Electron Paramagnetic Resonance

The same unpaired or odd electron that renders most radical intermediates unstable and highly reactive may be induced to leave a characteristic "calling card" by a magnetic resonance phenomenon called "electron spin resonance" (esr) or "electron paramagnetic resonance" (epr). Just as a proton (spin = 1/2) will occupy one of two energy states in a strong external magnetic field, giving rise to nmr spectroscopy; an electron (spin = 1/2) may also assume two energy states in an external field. Because the magnetic moment of an electron is roughly a thousand times larger than that of a proton, the energy difference between the spin states falls in the microwave region of the spectrum (assuming a moderate magnetic field strength). The lifetime of electron spin states is much shorter than nuclear spin states, so esr absorptions are much broader than nmr signals. One way of improving the signal to noise ratio in esr spectra is to display them as first derivatives rather than absorptions. These displays are illustrated on the left below. In practice, esr spectra may be quite complex, as shown by the derivative spectrum of triphenylmethyl radical on the right. This complexity is the result of hyperfine splitting of the resonance signal by protons and other nuclear spins, an interaction similar to spin-spin splitting in nmr spectroscopy. For example, the esr signal from methyl radicals, generated by x-radiation of solid methyl iodide at -200º C, is a 1:3:3:1 quartet (predicted by the n + 1 rule). The magnitude of signal splitting is much larger than nmr coupling constants (MHz rather than Hz), and is usually reported in units of gauss. The complexity of the triphenylmethyl spectrum is due to three different hyperfine splittings: 3 para hydrogens, 6 ortho hydrogens & 6 meta hydrogens. Ideally this should produce 196 lines, but imperfect resolution reduces the number observed.