A 13C-labeled triarylmethyl radical as EPR spin probe highly sensitive to molecular tumbling.

A stable triarylmethyl spin probe whose Electron Paramagnetic Resonance (EPR) spectrum is highly sensitive to molecular tumbling is reported. The strong anisotropy of the hyperfine coupling tensor with the central carbon of a 13 C1-labeled triarylmethyl radical enables the measurement of the probe rotational correlation time with applications to measure microviscosity and molecular dynamics.