Infrared (IR) spectroscopy measures the bond vibration frequencies in molecules to determine their functional groups such as OH, CHOH, CH2, CH, CH3, C=O, CHO, and aromatics. The IR region is just below the red visible region with wavelengths typically between 2.5-25 μm, which are commonly measured in wavenumbers or cm-1 as the reciprocal of the wavelength in centimeters. Wavenumbers are directly proportional to frequency and energy.