Triatomic non-linear H2O molecule makes an angle with oxygen at the centre. Being nonlinear, this molecule possesses oscillating dipole moments so H2O molecule is IR active. and will have, (3n – 6) observed absorption bands. Where n is the number of atoms. Thus, water molecule (n = 3 atoms) will have, 3n – 6 = 3(3) – 6 = 9 – 6 = 3 three vibrational degrees of freedom.
This indicates that there is a dipole moment for the three vibrations in the molecule. There is a periodic change of the electric dipole moment during the vibration. The dipole moment may occur either along or across the line of the symmetry axis.
All these modes of vibrations are associated with dipole moment and are Infra-red active. These modes of vibrations are:
Symmetrical Bending (Scissoring)
If we see the IR spectra of H2O molecule the frequencies 3652 cm-1 and 1595 cm-1 correspond to symmetric vibrations whereas 3756 cm-1, the highest one, represents asymmetric mode. The low frequency may be assigned to angle bending vibration.
However, the two O-H bonds in H2O are identical, hence their stretching vibrations cannot separately give rise to absorptions at different frequencies. Each absorption is not to be assigned to a separate vibration of each bond. The vibrations of the two bonds are coupled (overlapped) to give two different vibrations, one with symmetric stretching vibration and the other with asymmetric stretching vibration. These three vibrational motions are called normal vibrations, because all the three atoms in the molecule vibrate at the same frequency and in phase with each other.