17O Hyperfine Spectroscopy Reveals Hydration Structure of Nitroxide Radicals in Aqueous Solutions

The hydration structure of nitroxide radicals in aqueous solutions is elucidated by advanced O-17 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 O-17 hf spectra, where in-plane coordination over sigma ${\sigma }$ -type H-bonding leads to little spin density transfer on the water oxygen and small hf couplings, whereas pi ${{\rm \pi }}$ -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.

Automated summary

The hydration structure of nitroxide radicals in aqueous solutions is studied using advanced O-17 hyperfine (hf) spectroscopy, quantum chemical calculations, and MD simulations. Two different nitroxide radicals with piperidine and pyrrolidine-based groups are compared, revealing distinct differences in the preferred direction of H-bond formation. The O-17 hf spectra provide the best representation of these scenarios, showing that in-plane coordination leads to little spin density transfer and small hf couplings, while perpendicular coordination generates larger hf couplings. The quantitative analysis based on MD simulations and DFT predicted hf parameters confirms a distribution of solvating water molecules, where directionality is influenced by steric effects.