What is the Difference Between IR and Raman Spectra?

The main differences between IR and Raman spectra lie in the mechanism of interaction with the sample, the information they provide, and the experimental setup:

1. Mechanism:

• IR spectroscopy depends on the change in the dipole moment of a molecule.
• Raman spectroscopy depends on the change in polarizability of a molecule.

1. Information:

• IR spectroscopy measures the absolute frequencies at which a sample absorbs radiation.
• Raman spectroscopy measures the relative frequencies at which a sample scatters radiation.
• IR spectroscopy is sensitive to hetero-nuclear functional group vibrations and polar bonds, especially OH stretching in water.
• Raman spectroscopy is sensitive to homo-nuclear molecular bonds, such as C-C, C=C, and C≡C.
• IR spectroscopy provides a "fingerprint region" of the spectrum where intramolecular vibrations are observed.
• Raman spectroscopy yields information about intra- and intermolecular vibrations.

1. Experimental Setup:

• IR spectrometers typically employ a black body radiator.
• Raman spectrometers utilize a laser as the source, typically visible or near-IR laser.
• Raman spectra result from the scattering of light by vibrating molecules.
• IR spectra result from light absorption by vibrating molecules.
• In Raman spectroscopy, scattered light is observed at right angles to the direction of the incident beam.

Both techniques offer molecular information and can be used with microscopic techniques, but the choice between them depends on the nature of the application and the desired information.

Comparative Table: IR vs Raman Spectra

Here is a table comparing the differences between IR and Raman spectra:

Both IR and Raman spectroscopy are complementary techniques for analyzing the vibrational modes of molecules, and they provide information about the molecular structure and bonding. The main differences between the two techniques lie in the vibrational modes they emphasize and the interaction mechanisms involved.