Rotational correlation time
Rotational correlation time is the time it takes for a molecule to rotate one radian, on average. Rotational correlation times are in the order of picoseconds, for example 1.7 picoseconds for water or in the tens to hundreds range for small organic molecules. Rotational correlation times are employed in the measurement of microviscosity (viscosity at the molecular level) and in protein characterization.
Rotational correlation times may be measured by rotational (microwave) or dielectric spectroscopy. Rotational correlation times of probe molecules in media have been measured by fluorescence lifetime or for radicals, from the linewidths of electron spin resonances.