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IR Spectroscopy


Spectroscopy is the study of the interactions between electromagnetic radiation absorbed, scattered or emitted by matter. Usually when referring to infrared spectroscopy IR absorption spectroscopy is meant. Although this is the dominant method, occasionally it can be useful for sensing applications to also use IR emission spectroscopy.
Ir spectroscopy is used to study chemical, biological and related physical phenomena over the last decade. This surge of interest has largely been inspired by the increased availability of commercial, solid state, table-top pulsed laser equipment and nonlinear frequency conversion methods for generating high power, broadly tunable infrared pulses in the near to mid infrared wavelength regime.

Principle of IR Spectroscopy

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The atoms of any molecule are continuously undergoing vibrations and rotations of various kinds. The frequencies of these molecular motions are of the same order of magnitude as those of IR radiation. When the frequency of molecular motion is the same as that of IR radiation impinging on that molecule, and when there is a change in dipole moment during that motion, the molecule can absorb the radiation incident upon it.

The mid-infrared extends from about 2 to 25$\mu$m, the most useful range for chemical analysis. The most convenient unit for infrared wavelengths is microns or micrometers. It is more common now to express the infrared spectrum in terms of wave numbers, for which the units are reciprocal centimeters. The near infrared region of the spectrum occurring at higher frequency and shorter wavelengths than the mid infrared has found considerable use in recent years, particularly in process control and in monitoring relatively well defined materials.

IR Spectroscopy Instrumentation

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The experimental setup of IR is suitable for studying biological systems. The goal is to present guidelines for setting up IR according to specific types of experiments. The schematic diagram of an infrared spectrometer is shown below. 

IR Spectroscopy

As an example, we mention interferometers and the Fourier transform method, which are essential for modern IR spectroscopy and thus are covered in the IR spectroscopy. The source of radiation in the appropriate frequency range and then to get the light focused on the sample. In order to determine how much radiation has been absorbed and at what frequencies we need to separate the light according to wavelength ad measure its intensity.

In the instrumentation its is accomplished by a prism, which disperses the light according to wavelength or frequency. The light then focused onto a narrow slit that determines the resolution of the instrument. A narrow slit allows light in just a narrow frequency range to hit the detector permitting the separation of absorption bands that are close in frequency. Separate the light according to wavelength and measure how much light has been absorbed at each frequency by comparing intensities to light from the source that hasn't passed through the sample.

Application of IR Spectroscopy & Uses

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Among all the properties of an organic compound, no single property gives as much information about the compound's structure as its infrared spectrum. Thus IR spectroscopy is the most widely used method for structure determination of organic compounds. IR spectroscopy is especially used for detection of functional groups in organic compounds and for establishing the identity of organic compounds. Some important applications of IR spectroscopy to organic chemistry are given below.
  1. Detection of functional groups - All functional group absorb in a definite frequency region. Thus the presence or absence of a band in a definite frequency region tells the presence or absence of a particular functional group in the compound.
  2. Confirmation of the identity of compounds - The identity of a compound is often established by comparing its IR spectrum with that of an authentic sample. The large number of bands, especially in the fingerprint region are most useful for identification.
  3. Estimation of the purity of samples - The purity of a sample may be estimated by inspection of its IR spectrum and comparison with a reference spectrum.
  4. Study of hydrogen bonding - Ir spectroscopy is useful in detecting hydrogen bonding, in estimating the strength of hydrogen bonds and in distinguishing inter molecular and intra molecular hydrogen bonding.
  5. Calculation of Force constants - The frequencies of IR absorptions are commonly used to calculate the force constants of various bonds.
  6. Determination of orientations in aromatic compounds - Absorptions on the region 675-900cm-1 due to out of plane bending vibrations indicate the relative positions of substituents on the benzene ring. The position of absorption bands in thei region depends on the number of adjacent hydrogen atoms on the ring.
  7. Study of the progress of reactions - In most of the cases the progress of an organic reaction can be followed by IR spectroscopy. This is done by examining the IR spectra of portions of the reaction mixture withdrawn at certain time intervals.

IR Spectroscopy Table

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The absorption frequency of some of the compounds in IR spectra with their absorption frequency and their intensity are tabulated below.

Type Group Absorption frequency
Intensity Remarks
Halogen compounds
near 3000 s (strong) Asym and sym C-H stretch
Aromatic compounds Aromatic C-H
Aromatic C-C
1600 $\pm$ 5
C-H stretch
C=C skeletal stretch
Alcohols and phenols O-H 3590-3650
free O-H stretch
Intermolecular hydrogen bonded
O-H stretch
Ethers and epoxides -O-CH3
C-H stretch
Epoxide C-H stretch
 Aldehydes R-CHO 1720-1740 s C=O stretch; saturated, aliphatic
Ketones R-C(O)-R 1705-1725 s C=O stretch; saturated, acyclic
Carboxylic acids  R-COOH 1700-1725 s C=O stretch; saturated, aliphatic
Amides R-CO-NH2 1690 s Amide I, C=O stretch; free that is in dilute solution

Molecular Vibration

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Infrared spectroscopy is sensitive to a change in the dipole moment as a function of the vibrational motion. Vibrational spectroscopy provides detailed information on both structure and dynamics of molecular species.

1. Stretching vibrations

During stretching vibrations, the distance between two atoms increases or decreases but the atoms remain in the same bond axis.

Types of stretching vibrations

Stretching vibrations require higher energy and occur at higher frequency. There are two types 
  • Symmetrical stretching
In this stretching mode, both the atoms move in and out simultaneously. For example, symmetrical stretching of >CH2 group is shown below. 

Symmetrical Stretching
  • Asymmetrical stretching
In asymmetrical stretching one atom moves in while the other moves out and is represented for >CH2 group as shown below. 

Asymmetric Stretching

2. Bending vibration 

Bending vibration require lower energy and occur at lower frequency. There are two types 
  1. In plane bending
  2. Out plane bending 
Bending Vibration

Types of in-plane bending
  1. In plane scissoring deformation
  2. In plane rocking deformation
  • In plane scissoring deformation
In the scissoring deformation mode, both the atoms swing in concert toward opposite directions. 

Scissoring Vibration
  • In plane rocking deformation
In this deformation mode, both the atoms swing to the same side and then both to the other side. 

Rocking Vibration

Out of plane bending

When the atoms bend out of the nodal plane, the bending mode is called out of plane bending.

Types of out of plane bending vibrations
  1. Out of plane wagging deformation
  2. Out of plane twisting deformation
  • Out of plane wagging deformation
In this deformation mode, both the atoms swing up and down out of the plane of the paper in unison. 

Wagging Vibration
  • Out of plane twisting deformation
In twisting deformation one atom swings up and the other swings down related to the plane of the paper. 

Twisting Vibration