Analytical Instrument Documents

Application note ER260004E
Product used: Electron Spin Resonance (ESR)

ESR signal of nitroxide radical in solution

The linewidth of ESR signals from radicals in solutions is influenced by the molecular mobility of the radical species. In addition, hyperfine structure is observed because of interactions between the unpaired electron and surrounding nuclei. Three types of nitroxide radicals were dissolved in ultra-pure water and toluene at a concentration of 1.0 x 10^-4 mol/L, and ESR measurements were performed under identical experimental conditions. Comparison of the ESR signals revealed that, in ultra-pure water, the g-values are smaller, the A-values are larger, and the linewidths are narrower (Table 1 and Figure 1). This behavior is attributed to the lower viscosity and reduced intermolecular collisions in ultra-pure water compared with toluene, which allow solute molecules to undergo rapid rotation and diffusion. Nitroxide radicals exhibit anisotropic g-values and hyperfine interactions depending on their molecular orientation relative to the magnetic field. However, in aqueous solutions, molecular motion is sufficiently fast on the ESR timescale to result in effective averaging of this orientational anisotropy. Consequently, motional narrowing occurs, as all molecules experience an equivalent magnetic field, leading to sharp ESR signals.