Abstract
The hydration structure of nitroxide radicals in aqueous solutions is elucidated by advanced (17) O hyperfine (hf) spectroscopy with support of quantum chemical calculations and MD simulations. A piperidine and a pyrrolidine-based nitroxide radical are compared and show clear differences in the preferred directionality of H-bond formation. We demonstrate that these scenarios are best represented in (17) O hf spectra, where in-plane coordination over σ -type H-bonding leads to little spin density transfer on the water oxygen and small hf couplings, whereas π -type perpendicular coordination generates much larger hf couplings. Quantitative analysis of the spectra based on MD simulations and DFT predicted hf parameters is consistent with a distribution of close solvating water molecules, in which directionality is influenced by subtle steric effects of the ring and the methyl group substituents.