Infrared Spectroscopy - Definition, Principle, Hooke’s Law, IR Graph, Application with FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 05:03 PM IST

What is Spectroscopy?

Spectroscopy is the branch of science that deals with the study of responses of the molecule when it is exposed to certain kinds of radiations. A given molecule, when exposed to radiation, absorbs a part of it and gets excited and goes to higher energy. The amount and the type of wavelength of radiation absorbed by the molecule in order to reach the excited state depends upon the structural features of the molecule. Therefore, by studying the type of radiation absorbed, it is possible to predict what kind of structural features are present in the molecule.

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The absorption of different types of radiations such as UV, visible, infrared, microwaves, radio waves produce different kinds of excitations of the molecule and each excitation provides some important information about the structure. Since in all these methods, the nature of the radiation absorbed is studied, the spectroscopic methods are classified as “absorption spectroscopy”

IR Spectroscopy

IR spectroscopy is Infrared spectroscopy. IR spectroscopy of an organic compound reveals a good deal of information about the functional groups present in the compound. IR spectroscopy slideshare can be helpful in understanding the concept.

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IR Spectrum

In IR spectroscopy, the molecule is exposed to infrared radiations and the responses given by the molecule are recorded, generally in the form of a graph called the IR spectrum.

Principle of IR spectroscopy

Atoms in any molecule and molecule as a whole are never stationary. They continuously rotate, vibrate and also move from one point to another point. Even at 0 K, when the kinetic energy of the entire molecule is zero, the atomic nuclei vibrate about the bonds which connect them.

The atoms in the molecule can vibrate in a number of ways. Each vibration requires a definite amount of energy, that is the molecule has a number of vibrational energy levels, each of which is quantised. If a molecule is exposed to IR radiations, the molecules absorb the energy of the IR radiation and as result, gets excited to higher vibrational energy levels. The type of the IR wavelengths absorbed by the molecule depends on the type of the atoms and the chemical bonds in the molecule. When these absorptions are recorded, we get the IR spectrum.

A typical IR spectrum

IR region

The whole IR region is subdivided into three parts

IR region

The region between 2.5 to 16µ is very useful from the view of organic chemists because most of the fundamental vibrations of the molecule occur in this region.

The common functional groups show absorption bands in the region 2.5 to 7.7µ. This region is known as the functional group region. The bands that occur in this region are due to stretching vibrations of the functional groups in the molecule.
The region between 7.7 to 11µ is known as fingerprint region and is very complex and is a characteristic for a specific molecule. The bonds that appear in this region are due to skeletal vibrations of C-C, C-O, C-N bonds in the molecule. The region is very useful for sample comparison. If the fingerprint region of two samples are identical, then these samples are also identical.
The region between 11 to 16µ is known as the aromatic region. The band that occurs in this region is due to bending vibrations of C-H bonds in aromatic compounds.

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Hooke’s law in IR spectroscopy

The vibrational frequency of a bond may be related to the masses of vibrating atoms and the force constant (f) of the vibrating bond. This corresponds to a simple Hooke’s law model of two units connected by a spring in which the force constant is the restoring force provided by the spring.

IR spectroscopy instrumentation

The IR spectroscopy instrumentation is shown below.

Initially, a ray of IR radiation is passed through the reference from the source into the sample. The two beams are then reflected to pass first through a splitter and then second to a detector.

And lastly, the needed reading is taken out through printing out after the processor deciphers the passed data into the detector.

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IR working

Infrared spectroscopy requires infrared rays as a source of radiation to excite the compound molecules, generating a spectrum of infrared of the absorbed energy by a molecule as a function of the wavelength or frequency of light.

The important fact to know is that a dipole moment is needed for a molecule to absorb IR radiation. Basically, a bond with a dipole moment with stretching frequency causes IR spectrum absorption.

IR graph

Sample handling in IR spectroscopy

In IR spectroscopy three types of samples can be used that are gaseous, liquid and solid types.

Samples in liquid state are transferred between two plates of salt which are measured because they are transparent in presence of IR light. Potassium bromide, sodium chloride and calcium fluoride are generally used in making salt plates.
The samples of gaseous state should have long pathlength because gaseous state samples are in ppm concentration so that the light should travel, generally for a long distance into the sample cell.
Solid state samples are made by crushing the substance with an agent which has an oily layer.

FTIR spectroscopy

FTIR spectroscopy is also known as Fourier-Transform infrared. The FTIR principle is based on infrared spectroscopy in which an IR spectrum is obtained through an absorption of infrared samples which in continuation collects high resolution data spectrum over a long range of spectral networks.

