Infrared IR spectroscopy and Raman spectroscopy are two complementary techniques used to study the vibrational modes of molecules. Both techniques provide information about the molecular structure, but they are based on different principles and probe different types of vibrational modes.Infrared spectroscopy is based on the absorption of infrared light by a molecule, which leads to a change in the dipole moment of the molecule. This means that IR spectroscopy is sensitive to vibrational modes that involve a change in the molecular dipole moment. In contrast, Raman spectroscopy is based on the inelastic scattering of light usually in the visible or near-infrared range by a molecule, which leads to a change in the polarizability of the molecule. This means that Raman spectroscopy is sensitive to vibrational modes that involve a change in the molecular polarizability.There are some key differences between the vibrational modes probed by IR and Raman spectroscopy:1. Symmetry: IR-active modes are those that have a change in dipole moment during the vibration, while Raman-active modes are those that have a change in polarizability. Some vibrational modes may be both IR and Raman active, while others may be active in only one of the techniques.2. Selection rules: The selection rules for IR and Raman spectroscopy are different. In IR spectroscopy, only fundamental transitions v = 1 are allowed, while in Raman spectroscopy, both fundamental and overtones v = 2, 3, etc. can be observed.3. Signal intensity: The intensity of the signals in IR and Raman spectra depends on the magnitude of the change in dipole moment and polarizability, respectively. As a result, some vibrational modes may appear more intense in one technique compared to the other.Examples of when one technique may be more suitable than the other:1. IR spectroscopy is more suitable for studying polar molecules, as they have a significant change in dipole moment during vibrations. For example, IR spectroscopy is widely used to study the vibrational modes of molecules containing carbonyl C=O and hydroxyl O-H groups.2. Raman spectroscopy is more suitable for studying nonpolar or symmetric molecules, as they have a significant change in polarizability during vibrations. For example, Raman spectroscopy is widely used to study the vibrational modes of molecules containing carbon-carbon double bonds C=C and carbon-carbon triple bonds CC .3. In some cases, a molecule may have vibrations that are both IR and Raman active, but one technique may provide better resolution or more information about the vibrational modes. For example, the vibrational modes of water H2O can be studied using both IR and Raman spectroscopy, but the Raman spectrum provides better resolution and more information about the symmetric and antisymmetric stretching modes.4. In samples that are highly fluorescent or have a strong background signal, Raman spectroscopy may be more suitable, as it is less affected by these interferences compared to IR spectroscopy.In summary, the choice between IR and Raman spectroscopy depends on the molecular structure, the type of vibrational modes of interest, and the sample properties. Both techniques provide valuable information about the molecular structure and can be used in a complementary manner to obtain a comprehensive understanding of the vibrational modes of a molecule.