Molecular rotation
As molecular rotation , the rotation of is molecules to their center of gravity understood.
These rotations can occur in all molecules, but they have different effects on linear and non-linear molecules. They can be stimulated by the supply of energy , for example by heat . Molecular rotations are important as a model in infrared spectroscopy and microwave spectroscopy .
Influence on material properties
Molecular rotations, together with molecular vibrations and molecular translation, form the degrees of freedom of a molecule, which have an influence on the thermodynamic quantities of a substance. Molecular rotation, for example, determines the internal energy , entropy and heat capacity of a substance. These properties can also be calculated using statistical thermodynamics . The entropy of rotation can be determined, among other things, from the sum of the state of rotation .
Stimulation by heat
According to the Boltzmann statistics , more rotational states are excited at higher temperatures than at lower temperatures. At low temperatures, the rotations of molecules (or part of them) can also be frozen. This could be made directly visible by means of scanning tunneling microscopy , whereby the stabilization at higher temperatures could be explained by the entropy of rotation.
Direct observation
For a long time, molecular rotations could not be observed directly. Only measurement techniques with atomic resolution made it possible to detect the rotation of a single molecule. For example, the rotations of a hexa- (tert-butyl) decacyclene molecule adsorbed on a Cu (100) surface could be observed approximately directly in local space using scanning tunneling microscopy. The rotating molecule could only be depicted out of focus because it was not in phase due to the rotation.
See also
General textbooks
- PW Atkins : Physical Chemistry . Wiley-VCH, 2006, ISBN 978-3-527-31546-8 .
- G. Wedler : Textbook of Physical Chemistry . Wiley-VCH, 2004, ISBN 3-527-31066-5 .
- T. Engel, P. Reid: Physical chemistry . Pearson Studies, 2006, ISBN 978-3-8273-7200-0
Individual evidence
- ↑ Thomas Waldmann, Jens Klein, Harry E. Hoster, R. Jürgen Behm: Stabilization of Large Adsorbates by Rotational Entropy: A Time-Resolved Variable-Temperature STM Study . In: ChemPhysChem . 2012, p. n / a – n / a , doi : 10.1002 / cphc.201200531 .
- ↑ JK Gimzewski, C. Joachim, RR sledders, V. Langlais, H. Tang, I. Johannsen: Rotation of a Single Molecule Within a Supramolecular Bearing . In: Science . tape 281 , no. 5376 , July 24, 1998, p. 531-533 , doi : 10.1126 / science.281.5376.531 .