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Application of IR spectroscopy

Determination of structure of molecules

Functional group region: the observation of peaks in the functional group region will help us to understand the functional groups present in the molecule. If peaks are shifted to higher or lower values than the expected values, indicates the presence of either inductive or resonance or steric effects present in the molecule.
Fingerprint region: the peaks in this region are very characteristic for the molecule. No two different molecules can have identical IR spectra in this region and if they have then they are identical in nature. The comparison of the IR spectrum in the fingerprint region helps us to establish the identity of the molecule.
Aromatic region: the C-H bending vibrations in this region help us to know the substitution pattern in aromatic compounds.

Study of progress of chemical changes: IR spectroscopy is often used to follow the progress of chemical reaction, to control the chemical reaction, and to study the kinetics of various chemical reactions. These applications are based on the fact that most chemical reactions involve changes in the functional groups. This helps us to follow up a course of chemical reaction. A sample of the reaction mixtures is withdrawn and IR spectrum is recorded. The spectrum is used to determine how far the reaction has progressed and whether it is proceeding by the expected path or not.
Detection of hydrogen bonding: IR spectroscopy distinguishes between intramolecular “H” bonding, intermolecular “H” bonding and free -OH group. Free-OH group appears at higher frequency while “H” bonded alcohols and phenol appear at lower frequency.
Determination of size of ring ketones: Ring strain in cyclic ketones shifts the >C=O stretching frequency to a higher wavelength. As ring strain increases, >C=O frequency also increases. This enables us to differentiate the ring size of ketones.
Detection of impurity: we know that a particular functional group absorbs at particular frequency, known as group frequency. If the hydrocarbons contain a very small amount of ketone can be easily traced out by IR spectrum. This is because a ketone shows a strong absorption band at 1720 cm^-1, while hydrocarbons do not have an absorption band at 1720 cm^-1. Similarly, a small quantity of alcohol in hydrocarbon mixture can be easily detected by observing a broad absorption band at 3200-3600 cm^-1 due to OH stretching frequency, which is absent in hydrocarbons.

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Frequently Asked Questions (FAQs)

1. What is the chemical used in IR spectroscopy?

IR chemicals are calcium tetrachloride, methyl trichloride etc.

2. What are the Factors affecting IR spectroscopy?

Resonance, inductive effects, hydrogen bonding etc

3. Which Radiation source used in IR spectroscopy?

Infrared radiations are used as a source of radiations in IR spectroscopy.

4. What are the Types of molecular vibrations in IR spectroscopy?

Stretching vibrations

Bending vibrations.

5. Range of IR radiation?

800 nm to 10⁵ nm

6. How do functional groups affect infrared spectra?

Functional groups have characteristic absorption frequencies in infrared spectra. These appear as distinct peaks or bands in specific regions of the spectrum, allowing chemists to identify the presence of particular functional groups in a molecule based on their spectral patterns.

7. How does sample preparation affect IR spectroscopy results?

Sample preparation is crucial in IR spectroscopy as it can significantly impact the quality and interpretation of spectra. Factors like sample thickness, concentration, and physical state (solid, liquid, or gas) can affect peak intensity and resolution. Proper preparation ensures accurate and reproducible results.

8. What is the importance of the fingerprint region in an IR spectrum?

The fingerprint region, typically between 1500-400 cm⁻¹ in the IR spectrum, is crucial for molecule identification. It contains a complex series of absorptions that are highly specific to each molecule, much like a fingerprint. This region is often used to distinguish between molecules with similar functional groups.

9. How does IR spectroscopy differ from UV-Visible spectroscopy?

IR spectroscopy measures molecular vibrations and rotations, providing information about molecular structure and functional groups. UV-Visible spectroscopy, on the other hand, measures electronic transitions and is useful for determining conjugation in molecules and quantifying concentrations of absorbing species.

10. What is the significance of the IR graph in spectroscopy?

The IR graph, also known as an infrared spectrum, is a plot of infrared light absorption versus wavelength or frequency. It serves as a unique "fingerprint" for each molecule, showing characteristic absorption peaks that correspond to specific molecular vibrations and functional groups.

11. What is infrared spectroscopy?

Infrared spectroscopy is an analytical technique that uses infrared radiation to identify and study chemicals. It works by measuring how infrared radiation is absorbed by a sample, which provides information about the molecular structure and chemical bonds present.

