How does Raman Spectroscopy Differ from IR Spectroscopy?

Raman spectroscopy and infrared spectroscopy are techniques that are similar in nature and can be used to deduce the same type of information. Both spectroscopy techniques can be used to give information about the vibrational modes of molecules, but there are key differences between how the two instruments operate.

In this article, we look at the two techniques, how they both work and the differences between them.

Raman Spectroscopy

Raman spectroscopy is a form of vibrational spectroscopy used to identify vibrational, rotational, and other low-frequency modes of molecules.

These modes can then be used to determine the chemical structure of a molecule. However, most experiments are concerned with vibrational modes. Raman spectroscopy has found itself to be a very useful tool among inorganic chemists and material scientist in the analysis of oxygen-rich solid-state molecules.

In addition, Raman spectroscopy can also be used to identify polymorphs, track changes in molecular structure and crystallinity, evaluate the residual stress on a molecule and identify a molecule’s orientation.

The Raman Effect

Raman spectroscopy relies on a form of inelastic scattering of light, known as Raman scattering. Most scattering is elastic, i.e. the detected photons have the same energy and wavelength as the incident photon. However, in inelastic scattering, is when the light is scattered at a different frequency to the incident photon. This is known as the Raman effect.

The Raman effect occurs when the incident photon interacts with the electric dipole of the molecule undergoing analysis. This causes a perturbation of the molecules electric field and causes an excitation to a virtual energy state which is lower in energy than a real electronic transition state. The molecule then returns to its ground state and exhibits a change in its vibrational energy.

There are two types of Raman scattering mechanisms – Stokes and ant-Stokes scattering, although the former is generally the most commonly seen mechanism. This is only due to most experiments being performed close to room temperature, where the anti-Stokes scattering is weak compared to the Stokes scattering.

An example of a Raman spectrum showing typical Raman shifts for molecular functional groups. In this intance the molecule being studied is acetaminophen (paracetamol). Source: Wikimedia

Using a Raman Spectrometer

A beam of light is fired from a light source (commonly a LASER), whereupon it is collected by a lens and passed through a monochromator to isolate a single wavelength of light. Once the light hits the sample, a filter is used to collect the wavelengths that adhere to Rayleigh scattering principles (elastic scattered light at the same wavelength as the LASER light source). The rest of the light is then collected by a charge-couple device (CCD) detector, and the Raman spectrum is then analyzed.

There are also many advanced forms of Raman spectroscopy, including surface-enhanced Raman, resonance Raman, tip-enhanced Raman, polarized Raman, stimulated Raman transmission Raman, spatially offset Raman and hyper Raman spectroscopy.

Infrared (IR) Spectroscopy

Infrared (IR) spectroscopy measures the interactions between a sample and absorbed light. Unlike Raman spectroscopy, IR is concerned with detecting the stretching and vibrational modes of the covalent bonds in a molecule, and in particular, certain functional groups present in a molecule.

It is most commonly used for organic molecules, although some inorganic molecules can be analyzed as well. IR is not generally used to determine the whole structure of a molecule because there is too much noise in the IR peaks for the different (and sometimes vast number of) C-C and C-H bonds.

There are many ways in which a covalent molecule can stretch. These include symmetric stretching, where molecules can move through space; or asymmetric stretching, where a decrease or increase in the bond length is observed. In addition, molecules also exhibit symmetric stretching, antisymmetric stretching, scissoring, rocking, wagging and twisting vibrational modes.

The exact frequency at which the vibration occurs is determined by the strength of the bonds in the molecule, and different functional groups have specific absorption range that are known (so it is easy to deduce some groups from a spectrum).

An example of an IR spectrum showing typical absorptions for different molecular functional groups. Source: Chem Libre

Using an IR Spectrometer

To use an IR spectrometer, the sample must undergo one of many sample preparation methods. The type depends on the material being analyzed. Liquid samples are placed between two transparent salt plates, as they are transparent to infrared light and can encase the liquid. Solid samples are more versatile and can be prepared in a number of ways, including being ground up into KBr disks, dissolved in an appropriate solvent and being applied as a thin layer on salt plates.

The sample preparation is the most time-consuming part of using an IR machine. Once the sample is ready, two measurements are made. The first is a reference to eliminate any background noise that might affect the results, followed by the sample itself. Sometimes the reference can just be air, other times it may be the solvent that a compound is dissolved in.

An infrared light source is passed through a monochromator and is directed towards the sample. Once the light has been absorbed/reflected by the sample, the remaining wavelengths of light are detected at the infrared detector. How much energy was absorbed at each frequency can then be used to determine the functional groups in the molecule.

The Main Differences Between Raman and IR

The main difference between the two techniques is in the molecular vibrations that take place to determine the structure of the molecule. Raman spectroscopy relies on molecules which can undergo a polarizability change during the vibration (i.e. the electron cloud must undergo a positional change), whereas in IR, the molecule must undergo a dipole moment change during the vibration (i.e. a non-symmetrical molecule). In short Raman vs IR differs in the scattering vs absorption of light, respectively.

Another difference lies in the technique itself, especially in the type of monochromatic wave used. Raman spectroscopy uses a monochromatic beam in the visible, near-infrared or near-ultraviolet range of the electromagnetic spectrum. In comparison, IR only uses a beam in the infrared region of the electromagnetic spectrum.

Advantages of Raman Spectroscopy

Whilst the two are similar, Raman spectroscopy offers some advantages over IR, despite its higher cost.

Raman is more versatile and can be used to identify gases, and water can also be used a solvent (this is not possible in IR due to an intense light absorption) allowing aqueous solutions to be analyzed. Additionally, the samples in Raman spectroscopy don’t require much preparation, whereas an extensive preparation is required before a sample can be analyzed with IR.

Sources & Further Reading




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