Infrared spectroscopy is one the most powerful analytical techniques which offers the possibility of chemical identification. IR technique which coupled with intensity measurements may be used for quantitative analysis. Infrared has been of tremendous use to chemists and is currently more popular as compared to other physical techniques in the elucidation of the structure of unknown compounds.
The Range of Infrared Radiation:
The infrared radiation relates generally to that part of the electromagnetic spectrum which lies between the visible and microwave regions. From instrumentation and application point of view, the infrared region has been subdivided as follows:
- The near infrared region (overtone region). It ranges from 0.8 to 2.5μ (12500-4000 cm–1).
- The mind infrared region (vibration-rotation region). It range from 2.5 to 15μ (4000-667 cm–1).
- The far infrared region (rotation region). This region from 15 to 200μ (667 – 50 cm–1).
An infrared spectrum show downward peaks corresponding to absorption, plotted against wavelength (λ) or wave number (v). Wavelength is expressed in μ (microns) or μm (1μm =10-4 cm) Since λ is inversely proportional to energy, the wave number in cm–1 is mostly used to measure the position of a given infrared absorption.
Requirements for Absorption of Infrared Radiation.
For a molecule to absorb infrared (IR) radiation, it has to fulfill certain requirements, which are as follows (selection rules):
- Correct Wavelength of Radiation. A molecule absorbs radiation only when the frequency of vibration of some part of a molecule is the same as the frequency of the incident radiation.
- Electric Dipole. A molecule can absorb IR radiation when its absorption causes a charge in its electric dipole (dipole moment).
Each non-linear molecule has 3n-6 internal degrees of freedom and the linear molecules have 3n-5. Cn=No. of atoms in the molecule). For a vibration to be IR-active, there must be electrical coupling between oscillating electric field of electromagnetic radiation and molecular motion and should have a change in dipole moment.
Origin of Infrared spectra.
When a molecule is placed in an electromagnetic field, e.g., infrared radiation, a transfer of energy from the electromagnetic field to the molecule occurs when,
E is the difference in energy between two quantized states, h
is Planck’s constant (6.624 x 10-27
erg sec), v is the frequency of light. When the molecule is excited, it absorbs energy from the lower energy state E1
to higher energy state E2
and emits radiation of the same frequency when the molecule reverts from the higher energy states to lower energy state.
- ΔE = E1 –E2 (Absorption)
- ΔE = E2 –E1 (Emission)
Actually the energy of a molecule can be resolve into:
- The energy associate with the rotation of the molecule as a whole (rotational energy).
- The energy associated with the vibration of the constituent atoms in the molecule (vibrational energy).
The molecules undergo vibrations in the following ways.
A stretching vibration is a rhythmical activity along the connection axis such that the interatomic distance is increasing or decreasing. The symmetrical stretching vibration is inactive in the infrared since it produces no charge in the dipole moment of the molecule, e.g, O=C+O
The stretching vibrations are two types.
- Symmetrical stretching. When the stretching and compression occur in a symmetrical fashion, it is called symmetrical stretching.
- Asymmetric Stretching. When one bond is compressing while the other is stretching.
Bending or Deforming Vibrations.
A bending vibration may consist of a change in bond angles between bonds with a common atom, or the movement of a group of atoms with respect to the remainder of the molecule without activity of the atoms in the group. Bending vibrations are of four types:
When the two atoms join to a central atom move toward and away from each other with the deformation of the angle between them (in plane bending).
When in structural unit swings back and forth out of the plane of the molecule (in plane bending).
The structural unit swings back and forth out of the plane of the molecule (out of plane bending)
The structural units rotate about the bond which joins the rest of the molecule (out of plane bending)
In a molecule containing more than two atoms, all the four types of vibration may be possible. However, only those vibrations that result in a change in the dipole moment of the molecule are observed in the infrared. Various modes of vibrations for an AX2
group are show below.
Changes in the absorption frequencies foe a particular group take place when the substituents in the neighborhood of that particular group are changed. The frequency moved are due to electronic effects which include:
- Inductive effect
- Mesomeric effect and
- Field effect etc
The introduction of an electronegative atom or group causes the inductive effect which results in the increases of bond order. The force constant increases and hence the wave number of absorption rises. Consider the wave number of absorption in the following compounds:
- Acetone (CH3 COCH3) 1715 cm–1
- Chloroacetone (CH3 COCH2 Cl) 1725 cm–1
- Dichloroacetone (CH3 COCHCl2) 1740 cm–1
- Tetrachloroacetone (Cl2 CHCOCHCL2) 1750, 1778 cm–1
It causes lengthening or the weakening of a bond loading to the lowering of absorption frequency. In most of the cases, mesomeric effect works along with inductive effect.
In ortho substituted compounds, the lone pair of electron on two atoms influence each other through space interactions and change the vibrational frequencies of both the group. This effect is known as field effect.
Hydrogen bonding bring about remarkable downward frequency shifts. Stronger the bonding, greater is the absorption shif toward lower wave number than the normal value. Generally, bands due to intermolecular hydrogen bonds are sharp whereas intermolecular hydrogen bonds give rise to broad bands and depend on concentration.
Infrared technique is based upon the simple fact that a chemical substance show marked selective absorption in the infrared region. The frequency or wave length of consumption relies on the relative masses of the atoms, the power constants of the bonds and the geometry of the atoms. The absorption of energy is quantized. Then the molecules of a chemical substance vibrate at many rates of vibration, giving rise to close packed absorption bands. Vibrational spectra appear as bands rather than collections because a individual vibrational energy change is accompanied by a number of rotational energy changes. Thus IR
spectrum of a chemical substance is a Hand print
for its identification. Band intensities are expressed either as transmittance (T
) or absorbance (A
). Transmittance is the ratio of the radiant energy transmitted by a example to the radiant energy incident on the sample. Absorbance
is the logarithm, to the base 10, of the reciprocal of the transmittance, A=log10
These are either single beam or double beam. In a single beam spectrophotometer the radiations emitted from the source are passed through a cell containing the sample and through the prism which disperses the light. Single beam spectrophotometers
are simple, sensitive, accurate, versatile and are used to study details. But these instruments have two disadvantages.
- When the spectra of the solution is to be recorded, the absorption bands due to solvent are also obtained, thus, making the interpretation of the bands and identification of the compounds more difficult.
- The base line, i.e., the line obtained without the use of the sample in the light path, slopes because the intensity of the source change continuously with the wavelength.
Flow Sheet diagram of Infrared Spectrophotometers is given below: