The following are the selection rules for IR spectroscopy:
1. Only vibrational modes that result in a change in the dipole moment of the molecule are allowed. This is because IR spectroscopy is based on the absorption of infrared radiation, which is a type of electromagnetic radiation. When infrared radiation interacts with a molecule, it can cause the electrons in the molecule to vibrate. If the vibration results in a change in the dipole moment of the molecule, then the molecule will absorb the infrared radiation and an absorption peak will be observed in the IR spectrum.
2. Vibrational modes that are symmetric with respect to the center of symmetry of the molecule are not allowed. This is because the dipole moment of a symmetric molecule is zero, and therefore there can be no change in the dipole moment upon vibration.
3. Vibrational modes that are antisymmetric with respect to the center of symmetry of the molecule are allowed. This is because the dipole moment of an antisymmetric molecule is not zero, and therefore there can be a change in the dipole moment upon vibration.
In addition to these general selection rules, there are also specific selection rules for each type of vibrational mode. For example, the selection rules for stretching vibrations are different from the selection rules for bending vibrations.
The selection rules for IR spectroscopy are important because they allow us to predict which vibrational modes of a molecule will be observed in the IR spectrum. This information can be used to identify and characterize different molecules.
