Force constant

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As a force constant of is molecular spectroscopy a spring constant on molecular designated level. It serves u. a. as a mechanical model to describe a stretching vibration in a diatomic molecule.

description

A useful model for describing the normal vibrations of a molecule can be found if the atoms of the molecule are viewed as mass points and the bond between them as springs . Then the force required for a certain deflection of the atoms from their equilibrium position results in analogy to Hooke's law as the product of the deflection and the spring constant.

In general, there are numerous possibilities for the displacement of the atoms in a polyatomic molecule, which are described by internal coordinates . Every inner coordinate of a molecule is linked to a force constant:

  • Force constants that relate to changes in bond lengths refer to, valence force constants called
  • Force constants that involve changes in bond angles are called deformation force constants .

The valence force constants are particularly interesting from the point of view of bond theory, as they allow a statement to be made about the strength of bond or the degree of bond.

For a complete normal coordinate analysis, the introduction of interaction force constants is also necessary, since the deflection of a certain bond described by an inner coordinate also changes the bond conditions and thus the force constants of the neighboring bonds (slightly). A set of complete force constants of a molecule including all interaction force constants is called a force field .

disadvantage

The dependence of the force constant on the coordinates limits its usefulness in describing the strength of the chemical bond. Instead, the use of compliance constants ( reciprocal value of the spring constant) has become commonplace in recent years , as these guarantee significantly better transferability.

Individual evidence

  1. ^ Georg Wedler: Textbook of Physical Chemistry . 4th edition. Wiley-VCH, 1997, ISBN 3-527-29481-3 , pp. 614 .
  2. ^ Peter Atkins, Julio de Paula: Physical chemistry . 4th edition. Wiley-VCH, 2006, ISBN 3-527-31546-2 , pp. 333 .
  3. ↑ Douglas A. Skoog, James J. Leary: Instrumental Analytics: Fundamentals, Devices, Applications . Springer Verlag, 1996, ISBN 3-540-60450-2 , pp. 277 ( excerpt from Google book search).
  4. ^ Kai Brandhorst, Jörg Grunenberg: How strong is it? The interpretation of force and compliance constants as bond strength descriptors . In: Chemical Society Reviews . tape 37 , no. 8 , July 22, 2008, p. 1558-1567 , doi : 10.1039 / B717781J .
  5. ↑ Sason Shaik, David Danovich, Wei Wu, Peifeng Su, Henry S. Rzepa, Philippe C. Hiberty: Quadruple bonding in C2 and analogous eight-valence electron species . In: Nature Chemistry . tape 4 , no. 3 , March 2012, p. 195-200 , doi : 10.1038 / nchem . 1263 .
  6. ↑ Peter R. Schreiner, Hans Peter Reisenauer, Jaroslaw Romanski, Grzegorz Mloston: A Formal Carbon Sulfur Triple Bond: H-C ≡ S-O-H . In: Angewandte Chemie . tape 121 , no. 43 , 2009, p. 8277-8280 , doi : 10.1002 / anie.200903969 .

literature

  • Entry on force constants. In: Römpp Online . Georg Thieme Verlag, accessed on November 12, 2014.
  • Kai Brandhorst, Jörg Grunenberg. Characterizing Chemical Bond Strengths Using Generalized Compliance Constants. ChemPhysChem, 8, 1151-1156, 2007.