A molecule which contains an unpaired electron spin which can be detected with electron spin resonance (ESR) spectroscopy. Molecules are labelled when an atom or group of atoms which exhibit some unique physical property is chemically bonded to a molecule of interest. Groups containing unpaired electrons include organic free radicals and a variety of types of transition-metal complexes (such as vanadium, copper, iron, and manganese). Molecules with unpaired electron spins are readily detected with electron spin resonance spectroscopy. Through analysis of ESR spectra, rates of molecular motion whose motion is restrained by surrounding molecules can be determined. Measurement of rates of molecular motion and molecular orientation has proved to be very important in the study of a variety of types of biologically important problems.