Difference Between IR and Raman Spectra

The key difference between IR and Raman spectra is that IR spectra can be obtained from light absorption, whereas Raman spectra can be obtained from light scattering.

IR and Raman spectra are important in analytical chemistry for the determination of light-absorbing and light scattering properties of different molecules.

1. Overview and Key Difference
2. What is IR Spectra
3. What is Raman Spectra
4. Side by Side Comparison – IR vs Raman Spectra in Tabular Form
5. Summary

What is IR Spectra?

IR spectra or IR spectrum is the result of IR spectroscopy, where IR radiation is used to analyze a sample. Here, we can observe the interaction between matter and IR radiation. We can get IR spectra from absorption spectroscopy. IR spectroscopy is used for the identification and analysis of chemical substances in a given sample. Here, the sample can be solid, liquid or a gas. The instrument we can use to get an IR spectrum is the infrared spectrophotometer.

The IR spectrum is a graph. It has an absorbance of light by the sample in the y-axis and wavelength or the frequency of IR light in the x-axis. The units of frequency that we are using here is reciprocal centimetres (per centimetre or cm^-1). If we are using the wavelength instead of frequency, then the unit of measurement is micrometres.

Figure 01: A Sample IR Spectrum

An IR spectrum exploits the absorbance of different frequencies in IR radiation by the molecules in a sample and the characteristics features of the chemical structures. This is because the absorbed frequency of IR radiation is usually similar to the vibrational frequency of the analyte molecule. We can get the IR spectra for different molecules by passing a beam of IR radiation through the sample and detecting the transmitted light through the sample. It gives us details about the absorbed frequencies. Therefore, a typical IR spectrum is an absorption spectrum.

What is Raman Spectra?

Raman spectra or Raman spectrum is an analytical technique that lies on the inelastic scattering of photons in the sample. The inelastic scattering is called Raman scattering. This technique is very useful in determining the vibrational modes of molecules. Therefore, the Raman scattering effect is helpful in analytical chemistry for providing a structural fingerprint by which we can identify different molecules.

Figure 02: The different States involved in Raman Scattering

The radiation we can use in the detection of a Raman spectra include visible, near IR, or near UV range laser beams. However, near X-ray light beams can also be used here. In this process, the laser beam reacts with the molecular vibrations or phonons, resulting in the energy of the laser photons being shifted up or down.

What is the Difference Between IR and Raman Spectra?

IR and Raman spectra are important in analytical chemistry for the determination of light-absorbing and light scattering properties of different molecules. The key difference between IR and Raman spectra is that IR spectra can be obtained from light absorption whereas Raman spectra can be obtained from light scattering. Besides, Raman spectra is a highly expensive method compared to IR.

Below infographic shows more comparisons related to the difference between IR and Raman spectra.

Summary – IR vs Raman Spectra

IR and Raman spectra are important in analytical chemistry for the determination of light absorbing and light scattering properties of different molecules. The key difference between IR and Raman spectra is that we can obtain the IR spectra from light absorption and Raman spectra from light scattering.

Reference:

1. “Infrared Spectroscopy.” Chemistry LibreTexts, Libretexts, 3 June 2019, Available here.
2. “Infrared Spectroscopy.” Wikipedia, Wikimedia Foundation, 6 June 2020, Available here.
3. “Raman Spectroscopy.” Wikipedia, Wikimedia Foundation, 9 June 2020, Available here.

Image Courtesy:

1. “Bromomethane IR spectroscopy” By Unknown author – NIST (Public Domain) via Commons Wikimedia

2. “Raman energy levels” By Moxfyre, based on work of User:Pavlina2.0 – vectorization of File:Raman energy levels.jpg (CC BY-SA 3.0) via Commons Wikimedia