Raman vs Infrared Spectroscopy Measurement of Octasulfur
Infrared and Raman are both categorized as vibrational spectroscopies, which broadly speaking, measure the same basic material properties. Namely, they provide a molecular fingerprint by determining the vibrational energy levels of a given material. The term fingerprint, in this case, is used to highlight the fact that for two materials to have the same vibrational spectra, they must have identical reduced mass, bond configuration, and steric effects. To put it more succinctly, the only way two samples can have the same vibrational spectra is that they must be the same material. Therefore, Raman and infrared are among the most used analytical techniques for unknown material identification and reaction monitoring.
Despite the surface-level similarities of the two techniques, in practice, there are many subtle (and not so subtle) differences, which have led many in the community to get the false impression that one technique is superior to the other. This fracturing is rather unfortunate since the two techniques are highly complementary, and most laboratories can benefit from owning both Raman and infrared setups. To illustrate this, we have decided to focus this application note on determining which vibrational modes are better suited for Raman analysis and which ones are better suited for infrared.
A detailed analysis of the selection rules governing infrared and Raman spectroscopy can be somewhat overwhelming to those unfamiliar with quantum mechanics and physical chemistry. But most spectroscopists are familiar with the primary takeaways – infrared absorbance is a linear process dependent on dipole moment, and Raman scattering is a nonlinear process dependent on polarizability. Armed with this basic understanding, we can utilize molecular symmetries to simplify the process. While this requires group theory, fortunately, only 32 different point (symmetry) groups are needed to characterize any possible molecular configuration, each having a well-defined character table, which includes all possible translational, rotational, and vibrational operations for that group. Furthermore, the point group has already been identified and cataloged for most molecules.