Infrared Spectroscopy

Infrared (IR) spectroscopy is the most common and widely employed analytical technique in both inorganic and organic chemistry, primarily used for identifying compounds and analyzing molecular structures. It operates on the principle that chemical compounds have various functional groups that absorb IR radiation at different frequencies, allowing them to be identified based on their distinctive absorption spectra. At its core, IR spectroscopy relies on the vibrational behavior of molecules. Chemical bonds are not static; they oscillate, and these vibrations can absorb IR radiation, causing transitions between discrete vibrational energy levels—this gives rise to a molecule's vibrational spectrum. For a vibration to be IR-active, it must involve a change in the molecule's dipole moment. The instrumentation for IR spectroscopy includes several core components: an IR radiation source, sampling system, monochromator (or interferometer in Fourier Transform (FT) systems), detector (transducer), and signal processing unit. There are three main types of IR spectrometer: dispersive IR spectrometer, Fourier transform IR spectrometers (FT-IR), and non‐dispersive IR spectrometer. These instruments operate across the IR portion of the electromagnetic spectrum, which is typically divided into near, mid-, and far IR regions. Among these, the mid‐IR region is most frequently used in chemical analysis because of its sensitivity to functional group vibrations. This chapter offers a comprehensive overview of IR spectroscopy, including its fundamental principles, the theory behind molecular vibrations, and the challenges involved in spectral interpretation, particularly for complex molecules. It also outlines the instrumentation and analytical methodologies that enable precise chemical characterization using IR spectroscopy.