IUPAC > Gold Book > alphabetical index > R > rotational correlation time, τ c or θ


rotational correlation time, τ c or θ

Parameter describing the time dependence of the tumbling of a molecular entity in a medium of viscosity η. The rotational correlation time can be obtained from the decay of the fluorescence or phosphorescence anisotropy and is related to the average molecular rotational diffusion coefficient, D r, in turn related to the hydrodynamic molecular volume of the fluorophore, V, and to η (see Note 3).
  1. Mathematical definition: r t = r 0 exp ( − t τ c ) with r t the emission anisotropy at time t and r 0 the fundamental emission anisotropy.
  2. In the case of a spherical emitting species reorienting itself in a homogeneous fluid, τ c = 1 6 D r.
  3. Often, the StokesEinstein relationship is used for the calculation of D r, i.e., D r = R T / 6 V η with R the gas constant, T the absolute temperature and V the hydrodynamic molecular volume. However, the use of this relationship at a molecular level is questionable, and D r should be independently determined by time-resolved fluorescence polarization methods. Compare with rotational relaxation time.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 416
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IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook.
Last update: 2014-02-24; version: 2.3.3.
DOI of this term: doi:10.1351/goldbook.RT07474.
Original PDF version: http://www.iupac.org/goldbook/RT07474.pdf. The PDF version is out of date and is provided for reference purposes only. For some entries, the PDF version may be unavailable.
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