# Rotational correlation time

Rotational correlation time (${\displaystyle \tau _{c}}$) is the average time it takes for a molecule to rotate one radian.[citation needed] In solution, rotational correlation times are in the order of picoseconds. For example, the ${\displaystyle \tau _{c}=}$ 1.7 ps for water,[1] and 100 ps for a pyrroline nitroxyl radical in a DMSO-water mixture.[2] Rotational correlation times are employed in the measurement of microviscosity (viscosity at the molecular level) and in protein characterization.