
In this article at OpenGenus, we have explored the concept of Infrared Spectroscopy along with related concepts like Hooke's law and others.
Table of contents:
- Principle
- Infrared region is divided into following regions
- Infrared light can produce 3 types of controlled vibrations
- Symmetrical stretching
- Asymmetrical stretching
- Bending
- Selection route for IR Spectroscopy
- Regions of IR spectra
- Vibrational frequency (Hooke's Law)
- Difference between IR and UV spectrum
Principle
- "Vibrational(IR) spectroscopy involves the transitions between the vibrational energy levels of a molecule on the absorption of radiations falling in the spectral range of 500/cm- 4000/cm(infrared region)."
- Vibrational spectra appear as vibrational-rotational bands as a single vibrational energy change is accompanied by a large number of rotational energy changes.
Infrared region is divided into following regions:
- Near-infrared region- Part of IR spectrum closest to visible light(400nm-700nm)
- Far-infrared region- Part of IR spectrum closest to microwave region(3mm-30cm)
- Mid-infrared region- Part of IR spectrum between near and far infrared regions
Thus, wavelength of infrared waves is longer than visible and shorter than microwaves.
Infrared light can produce 3 types of controlled vibrations:
- Stretching
- Symmetrical
- Asymmetrical
- Bending
Symmetrical stretching
- Stretching and compression of bonds in same direction/ symmetrical or alternate manner.
- Bond axis remains fixed.
- Only bond length changes.
Asymmetrical stretching
- Bonds are stretched in opposite directions or in simple words one bond is stretched and another is compressed.
- Bond axis remains fixed.
- Only bond length changes.
Bending
- Bond angle changes but bond length remains constant
- Takes place within the plane and out of plane
- In plane bending vibrations
- Rocking: Involve movement of atoms in same direction
- Scissoring: Two atoms are joined to a central atom and move towards or away from each other with change in bond angle
- Out of plane bending vibrations
- Wagging: Both atoms move up and below the plane with respect to central atom.
- Twisting: Movement of one atom up and the other atom down the plane with respect to the central atom.
Selection route for IR Spectroscopy
- Vibration must cause a change in dipole moment of the molecule.
- If a molecule is polar, then only it can show IR spectra and the molecule is IR active.
- Example:HCl, CO
- If a molecule is non-polar, then it cannot show IR spectra and the molecule is IR inactive.
- Example: H2, N2, O2, Cl2
- Exception: CO2, being non-polar, is IR- active
- Reason: Due to asymmetric stretching of CO2, a change occurs in dipole moment and it becomes IR- active.
Regions of IR spectra
- Functional group region:
- High frequency region, between 4000/cm-1300/cm
- Characteristic stretching frequencies for important functional groups such as CO, OH and NH occur in this region.
- Finger print region:
- Middle frequency region, between 1300/cm- 900/cm.
- Complex absorptions occur due to combinations of interacting vibrational modes, providing unique fingerprint to each molecule.
- Pattern of bands are dependent on the structure of molecule.
- This pattern is highly sensitive and even to minor changes in structure and stereochemistry.
- Low- frequency region
- Region between 900/cm-650/cm
- Provides general classification of molecules from the pattern of absorption, such as substitution patterns on benzene ring.
- Absence of absorptions provide a good evidence for absence of an aromatic compound.
Vibrational frequency (Hooke's Law)
- Hooke's Law helps to determine stretching frequency.
- Vibrational frequency= 1/(2Ï€c)xsqrroot(k/u)
c= velocity of light
k= force constant
u= reduced mass
u=m1m2/m1+m2
Difference between IR and UV spectrum
With this article at OpenGenus, you must have the complete idea of Infrared Spectroscopy.