Many factors contribute to the observed lineshape and linewidth of an ESR resonance. One important factor is how quickly the "spins" relax back to equilibrium. In ESR, as in NMR, most systems lose the energy they gained thermally: interactions, such as collisions, with their surroundings and each other. The faster the system relaxes back, the broader the lines.
Magnetic field inhomogeneity can cause both line broadening and lineshape distortion. The presence of a paramagnetic species like dissolved O2 and phenomena such as spin-spin exchange can also broaden lines because they provide another effective relaxation pathway for electrons. The shape of ESR lines also gives important information about molecules and their environment. In a non-viscous liquid, small molecules such as TEMPOL can tumble freely. As the solution becomes more viscous, the molecular motion becomes hindered. The ESR spectrum of the nitroxide changes as the rotational motion becomes restricted. ESR can be used to measure rotational correlation times. The technique referred to as spin labeling is based on this phenomenon.
This laboratory exercise looks at line broadening through spin-spin exchange and students calculate the collision frequency at different solution concentrations based on the ESR linewidth. The effect of dissolved oxygen on linewidths is also discussed. The second part of this lab looks at the effect of increasing viscosity on TEMPOL lineshapes.