THEORY

Raman is a vibrational spectroscopic technique; it is complementary to mid-infrared (IR) and near-infrared (NIR) spectroscopy where there is center of symmetry, because both techniques rely on interaction with the molecular structure of the material. The Raman effect itself arises as a result of a change in the polarizability of molecular bonds during a given vibrational mode and is measured as inelastically scattered radiation, whereas absorption intensity at IR or NIR wavelengths occurs due to differences in dipole moments. Raman spectroscopy is particularly sensitive to non-polar bonds (e.g., C–C, single or multiple bonds), and thus, strong dipole oscillators do not show Raman scattering because the molecular geometry does not give rise to polarizability. The appearance of a Raman spectrum is much like an IR absorbance spectrum. The intensities, or the number of Raman photons counted, are plotted against the shifted energies. The x-axis is generally labeled “Raman shift/cm⁻¹” or “wavenumber/cm⁻¹”. The Raman shift is usually expressed in terms of wavenumber and represents the energy difference between the incident photon energy and the inelastically scattered photon energy. In addition, water, which has a strong IR absorption spectrum, only exhibits a weak Raman signature and is a useful solvent for this technique. The spectral features and intensities of Raman and IR/NIR are therefore different for most organic molecules, and thus the spectra of the different techniques may be used to complement specificity and characterization.