Preparation and Spectroscopic, Magnetic, and Electrochemical Studies of Mono-/Biradical TEMPO Derivatives

Abstract

A comparison set of mono-/biradical TEMPO derivatives was prepared, novel compounds were fully characterized, and their physicochemical properties were determined. Cyclic voltammetry revealed reversible redox behavior for all studied nitroxides. Moreover, the electron-withdrawing substituents increased the oxidation potential of the respective nitroxides in comparison to electron-donating groups. While EPR spectra of monoradicals in dichloromethane at 295 K reveal the expected three-line signal, spectra of biradicals show more complex features. DFT and MP2 calculations indicate that the EPR splitting pattern of dinitroxide 7 could be explained by its interactions with solvent molecules. In the solid state, mononitroxides 4 and 5 behave as a Heisenberg antiferromagnetic chain, whereas dinitroxides 6-8 are almost isolated paramagnetic diradicals coupled in an antiferromagnetic manner.

Synthesis, physicochemical properties and antimicrobial activity of mono-/dinitroxyl amides

“…Crystallisation of a small amount of 4 from chloroform-hexanes yielded orange crystals suitable for X-ray analysis (Fig. 5 4-tert-Butoxycarbonylamino-2,2,6,6-tetramethylpiperidin-1-oxide (5). Prepared according to the described procedure.…”

“…5 In general, nitroxides are readily electrochemically oxidised to the corresponding N-oxoammonium cations. 22 Cyclic voltammograms of 4-13 ( Fig.…”

“…However, the specific interactions of the NO • group with solvent molecules may cause small asymmetry (g (1) ≠ g (2) and A (1) N ≠ A (2) N ) which is also the case for symmetric dinitroxide molecules. 5,30 In general, the EPR spectra of nitroxide biradicals are substantially influenced by the value of the exchange coupling constant. 30 Two limiting situations may be considered here: J = 0 where each nitroxide moiety separately contributes to the EPR signal giving a three-line spectrum (intensity ratio 1 : 1 : 1), and J ≫ A N representing the interacting electrons with a five-line EPR spectrum (intensity ratio 1 : 2 : 3 : 2 : 1).…”

“…3, Table 3); the asymmetry found for the A N couplings is most probably caused by the interaction of nitroxide moieties with polar solvent molecules (DCM); an associated phenomenon was generally observed earlier by others 30 and confirmed recently by our DFT and/or MP2 calculations. 5 On the other hand, in order to fit with the experiment, the simulated EPR spectrum of 11 ( Fig. 4) was constructed as a superposition of two different signals corresponding to interacting and non-interacting radicals 31 within the molecule of the respective dinitroxide.…”

Voltammetric determination of harmaline in natural food products using boron-doped diamond electrode

“…In addition, they provide information about the kinetics and charge transfer mechanism of the electrode reaction [22]. Regarding the electrochemistry of HME, however, there is only one study available on its voltammetric characterization and amperometric determination using HPLC with a carbon nanotube (CNT)-modified electrode as electrochemical detector [10,11] although this molecule is electrooxidisable at several conventional electrodes.…”