Meckesche symbols

from Wikipedia, the free encyclopedia

The meckeschen symbols is a seldom used designation for a symbolism named after Reinhard Mecke (1895–1969) of molecular vibrations of simple groups, that is two to four atoms.

Mecke presented his idea of ​​the stretching and deformation vibrations of molecular groups for the first time in 1930 at the Bunsen conference in Heidelberg and again more precisely at a conference in Leipzig in 1931. His idea was heavily criticized at the time, because it was believed that normal vibrations are only possible if all atoms of the molecule are included. In the following decades, Meckes' classification of normal vibrations became more and more popular and is nowadays generally accepted in molecular spectroscopy .

Assignments

For simple groups of molecules, Mecke first introduced a division of the vibrations into valence and deformation vibrations. For him, valence vibrations were vibrations in which the forces of the bond are mainly stressed (the individual bonds are compressed or plugged). These were generally designated with the symbol ν (lowercase Y). Deformation vibrations ( δ , lowercase delta) - also drawn as bending vibrations - are the remaining vibrations, in which the shape of the molecule changes (the bond angle changes).

Based on the maximum number of normal vibrations of an angled or linear molecule ( or , where the number of atoms is), according to Mecke, stretching vibrations (ν i ) and deformation vibrations (δ i ) result for this molecule . The individual valence and deformation vibrations were numbered according to their energetic position in the spectrum, with the respective vibration with the lowest energy being assigned the index 1.

An alternative designation can be made on the basis of the molecular groups considered and the existing symmetries of the vibrations. There are symmetrical and asymmetrical (or anti-symmetrical) vibrations, e.g. B. the symmetrical and the asymmetrical stretching vibration (ν s and ν as ). This is shown for ethene in the adjacent example, resulting in the stretching vibrations ν s (C = C), ν as (CH) and ν s (CH) as well as the deformation vibrations δ as (C – H) and δ s (C – H ).

Later, the deformation vibrations were designated more precisely as a function of the characteristic change in relation to the molecule and further symbols were introduced:

  • In the plane (engl. Plane in- )
    • δ… spreading vibration ( bending ), sometimes also called scissoring vibration
    • γ ... pendulum oscillation (English rocking ), sometimes also called rocking oscillation
  • Out of the plane (Engl. Out-of-plane )
    • τ ... torsional vibration with respect to an axis in the molecule ( twisting )
    • ϰ … wagging , sometimes also called pitching vibration

Overviews of symbols can be found in many standard works on molecular spectroscopy. However, it can be said that the assignment of oscillation and symbol has changed in some cases, so some authors of standard works such as Günzler, Nakamoto, Schrader use a different assignment and additional indices. For example, the symbols ω for wagging and ρ for rocking are used very often in current literature .

Individual evidence

  1. ^ Robert T. Conley: Infrared spectroscopy . Allyn and Bacon, 1972, p. 22 .
  2. ^ A b Johann Weidlein, Ulrich Müller, Kurt Dehnicke: Schwingungsspektoskopie: An introduction . 2., revised. Edition. Thieme, Stuttgart 1988, ISBN 3-13-625102-4 , p. 42 .
  3. ^ Arnold Eucken, KH Hellwege (ed.): Landolt - Börnstein: Numerical values ​​and functions from physics, chemistry, astronomy, geophysics and technology. Vol. 1: Atomic and Molecular Physics. Part 2: Molecules I (core structure). 6th edition, Springer, Berlin 1951, pp. 212-571
  4. ↑ Bernhard Schrader (Ed.): Infrared and raman spectroscopy: methods and applications . VCH, Weinheim u. a. 1995, ISBN 3-527-26446-9 , pp. 33, 71, 88 .
  5. ^ A b W. Lüttke, GAA Nonnenmacher: Reinhard Mecke (1895 - 1969): Scientific work and personality . In: Journal of Molecular Structure . tape 347 , 1995, pp. 1-17 , doi : 10.1016 / 0022-2860 (95) 08532-Z .
  6. ↑ R. Mecke: Experimental results for band spectroscopy of polyatomic molecules . In: Peter Josef William Debye (Ed.): Molecular structure (=  Leipzig lectures ). Verlag von S. Hirzel, Leipzig 1931, p. 23-49 .
  7. ^ R. Mecke: valence and deformation vibrations of simple molecules I. General theory . In: Journal for physical chemistry / Dept. B, chemistry of elementary processes, structure of matter . tape 16 , no. 6 , 1932, ISSN  0372-9664 , p. 409-420 ( limited preview in Google Book search).
  8. ↑ a b R. Mecke: The rotational vibration spectrum of acetylene. III . In: Journal of Physics . tape 64 , no. 3-4 , 1930, pp. 173-185 , doi : 10.1007 / BF01397589 .
  9. ↑ Reinhard Mecke, Hilmar Spiesecke: The infrared and Raman spectra of highly symmetrical molecules with a urotropin structure . In: Chemical Reports . tape 88 , no. 12 , 1955, pp. 1997-2002 , doi : 10.1002 / cber.19550881231 .
  10. ↑ a b cf. Hans-Ulrich Gremlich, Helmut Günzler: IR Spectroscopy: An Introduction . 4th edition. Wiley-VCH, 2003, ISBN 3-527-30801-6 , pp. 168 .
  11. ↑ K. Nakamoto (Ed.): Infrared and Raman spectra of inorganic and coordination compounds . 4th edition. Wiley, 1986, ISBN 0-471-01066-9 , pp. 2 .
  12. ↑ a b Bernhard Schrader (Ed.): Infrared and Raman Spectroscopy. Methods and Applications . VCH-Wiley, 1995, ISBN 3-527-26446-9 , pp. 33 .