Question

For a molecule with three atoms it is predicted that there will be three IR-active vibrational modes. Why might there be less than three bands in its IR spectrum? Why might there be more than three?

Answer 1

For a molecule with three atoms it is predicted that there will be three IR-active vibrational modes. Why might there be less than three bands in its IR spectrum? Why might there be more than three?

The number of molecular vibrational modes =3n-6 For non linear molecule

The number of molecular vibrational modes = 3n-5 for linear molecules

here n is the number of atoms.

For example H2O which is non-linear than the number of molecular vibrational modes = 3n-6

=3*3-6

=9-6

=3

For example CO2 which is linear than the number of molecular vibrational modes = 3n-5

=3*3-5

=9-5

=4