Simple Test of the Effect of an Electric Field on the ¹⁴N-Hyperfine Coupling Constant in Nitroxide Spin Probes

The sensitivity of the 14 N-hyperfine splitting of nitroxide spin to the local electric field, E loc, has often been used to probe the polarity of microenvironments; however, two uncertainties have been faced at once as follows: (i) the value of E loc in a complex microenvironment and (ii) the effect of E loc on the hyperfine splitting. In this paper, we study nitroxide free radicals that may be charged (positively or negatively) by varying the pH. The additional charge produces an additional E loc, whose value we may calculate with confidence, allowing us to focus attention on the effect that E loc has on the hyperfine splitting. A simple theory is developed that provides a link between the shift in the 14 N-hyperfine splitting, Δ A N, and the local electric field, E loc . To test this approach, we have calculated the Δ A N for several charged nitroxide spin probes and found good agreement (both magnitude and sign) with the measured values using Δ A N = −3.8 × 10 -8 E loc (Δ A N in Gauss and E loc in V/cm), where E loc is the component of E loc directed along the N−O bond from the nitrogen to the oxygen.