kinds of polyatomic molecules in spectroscopy

kinds of polyatomic molecules in spectroscopy

Polyatomic molecules are those molecules which contain more than two atoms in their structure. Spectroscopy is a technique which is used  the study of reaction when light interacts with matter and is used to explain the characteristics of molecules. The spectra of polyatomic compounds is quite difficult to study in spectroscopy then the spectra of diatomic molecules due to its more complexity.

Many  forms of polyatomic molecules can be characterised from one another owing to their symmetry and molecular geometry:


These molecules are known as linear ones and lack a constant dipole moment.

Linear molecules include:

  • hydrogen cyanide (HCN)
  • and carbon dioxide (CO2).


Nonlinear molecules have a constant dipole moment and a bent or angular form.

Non linear molecule includes:

  • Water (H2O)
  • and ammonia (NH3).


Symmetric top molecules lack a persistent dipole moment and have a symmetric top shape.

For example:

  • Methane (CH4)
  • and carbon tetrachloride (CCl4).


Asymmetric top molecule contain these two characteristics

  1. constant dipole moment.
  2. and an asymmetric top shape.

Asymmetric top molecule include:

  • acetic acid (CH3COOH)
  • and ethanol (C2H5OH).

Effect in spectroscopy:

The kind of polyatomic molecule and its molecular geometry both have a significant impact on how it behaves spectroscopically. For instance, due to the absence of a permanent dipole moment, symmetric top molecules have a simpler rotational spectrum than asymmetric top molecules. Because of their persistent dipole moment and bending modes, nonlinear molecules have more complicated rotational and vibrational spectra than do linear molecules.