5. Infrared absorption
Selection rule:
A molecule will absorb infrared radiation if the
change in vibrational states is associated with
a change in the dipole moment () of the
molecule.
Vibrations which do not change the dipole
moment are Infrared Inactive (homonuclear
diatomics).
7. • In the region from 1300 to 400 cm-1,
vibration frequencies are affected by the
entire molecule, as the broader ranges for
group absorptions in the figure below –
fingerprint region.
• Absorption in this fingerprint region is
characteristic of the molecule as a whole. This
region finds widespread use for identification
purpose by comparison with library spectra.
8. Effect of hydrogen bonding
Hydrogen bonding alters the force constant of both
groups:
– the X–H stretching bands move to lower frequency
– the stretching frequency of the acceptor group (B) is also
reduced, but to a lesser degree
– The X–H bending vibration usually shifts to a shorter
wavelength
9. Stretching vibrations (carbonyl groups)
Structural unit Frequency, cm-1
Aldehydes and ketones1710-1750
Carboxylic acids 1700-1725
Acid anhydrides 1800-1850 and 1740-
1790
Esters 1730-1750
Amides 1680-1700