Vibrational coupling
In addition to the vibrations mentioned above, interaction between vibrations can occur (coupling) if the vibrating bonds are joined to a single, central atom. Vibrational coupling is influenced by a number of factors;
Strong coupling of stretching vibrations occurs when there is a common atom between the two vibrating bonds
Coupling of bending vibrations occurs when there is a common bond between vibrating groups
Coupling between a stretching vibration and a bending vibration occurs if the stretching bond is one side of an angle varied by bending vibration
Coupling is greatest when the coupled groups have approximately equal energies
No coupling is seen between groups separated by two or more bonds