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.
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's 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.
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.
The types of vibrations measured in Raman and IR spectroscopy are useful for different applications. The result of Raman spectroscopy is a much cleaner fingerprint region, as there isn't as much interference, so it's used when we want to easily identify the peaks in a compound. It's often used to initially determine possible compounds, but it often can't determine the exact compound because it isn't as specific as IR spectroscopy. Raman spectroscopy won't tell us about as many of the bonds in the molecule. For example it won't often tell us about a carbon-oxygen double bond because the dipole is so strong that the polarization won't change, while IR spectroscopy will tell us about all of these bonds.
Lesson Summary
Vibrational spectroscopy is a non-destructive method of characterizing and identifying compounds that works by measuring the vibrations of compounds. Each compound has a unique fingerprint, or measurement of the vibrations, allowing compounds to be identified. The two main types of vibrational spectroscopy are infrared spectroscopy, which means the absorption and transmission of IR light due to the vibration of the molecule, and Raman spectroscopy, which measures the light scatterings caused by the vibrations of the molecule.
