Infrared radiation brings about changes in the stretching and bending motions of chemical bonds. In order for a bond to absorb infrared radiation, the dipole moment of the bond must change during the motion. Animations of motions are published in the WWW J. Chem Educ. A classic physical chemistry laboratory experiment involves measuring and interpreting the vibration-rotation spectrum of HCl.

Since vibrations and rotations are a major part of the kinetic energy of molecules, infrared radiation characteristic of the temperature is emitted from all objects. There are many remote temperature sensing devices that measure infrared energy. An ear thermometer uses an infrared waveguide with a sensor to detect infrared radiation from the eardrum. Industrial processes sense temperatures remotely with infrared detectors. Night vision goggles and cameras depend on detecting the heat from objects as infrared radiation. For night vision, objects must radiate different amounts of infrared energy.

Organic substances often are identified by infrared spectra. An example is the infrared spectrum of acetone.

An on-line IR spectroscopy tutorial is available. Infrared spectral assignments are addressed in more detail in the Instrumentation course. The units used in infrared spectra are often micrometers for wavelength or cm^-1 which is the reciprocal of wavelength. To convert cm^-1 to frequency the term must be multiplied by the speed of light (c = 2.998 x 10⁸ cm/sec). Most tables of organic stretching frequencies are expressed as cm^-1.