vibrational spectroscopy otherwise IR-spectroscopy; FTIR-spectroscopy (rus. спектроскопия, колебательная otherwise ИК-спектроскопия) — a family of methods to study the structure of matter by vibrational spectra of absorbed or emitted light due to transitions between energy states which are characterised by different vibrations of or groups of atoms relative to their equilibrium positions.

Description

Vibrational spectra are extremely sensitive and contain data on specific characteristics of molecules, so they are widely used in chemical research. The advantage of the of vibrational spectroscopy methods is that they can be used to study virtually any substance in any state of aggregation (gas, liquid or solid including crystals and amorphous bodies). To obtain vibrational spectra, it is sufficient to have a very small amount of material (depending on spectrometer type and sensitivity, up to several milligrams).

Most of the known vibrational frequencies lies within the range 1012-1014 Hz which corresponds to the infra-red (IR) range of electromagnetic radiation. Absorption or emission bands corresponding to vibrations of atom pairs relative to each other (the so-called stretching vibrations) as well as vibrations of atom groups are studied both experimentally and theoretically for different classes of substances and are available in reference books (the so-called characteristic frequencies).

The main methods of vibrational spectroscopy are infra-red spectroscopy and Raman light scattering spectroscopy. IR spectra are absorption spectra, and interpreting requires scientists to operate with categories of the absorption bands concept. Raman spectra are the spectra of fluorescence (emission).

Infra-red spectrometers or, more precisely, IR-Fourier spectrometers allow one to quickly obtain vibrational spectra of liquids and gases as well as solid bodies which are sufficiently transparent in the studied region of the emission frequency (~40-6000 cm-1 for the most common devices) which gives the possibility to perform qualitative and quantitative analysis of samples. To study strongly absorbing materials scientists can also use the attenuated total reflectance (ATR) method which makes it possible to obtain spectra of substances on the samples' surface.

Due to the fact that some vibrational transitions may be forbidden by the selection rules (see "Electronic-Vibrational Spectroscopy") a series of vibrational frequencies inherent to characteristic structural groups may be absent in spectra registered by infra-red spectrometers. In this case, the data from Raman light scattering spectroscopy may be complementary, is the corresponding spectral bands are allowed by the selection rules for luminescence transitions.

One of the most illustrative examples of the efficient application of vibrational spectroscopy today is the determination of graphene's internal structure by the Raman scattering peak in the 1600 cm-1 region characteristic for group vibrations of the carbon lattice atoms.

Illustrations

К (above) and КР (below) spectra of benzene C
К (above) and КР (below) spectra of benzene C6H6 [1].

Author

  • Lourie Sergey

Source

  1. Pentin Yu. A., Vilkov L. V. Physical Methods in Chemistry. Textbook (in Russian) // Moscow: Mir, 2003. - 683 p.

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