12. How can IR spectroscopy be used to determine molecular structure?

IR spectroscopy can determine molecular structure by identifying functional groups present in a molecule. The position, intensity, and shape of absorption bands in the spectrum provide information about the types of bonds and their environment within the molecule, helping to elucidate its overall structure.

13. How does hydrogen bonding affect IR spectra?

Hydrogen bonding typically causes a broadening and shifting of certain absorption bands, particularly those associated with O-H and N-H stretching vibrations. The hydrogen bond weakens these bonds, resulting in absorption at lower frequencies (longer wavelengths) compared to non-hydrogen bonded molecules.

14. What are combination bands in IR spectroscopy?

Combination bands occur when two or more fundamental vibrations are excited simultaneously, resulting in absorption at a frequency that is the sum of the individual fundamental frequencies. These bands are typically weaker than fundamental absorptions but can provide additional structural information.

15. How does deuteration affect IR spectra?

Deuteration, the replacement of hydrogen atoms with deuterium (²H), significantly affects IR spectra. Because deuterium is heavier than hydrogen, bonds involving deuterium vibrate at lower frequencies. This shift can help in peak assignment and in studying hydrogen bonding interactions.

16. What is the role of Fourier Transform in modern IR spectroscopy?

Fourier Transform Infrared (FTIR) spectroscopy uses a mathematical process called Fourier Transform to convert raw data into the actual spectrum. This technique allows for faster data collection and improved signal-to-noise ratio compared to traditional dispersive IR spectroscopy.

17. What are overtones in IR spectroscopy and why do they occur?

Overtones are absorption bands that appear at approximately integer multiples of the fundamental absorption frequency. They occur when a molecule absorbs enough energy to jump more than one vibrational energy level. Overtones are usually weaker than fundamental absorptions but can provide additional structural information.

18. What is the significance of the 4000-1500 cm⁻¹ region in IR spectra?

The 4000-1500 cm⁻¹ region is often called the functional group region. It contains absorption bands characteristic of specific functional groups like O-H, N-H, C=O, and C≡N. This region is particularly useful for identifying the presence of these groups in unknown compounds.

19. What is the Beer-Lambert Law and how does it apply to IR spectroscopy?

The Beer-Lambert Law relates the absorption of light to the properties of the material through which it is traveling. In IR spectroscopy, it states that the absorbance is directly proportional to the concentration of the absorbing species and the path length of the sample. This relationship allows for quantitative analysis in IR spectroscopy.

20. How does temperature affect IR spectra?

Temperature can affect IR spectra in several ways. Higher temperatures can lead to band broadening due to increased molecular motion. It can also cause shifts in peak positions and changes in relative intensities of bands. In some cases, temperature changes can reveal information about molecular conformations and intermolecular interactions.

21. What is the principle behind infrared spectroscopy?

The principle of infrared spectroscopy is based on the fact that molecules absorb specific frequencies of infrared radiation that match their vibrational frequencies. This absorption causes changes in the vibrational energy levels of the molecules, which can be detected and analyzed to identify the molecular structure.

22. How does Hooke's Law apply to infrared spectroscopy?

Hooke's Law is relevant to infrared spectroscopy because it describes the behavior of chemical bonds as spring-like. The law states that the force needed to extend or compress a spring is directly proportional to the distance of extension or compression. In molecules, this relates to the vibration of bonds, which absorb infrared radiation at specific frequencies.

23. Why are some bonds IR active while others are not?

Bonds are IR active if they have a dipole moment that changes during vibration. This typically occurs in polar bonds or asymmetric molecules. Symmetric stretching of completely non-polar bonds (like those in diatomic molecules like N2) are IR inactive because their dipole moment doesn't change during vibration.

24. How does infrared spectroscopy relate to the electromagnetic spectrum?

Infrared radiation falls between visible light and microwave radiation in the electromagnetic spectrum. It has longer wavelengths than visible light but shorter wavelengths than microwaves, typically ranging from about 700 nanometers to 1 millimeter.

25. What is the difference between transmission and absorption in IR spectroscopy?

In IR spectroscopy, transmission refers to the amount of infrared radiation that passes through a sample, while absorption is the amount of radiation absorbed by the sample. The two are inversely related: high absorption results in low transmission, and vice versa.

26. How does IR spectroscopy compare to NMR spectroscopy in structural determination?

While both techniques are valuable for structural determination, they provide complementary information. IR spectroscopy primarily gives information about functional groups and bond types, while NMR provides detailed information about the carbon skeleton and hydrogen environments. IR is generally faster and requires less sample, but NMR often provides more comprehensive structural information.

