This document discusses infrared spectroscopy and the types of vibrations that can be observed using this technique. It provides an overview of the selection rules for infrared absorption and notes that vibrations that do not change the dipole moment of a molecule will not be observed. Typical absorption regions are highlighted, with the fingerprint region from 1300-400 cm-1 noted as providing information about the overall molecule. The effects of hydrogen bonding and common stretching frequencies for functional groups are summarized.

1. Infrared Spectroscopy

2. CONTENTS
Fundamental
Overtone and combination bands
Vibration coupling
Group frequencies for common functional group
References

4. Types of Vibrations

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).

6. Absorption region

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

10. Stretching vibrations (single bonds)
sp C—H 3310-3320
sp2 C—H 3000-3100
sp3 C—H 2850-2950
sp2 C—O 1200
sp3 C—O 1025-1200

11. Stretching vibrations (single bonds)
O—H (alcohols) 3200-3600
O—H (carboxylic acids) 3000-3100

12. Stretching vibrations (single bonds)
sp2 C—O 1200
sp3 C—O 1025-1200

14. REFERENCES
Organic Spectroscopy, Principles and
Applications, Jag Mohan.
Infrared Spectroscopy, Fundamental and
Applications, Barbara Stuart.