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Energy and wavenumber of infrared absorption by molecular vibration

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  422-423

2026-11-11

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Energy and wavenumber of infrared absorption by molecular vibration

Molecules can undergo two types of vibrations, namely stretching vibrations that involve changes in bond length and bending vibrations that involve changes in bond angles. The vibrational modes associated with the methylene group -CH₂- . Theoretically, a ‘nonlinear’ molecule has (3N 6) such modes of vibration, where N is the number of atoms in the molecule. These are called fundamental modes and require IR energy in the range 4000–400 cm^-1 (mid IR) to become excited. Not all vibrations in a molecule are assignable in an IR spectrum; generally, only the most prominent are readily assigned to a given vibrational mode. These characteristic vibrations are a good way to detect the existence of functional groups in a chemical compound. The precise wavenumber at which a particular vibration absorbs light is associated with bond strength and the atomic masses of the atoms in the bond. The wavenumbers required for the excitation of typical vibrations are given in Table 16.1.

In addition, there are overtones (the excitation of a vibration to a double or higher frequency) and combinations that are the sum or difference of two or more fundamental bands. No funda mental vibrations require energy greater than 4000 cm1 to become excited. All vibrations in the near IR are therefore overtones or combination bands. The reader is referred to spectroscopic texts for a more detailed explanation of the origin of IR bands (e.g. Williams and Fleming 1995, and others listed in the Further Reading). Changes in the wavenumber of a band can be related to changes in either the structural environment or the physical state of the molecule. However, many bands in the complex region from 1800 to 400 cm^-1, which is usually referred to as the ‘fingerprint region’, remain of

unconfirmed origin. Many of the bands are characteristic of the molecule as a whole and cannot be assigned directly to particular bonds. Nevertheless, inspection of IR spectra can form the basis of qualitative analytical work in IR spectroscopy to confirm the identity of a sample. A complete molecular structure cannot be deduced directly from an IR spectrum. Rather, functional groups are identified and total molecular identity is confirmed by comparison with IR spectra in a compendium or by comparison with a reference substance of known provenance run under the same conditions.