27. How does IR spectroscopy contribute to the field of astrochemistry?

In astrochemistry, IR spectroscopy is crucial for identifying molecules in space. Many molecules in interstellar clouds and planetary atmospheres are detected through their IR absorption or emission spectra. This technique has led to the discovery of complex organic molecules in space and helps in understanding the chemical composition of distant celestial bodies.

28. How can IR spectroscopy distinguish between aldehydes and ketones?

IR spectroscopy can distinguish between aldehydes and ketones based on their carbonyl (C=O) stretching frequencies. Aldehydes typically show a C=O stretch around 1725-1740 cm⁻¹, while ketones absorb slightly lower, around 1705-1725 cm⁻¹. Aldehydes also show a weak C-H aldehyde stretch near 2850 cm⁻¹.

29. How can IR spectroscopy be used to monitor chemical reactions?

IR spectroscopy can monitor chemical reactions by tracking changes in the spectra over time. As reactants are consumed and products are formed, characteristic absorption bands will decrease and increase, respectively. This allows for real-time analysis of reaction progress and kinetics.

30. What is attenuated total reflectance (ATR) in IR spectroscopy?

Attenuated total reflectance (ATR) is a sampling technique used in IR spectroscopy. It works by measuring the changes that occur in an internally reflected infrared beam when it comes into contact with a sample. ATR allows for easy analysis of solids and liquids with minimal sample preparation.

31. What is the difference between near-IR and mid-IR spectroscopy?

Near-IR spectroscopy uses higher energy radiation (shorter wavelengths) than mid-IR, typically in the range of 14000-4000 cm⁻¹. Near-IR mainly measures overtones and combinations of fundamental vibrations, while mid-IR (4000-400 cm⁻¹) measures fundamental vibrations. Mid-IR is more commonly used for structural determination.

32. How can IR spectroscopy be used in quality control?

IR spectroscopy is valuable in quality control as it can quickly and non-destructively analyze materials. It can verify the identity and purity of raw materials, monitor product consistency, detect contaminants, and ensure that final products meet specifications. The technique's speed and sensitivity make it ideal for routine quality checks.

33. What is the concept of group frequencies in IR spectroscopy?

Group frequencies are characteristic absorption frequencies associated with specific functional groups. These frequencies remain relatively constant regardless of the rest of the molecule's structure. Understanding group frequencies allows chemists to quickly identify the presence of particular functional groups in unknown compounds.

34. How does IR spectroscopy compare to Raman spectroscopy?

While both techniques provide information about molecular vibrations, they are complementary. IR spectroscopy measures absorbed radiation and is sensitive to polar bonds, while Raman spectroscopy measures scattered light and is more sensitive to symmetric vibrations. Some vibrations may be IR active but Raman inactive, or vice versa.

35. What are some limitations of IR spectroscopy?

Limitations of IR spectroscopy include: difficulty in analyzing aqueous solutions due to strong water absorption; inability to detect atoms or monoatomic ions; challenges in analyzing complex mixtures; and the fact that some vibrations may be IR inactive. Additionally, the technique provides limited information about molecular geometry compared to some other spectroscopic methods.

36. How can IR spectroscopy be used to determine the purity of a sample?

IR spectroscopy can assess sample purity by comparing the spectrum of the sample to that of a known pure standard. Any additional peaks or variations in peak intensities can indicate the presence of impurities. The technique can also be used quantitatively to determine the concentration of impurities if proper calibration is performed.

37. What is the role of IR spectroscopy in studying hydrogen bonding?

IR spectroscopy is particularly useful for studying hydrogen bonding because hydrogen bonds significantly affect the stretching frequencies of O-H, N-H, and sometimes C-H bonds. Hydrogen bonding typically causes these stretching bands to broaden and shift to lower frequencies, providing information about the strength and nature of hydrogen bonding interactions.

38. How does pressure affect IR spectra?

Pressure can significantly impact IR spectra, especially for gases. Increased pressure can lead to band broadening due to increased molecular collisions. It can also cause shifts in peak positions and changes in relative intensities. In some cases, high pressure can reveal new absorption bands or cause changes in molecular conformations.

39. What is the significance of the C=O stretching frequency in IR spectroscopy?

The C=O stretching frequency is one of the most important and easily recognizable features in IR spectroscopy. It typically appears as a strong band between 1650-1750 cm⁻¹. The exact position of this band can provide valuable information about the type of carbonyl compound (e.g., ketone, aldehyde, ester) and its molecular environment.

