Rotational correlation time is the time it takes for a molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 to rotate one radian

Radian

Radian is the ratio between the length of an arc and its radius. The radian is the standard unit of angular measure, used in many areas of mathematics. The unit was formerly a SI supplementary unit, but this category was abolished in 1995 and the radian is now considered a SI derived unit...

, on average. Rotational correlation times are in the order of picosecond

Picosecond

A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....

s, 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

Microviscosity

Microviscosity, also known as microscopic viscosity, is the friction experienced by a single particle undergoing diffusion because of its interaction with its environment at the micrometer length scale. The concept of microviscosity is intimately related to the concept of single particle diffusion...

 (viscosity at the molecular level) and in protein characterization.

Rotational correlation times may be measured by rotational (microwave)

Rotational spectroscopy

Rotational spectroscopy or microwave spectroscopy studies the absorption and emission of electromagnetic radiation by molecules associated with a corresponding change in the rotational quantum number of the molecule...

 or dielectric spectroscopy

Dielectric spectroscopy

Dielectric spectroscopy , and also known as electrochemical impedance spectroscopy, measures the dielectric properties of a medium as a function of frequency...

. Rotational correlation times of probe molecules in media have been measured by fluorescence

Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

 lifetime or for radical

Radical (chemistry)

Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...

s, from the linewidths of electron spin resonances.