Preparation of Some Homologous TEMPO Nitroxides and Oxoammonium Salts; Notes on the NMR Spectroscopy of Nitroxide Free Radicals; Observed Radical Nature of Oxoammonium Salt Solutions Containing Trace Amounts of Corresponding Nitroxides in an Equilibrium Relationship

Abstract

Three new homologous TEMPO oxoammonium salts and three homologous nitroxide radicals have been prepared and characterized. The oxidation properties of the salts have been explored. The direct C NMR and EPR spectra of the nitroxide free radicals and the oxoammonium salts, along with TEMPO and its oxoammonium salt, have been successfully measured with little peak broadening of the NMR signals. In the spectra of all ten compounds (nitroxides and corresponding oxoammonium salts), the carbons in the 2,2,6,6-tetramethylpiperidine core do not appear, implying paramagnetic properties. This unpredicted overall paramagnetism in the oxoammonium salt solutions is explained by a redox equilibrium as shown between oxoammonium salts and trace amounts of corresponding nitroxide. This equilibrium is confirmed by electron interchange reactions between nitroxides with an N-acetyl substituent and oxoammonium salts with longer acyl side chains.

Evidence for a Proton‐Coupled Electron Transfer Mechanism in a Biomimetic System for Monoamine Oxidase B Catalysis

“…EPR spectrum of the reaction of MMTP and Fl + under anaerobic conditions. The spectrumm atches that reported for FlC in reference[17].…”

“…Gould and Melville dubbed the term" NMR silent" [16] to describe situations wherein diamagneticc ompounds do not exhibit NMR signals (under any conditions) because they are in rapid equilibria with radicals. [16,17] The rapid equilibrium between diamagnetic( MMTP-Fl) and paramagnetic (MMTPC and FlC)b roadens the NMR lines associated with MMTP-Fl, to the point that (when the equilibrium is rapid), no signals are observed. Because no signals are observed for MMTP-Fl,a tt his point,w ec an only speculate on its possible structure(s).…”

Efficient Building Blocks for Solid‐Phase Peptide Synthesis of Spin Labeled Peptides for Electron Paramagnetic Resonance and Dynamic Nuclear Polarization Applications

“…Furthermore, the comparison shows that all signals are preserved during the coupling step. Some signals are shifted which implies a proximity of these nuclei to the radical in the sample . This particularly applies to hydrogens close to the spin label such as the α (β, γ, δ) groups of the amino acids.…”

A novel profluorescent paramagnetic diaza-crown ether: synthesis, characterization and alkaline metal-ion complexation

“…Recently, 19 TEMPO-derived compounds were investigated by 1 H and 13 C NMR experiments, indicating that carbon spectra can be very informative for the atoms located far enough from the free radical site. This was also conrmed in our case (see ESI † for full spectra and Fig.…”