40. How can IR spectroscopy be used to distinguish between cis and trans isomers?

IR spectroscopy can distinguish between cis and trans isomers based on differences in their vibrational modes. For example, in alkenes, the out-of-plane C-H bending vibration typically occurs at higher frequencies for trans isomers compared to cis isomers. Additionally, cis isomers often show more complex spectra due to their less symmetrical structure.

41. What is the importance of baseline correction in IR spectroscopy?

Baseline correction is crucial in IR spectroscopy to ensure accurate peak intensities and positions. Factors like sample thickness, scattering, and instrumental drift can cause baseline shifts or slopes. Proper baseline correction improves the accuracy of both qualitative and quantitative analyses, allowing for more reliable comparisons between spectra.

42. How does IR spectroscopy contribute to the study of protein structure?

IR spectroscopy is valuable in studying protein structure, particularly secondary structure. The amide I band (around 1650 cm⁻¹) is sensitive to the protein's secondary structure, with different structures (α-helix, β-sheet, random coil) showing characteristic frequencies. This allows researchers to estimate the proportions of different secondary structures in proteins.

43. What is the principle behind diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)?

DRIFTS is a technique used for analyzing powdered or rough samples. It works by measuring the diffusely reflected IR radiation from a sample surface. The incident IR beam penetrates the sample, undergoes multiple reflections, scattering, and absorption before being collected and analyzed. This technique is particularly useful for samples that are difficult to analyze by transmission methods.

44. How can IR spectroscopy be used to study reaction mechanisms?

IR spectroscopy can provide insights into reaction mechanisms by allowing real-time monitoring of changes in functional groups. By observing the disappearance of reactant peaks and the appearance of product peaks over time, researchers can infer the order of bond breaking and formation, identify intermediates, and determine rate-limiting steps in chemical reactions.

45. What is the significance of the OH stretching region in IR spectra?

The OH stretching region, typically between 3200-3600 cm⁻¹, is significant because it provides information about hydrogen bonding and the environment of OH groups. Free OH groups show sharp peaks at higher frequencies, while hydrogen-bonded OH groups show broader peaks at lower frequencies. The shape and position of these bands can indicate the strength and nature of hydrogen bonding.

46. What is the principle behind time-resolved IR spectroscopy?

Time-resolved IR spectroscopy is a technique that allows the observation of short-lived species or rapid changes in molecular structure. It typically involves using a short pulse of light to initiate a reaction or change, followed by rapid IR measurements to track the subsequent molecular changes. This technique is valuable for studying reaction dynamics and intermediate species.

47. How can IR spectroscopy be used to study the degree of crystallinity in polymers?

IR spectroscopy can assess the degree of crystallinity in polymers by comparing the intensities of bands associated with crystalline and amorphous regions. Crystalline regions often show sharper, more intense peaks, while amorphous regions tend to have broader, less intense peaks. The ratio of these peak intensities can be used to estimate the degree of crystallinity.

48. What is the concept of characteristic group frequencies in IR spectroscopy?

Characteristic group frequencies are specific absorption frequencies associated with particular functional groups or molecular vibrations. These frequencies remain relatively constant across different molecules containing the same functional group. Understanding these frequencies allows chemists to quickly identify the presence of specific structural features in unknown compounds.

49. What is the principle behind photoacoustic IR spectroscopy?

Photoacoustic IR spectroscopy is based on the photoacoustic effect, where absorbed IR radiation is converted into heat, causing thermal expansion that generates acoustic waves. These waves are detected by a microphone. This technique is particularly useful for analyzing opaque or highly absorbing samples that are difficult to analyze by traditional transmission methods.

50. How can IR spectroscopy be used to study the secondary structure of nucleic acids?

IR spectroscopy can provide information about the secondary structure of nucleic acids by analyzing specific regions of the spectrum. For example, the position and shape of the phosphate stretching bands (around 1220 cm⁻¹) and the sugar-phosphate backbone vibrations can indicate whether the DNA is in an A-, B-, or Z-form. Additionally, base-pairing interactions can be studied through hydrogen bonding patterns.

51. What is the significance of the fingerprint region in forensic applications of IR spectroscopy?

In forensic science, the fingerprint region of the IR spectrum (approximately 1500-600 cm⁻¹) is crucial for identifying and comparing unknown substances. This region contains a complex pattern of absorption bands that is unique to each molecule, much like a fingerprint. It allows forensic scientists to identify or compare drugs, fibers, paints, and other materials found at crime scenes.

52. How does IR spectroscopy contribute to the study of atmospheric chemistry?

IR spectroscopy plays a vital role in atmospheric