Multifrequency electron paramagnetic resonance of ultramarine blue

Electron paramagnetic resonance spectra of the S-3(-) radical center in ultramarine blue over a factor of about 2500 in frequency (258 MHz to 670 GHz) reveal a substantially Lorenizian shape, without resolution of g anisotropy. Variable temperature measurements found that the line width is independent of temperature, within experimental uncertainty, up to about 90 K at 9.5 GHz and between ca. 5 K and room temperature at 95 and 217 GHz, as expected for an exchange-narrowed signal. Analysis of the increase in the low-temperature line width as a function of frequency above 9 GHz is consistent with an exchange interaction of about 2. 10(-2) K. The line width increases as frequency is decreased from 2.7 GHz to 258 MHz which is attributed to the contribution from nonsecular terms that has been denoted the "10/3" effect.