Figure 1.

Energy levels of a radical containing different numbers of hyperfine-coupled spin-1/2 nuclei. (a) In the absence of hyperfine, exchange, and dipolar interactions, the electron experiences only the external magnetic field, resulting in a Zeeman splitting equal to the Larmor frequency, ${\nu }_{\mathrm{L}}$. (b) A small number of hyperfine interactions results in a variety of energy-level spacings. (c) A large number of hyperfine interactions gives rise to many energy levels, a situation representative of the majority of organic radicals. Only in case (a) would a radiofrequency field at the Larmor frequency be expected to have a strong effect on the spin dynamics. To see this figure in color, go online.