Vibrational spectroscopy
The vibrational spectroscopy is a group of the analytical measurement methods based on the excitation of the normal vibrations of molecules based. Vibrational spectroscopy can thus be viewed as a sub-area of molecular spectroscopy . Typical methods are infrared spectroscopy (IR) and Raman spectroscopy as well as HREEL spectroscopy .
basis
In simple terms, molecules consist of atoms of the same or different mass, which are connected to one another by "elastic" bonds. Similar to a comparable structure made of metal balls that are connected by springs, every molecule is capable of vibrations in which the atoms periodically shift against each other (see normal vibration ). The frequency range of these vibrations extends from the far-infrared (mostly inorganic compounds) through the mid-infrared (usually for organic compounds) to the near-infrared range (combination and harmonics of the vibrations from the other two ranges, see near-infrared spectroscopy ).
Due to the mass of the atoms involved and the strength, length and angle of the bond, the vibrations and the associated vibration frequencies in a molecule are characteristic of the respective molecule or of a molecular group ( group frequency ) such as a hydroxyl group . Therefore, the investigation of absorption and emission spectra enables the determination of substances in a sample as well as the identification of reaction processes and the elucidation of structure (even if there are further analytical methods here that better fulfill these tasks). A substance is usually determined by comparing a spectrum with the spectrum of a reference sample, but it can be done by comparing it with itself (for example a difference spectrum before a reaction) or with a calculated spectrum.
Methods and areas of application
Vibration spectroscopy is widely used in research and industry. The respective sub-methods each have their own specific restrictions, so that they can sometimes only be applied to very specific problems. Industrial areas of application can be found primarily in the area of quality and process control .
One of the two most widely used methods of vibrational spectroscopy is infrared spectroscopy. It is based on the interaction of the molecular vibrations with infrared radiation, for example absorption and emission. This is always the case when the molecule has either a changeable or an inducible dipole moment (IR-active). Typical areas of application are both the qualitative and the quantitative analysis of mostly organic compounds, for example for the identification of solids and liquids or the quality control of raw materials. Infrared spectroscopy is less suitable for more complex tasks, such as the analysis of protein conformations, since the coupling of individual groups of molecules with neighboring groups of molecules quickly leads to very complex spectra. Nowadays, measurements are mostly carried out using Fourier transform spectrometers , which can measure faster, more sensitively and with greater accuracy than dispersive spectrometers. The short measurement time also enables the lateral rasterization of a sample to analyze material distributions by means of FTIR microscopy .
In the case of oscillations symmetrical to the center of symmetry, there is no change in the dipole moment, and therefore only difficult or impossible to detect with infrared spectroscopy. They are also known as "IR inactive". Such “forbidden” vibrations are often Raman-active, that is, they can be detected by means of Raman spectroscopy . In contrast to infrared spectroscopy, Raman spectroscopy does not use infrared radiation, but visible laser light. The light is directed onto a sample and the light indirectly scattered by the sample ( Raman effect ) is detected. As a result of the indirect scattering, in addition to the irradiated frequency, other frequencies are observed whose frequency differences to the irradiated light correspond to the energies of rotation and oscillation processes characteristic of the material. The spectra obtained allow conclusions to be drawn about the substance being examined. Typical sample materials are inorganic substances, as they are usually easier to analyze than organic materials. An advantage over infrared spectroscopy, which is particularly evident in the area of process control, is the possibility of using optical fibers. Fourier transform spectrometers are also increasingly being used in Raman spectroscopy.
Other methods that are mostly only used in research due to their complexity or a narrowly limited area of application are, for example, HREEL and sum frequency spectroscopy .
literature
- Claus Czeslik, Heiko Seemann, Roland Winter: Basic knowledge of physical chemistry . 4th updated edition. Vieweg + Teubner, Wiesbaden 2010, ISBN 978-3-8348-0937-7 ( limited preview in the Google Book Search - Chemistry Study Books ).
- Helmut Günzler and others: Analytiker-Taschenbuch 21 . Springer, Berlin et al. 2000, ISBN 3-540-66232-4 ( limited preview in the Google book search).
- Ingolf V. Hertel, Claus-Peter Schulz: Atoms, molecules and optical physics. Volume 2: Molecules and Photons - Spectroscopy and Scattering Physics . Springer, Berlin et al. 2010, ISBN 978-3-642-11972-9 , pp. 247 ( limited preview in Google Book Search - Springer Textbook ).
Individual evidence
- ↑ Joseph B. Lambert, Scott Gronert, Herbert F. Shurvell, David A. Lightner: Spectroscopy - structure clarification in organic chemistry . 2nd edition, Pearson Germany, Munich 2012, ISBN 978-3-86894-146-3 , pp. 485-590.