in measuring the

degree of polymerization in polymer manufacture

8. Forensic Analysis and Crime Investigation

Since infrared spectroscopy is useful for the identification and confirmation of the identity of

materials and substances, the method is beneficial to the field of forensic analysis. With the

aid of integrated computer databases and machines capable of performing infraredspectroscopy, almost any substance or material can be identified. Computer databases have

records of known infrared absorbance graphs. Infrared spectroscopy plays an important in

crime investigation because it can help authorities to solve crimes and locate criminal

offenders. The evidence gathered from the scene of the crime can be examined closely with

the use this method. The results can provide clues to a criminal's whereabouts. For example,

infrared spectroscopy can be used to find a car model by simply subjecting a paint chip to

infrared spectroscopy.

9. Chemical Analysis: Testing Pill Quality

According to "Medical News Today," scientists at the University of Maryland have been

successful in using the method of near-infrared spectroscopy (NIR) to make a prediction

regarding quick dissolution of pills inside the body. The success of the experiment can help

drug manufacturers in checking the quality of pills to benefit consumers in the health

industry. Pills can be tested for consistencies because any imbalance in pill ingredients can

prove to be lethal. The Food and Drug Association (FDA) can also use the method of infrared


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spectroscopy for checking and identifying materials in the manufacture of medicines. The

FDA regulates drug companies and protects consumers from potential health disasters.

10. Chemistry Applications

Using infrared spectroscopy, it is possible to measure the degree of polymerization in

chemical compounds. Polymerization happens when monomer molecules undergo chemical

reaction to form polymer chains. Infrared spectroscopy can measure the changes in the nature

and quantity of molecular bonds. Portable instruments that can measure infrared spectroscopy

are used in field trials. This method is important for researchers in identifying more uses of

different substances to improve the lives of modern society. Medical breakthroughs are not

far behind. The analysis of molecular compounds can lead to the discovery of new chemical

compounds that can produce useful products.

11. Geometrical Study:

Knowledge of mineral composition is essential to characterize the geochemical and physico-

mechanical properties of rocks. Nature and content of minerals (especially clay minerals)

present in rocks have a significant influence on the behavior and properties of rocks as well

as on the whole rock massif.There are several conventional analytical methods exist that can

be used to examine the mineral composition of rocks, like, optical microscopy, electron

microscopy, X-ray diffraction (XRD), thermal analysis and bulk chemistry analysis. But,

unfortunately, these methods are rather complicated and often inaccurate. The current Fourier

Transform Infrared (FTIR) spectroscopy makes it possible to analyze individual minerals,

noncrystalline admixtures and, simultaneously, to detect the presence of organic matter.

Advantages and Disadvantages of Infrared Spectroscopy:

Some advantages and disadvantages of infrared spectroscopy are listed in below.

Advantages Disadvantages

Solids, Liquids, gases, semi-solids, powders

and polymers are all analyzed

The peak positions, intensities, widths, and

Atoms or monatomic ions do not have

infrared spectra

Homonuclear diatomic molecules do not


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shapes all provide useful information

Fast and easy technique

Sensitive technique (Micrograms of

materials can be detected routinely)

Inexpensive

posses infrared spectra

Complex mixture and aqueous solutions

are difficult to analyze using infraredspectroscopy

Application of IR (Infra-Red) Spectroscopy

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