Vibrational Spectroscopy: Definition & Types

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Vibrational spectroscopy is a method for characterizing and identifying compounds. In this lesson we will learn what vibrational spectroscopy is and the two types, Raman and IR.

What is Vibrational Spectroscopy?

How do we identify an unknown substance, or ensure a specific substance is what we think? There are several ways to do this that will destroy the sample, which unfortunately means further testing cannot be performed on it. Vibrational spectroscopy is a non-destructive identification method available that measures the vibrational energy in a compound.

Each chemical bond has a unique vibrational energy. Even a carbon-carbon bond will be different from one compound to another depending on what other compounds each carbon is bound to. Due to this unique vibrational energy, each compound will have a unique fingerprint, or the output identifying the peak strengths at specific vibrations. This fingerprint can be used to determine compound structures, identify and characterize compounds and identify impurities. This is done by comparing the fingerprint with the fingerprints of known compounds.

There are two types of vibrational spectroscopy: infrared and Raman. The main difference between these is the types of vibrations and transitions that are measured. Typically these two forms will be used in connection with each other in order to get a more complete picture of the compounds.

Infrared Spectroscopy

Infrared spectroscopy (IR spectroscopy) uses a beam of light within the infrared spectrum (700 nm - 1000 nm) to excite the bonds and measure the absorption and transmission of light due to the vibrations of the bonds. IR spectroscopy output will have % transmittance on the y-axis and wavelength on the x-axis. There are specific functional groups which will have specific peaks.

It often takes a while to learn how to read an IR spectra due to the many interfering peaks
IR spectra

When looking at an IR spectra fingerprint we can identify certain peaks. Spectra refers to the chart or measurement results from a spectroscopy test. For example, an alcohol (OH) peak will be wide and within the 3650-3200 cm wavelength range. A terminal alkyne will be in a similar wavelength range (3340-3250 cm), but it will be a very narrow peak. These specific functional groups can first be identified in order to begin characterizing the compound. It is specifically useful if we don't already have an IR spectra to which we can compare the compound.

Raman Spectroscopy

Raman spectroscopy can use infrared, visible, or UV light beams (10 nm - 1000 nm) to excite the bonds and specifically measure the light scatterings caused by the vibrations. So, Raman spectroscopy can use a much wider range of wavelengths, but it only measures a narrow type of vibration.

The Raman spectra is much cleaner than IR spectra but does not give much information
Raman spectra

In order for Raman spectroscopy to measure the transitions taking place, the vibrations need to be strong enough to actually change the polarization of the bond. For IR to measure the vibrations there only needs to be a change in the dipole, instead of the actual polarization.

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