Searching for the Exchange Shift: a Set of Test Systems

We suggest that intramolecular exchange coupling J in solutions can possibly be measured with just a ruler using an analogue of the nuclear magnetic resonance paramagnetic shift. If one of the two coupled electron spins is rapidly relaxing, the resulting electron paramagnetic resonance (EPR) spectrum of the system is the resolved spectrum for the second spin with a shifted g-factor. The amount and the sense of the shift are both linear in J, which thus can be obtained from the measured shift by simple algebra. We prepared a set of complexes of rare-earth ions with nitroxides to probe this idea, but the sought effect was not yet found indicating J < 0.1 cm(-1) for these systems. Either the radical centers should be brought closer to the central lanthanide ion, or the ion itself should be substituted for a rapidly relaxing transition-metal ion, e.g., Cr2+ or Co2+. Furthermore we report evolving EPR spectra that reflect the inner life of the complexes in solution.