Infrared spectroscopy is an absorption method in the wavelength region of 1 to 100 µm in that extends the region of the visible light to longer wavelengths and smaller frequencies/energies. The energy of infrared light is no longer sufficient to induce transitions of valence electrons. Instead, infrared radiation excites vibrational and rotational motions in molecules. Except for the differences in the energy transfer from the radiation to the molecule, the principles of IR spectroscopy are the same as those of VIS/UV spectroscopy or other spectroscopic techniques. The absorption of infrared light is again characterized by the BougerLambert-Beer Law. However, infrared spectra are usually presented by a plot of the percentage of transmission vs the wavenumber in cm–1 (as opposed to a plot of absorbance vs. the wavelength in nm in UV/VIS spectroscopy). A typical IR spectrum is therefore recorded from about 4 000 to 10 000 cm–1 (upper limit) to about 100-800 cm–1 (lower limit