This chapter introduces how both mid-infrared (mid-IR) and Raman spectroscopy provide characteristic fundamental vibrations that are employed for the elucidation of molecular structure. Mid-IR, near-IR, and Raman spectroscopy are part of vibrational spectroscopy. Raman spectroscopy is best at symmetric vibrations of nonpolar groups while IR spectroscopy is best at the asymmetric vibrations of polar groups. Infrared and Raman spectroscopy involve the study of the interaction of radiation with molecular vibrations but differ in the manner in which photon energy is transferred to the molecule by changing its vibrational state. IR spectroscopy measures transitions between molecular vibrational energy levels as a result of the absorption of mid-IR radiation. The IR and Raman vibrational bands are characterized by their frequency (energy), intensity, and band shape (environment of bonds). The frequencies of these molecular vibrations depend on the masses of the atoms, their geometric arrangement, and the strength of their chemical bonds. Two different approaches are used for the interpretation of vibrational spectroscopy and elucidation of molecular structure. The first approach is the use of group theory with mathematical calculations of the forms and frequencies of the molecular vibrations, and the second approach is the use of empirical characteristic frequencies for chemical functional groups. IR spectroscopy was the first structural spectroscopic technique widely used by organic chemists. An extensive user base resulted in a great increase in available IR interpretation tools and the eventual development of FT-IR instrumentation.
