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

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 s-type H-bonding leads to little spin density transfer on the water oxygen and small hf couplings, whereas p-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.

Dissecting the Molecular Origin of g-Tensor Heterogeneity and Strain in Nitroxide Radicals in Water: Electron Paramagnetic Resonance Experiment versus Theory

“…On the other hand, the experimental sample contained 10% (v/v) glycerol, which is absolutely necessary as a cryoprotectant to avoid clustering of HMI in the ice. We expect only minor effects on g - or A -, supported by the finding of Hecker et al that glycerol barely forms hydrogen bonds with nitroxides …”

“…The characterizing parameters of a nitroxide’s EPR spectrum, namely, the g - and A -tensors, are accessible at different levels of theory . While many studies are based on density functional theory (DFT), − the development of electron correlation methods gives access to more accurate predictions. Recent advances in local correlation theory, such as the DLPNO-CCSD method, − enable coupled cluster-level results of A -tensors for larger and more complex systems, for instance, molecular clusters of an organic radical surrounded by a fair number of solvent molecules, as we previously did for an HMI molecule in water .…”

17O hyperfine spectroscopy in surface chemistry and catalysis

“…7b) graphically shows the negative spin density (blue) in the oxygen hybrid orbitals aligned along the V-O bonds and the positive (cyan) spin density in the p orbitals perpendicular to the bond. This is opposite to water coordination to nitroxide spin labels, where a negative a iso (positive spin density at the oxygen) is observed because of a direct spin density transfer via overlap of the SOMO with the H atom of a hydrogen bonded water molecule [56].…”

Hydrogen sharing between two nitroxyl radicals in the gas phase and other microsolvation effects on the infrared spectrum of a bulky hydroxylamine

“…8 It again appears desirable to extend the study of such neutral XHX species to organic radicals X, where conformational isomerism is expected to influence the energetics and dynamics of the shared hydrogen atom. 9 A natural choice for X is the persistent, sterically hindered TEMPO radical 10 (T) or some of its derivatives, 11 which have shown to be relevant in many fields of molecular science such as controlled radical polymerisation, 12 spin labeling, 13 and redox reactions. 14 T can accept an OH• • • O hydrogen bond from its associated hydroxylamine TEMPO-H (TH).…”

“…A natural choice for X is the persistent, sterically hindered TEMPO radical 10 (T) or some of its derivatives, 11 which have shown to be relevant in many fields of molecular science such as controlled radical polymerisation, 12 spin labeling, 13 and redox reactions. 14 T can accept an OH⋯O hydrogen bond from its associated hydroxylamine TEMPO-H (TH).…”