Basics of vibrational spectroscopy

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IR spectroscopy is based on the absorption by the sample of a light beam whose radiations are divided into three ranges:

  • Near-infrared
  • Mid-infrared
  • Far-infrared

IR ranges and types of movements observed

(click on each range for more details)
Table of infrared range. In the far IR (100 - 400 cm-1), pure rotations (gas). In the middle IR (400 - 4000 cm-1) stretching and deformations of covalent bonds, harmonics and combinations. In the near IR (4000 - 12500 cm-1), harmonics and combinations.

In the far-IR range, pure rotations of molecules in their gaseous form are mainly observed. Their interpretation remains difficult and often requires numerical simulation.

Mid-IR spectroscopy is the most common and provides access to the identification of chemical functions in a molecule.

In the near-IR, the wide and low-intensity bands observed correspond to combinations and overtones of vibrational modes. The applications of these analyses, found in quality control, often require data processing by chemometrics.

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