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A different type of MnO

This article presents the results of a study of electrochemical transformations in aqueous and aprotic media of 5-methyl-6-nitro-7-oxo-4,7-dihydro-1,2,4-triazolo[1,5-a]pyrimidinide l-arginine monohydrate (1a, Triazid) obtained by electrochemical methods and ESR spectroscopy. The effect of pH on the current and the reduction potential of 1a in an aqueous Britton–Robinson buffer solution was studied. It was found that 1a is irreversibly reduced in aqueous acidic media on a glassy carbon electrode in one stage with the participation of six electrons and the formation of 5-methyl-6-amino-7-oxo-1,2,4-triazolo[1,5-a]pyrimidin. The electroreduction of 1a in DMF on a background of tetrabutylammonium salts proceeds in two stages, controlled by the kinetics of second-order reactions. In the first stage, the reduction of 1a is accompanied by protonation by the initial compound of the basic intermediate products formed in the electrode reaction (self-protonation mechanism). The second quasi-reversible stage of the electroreduction 1a corresponds to the formation of a dianion radical upon the reduction of the heterocyclic anion 5-methyl-6-nitro-7-oxo-4,7-dihydro-1,2,4-triazolo[1,5-a]pyrimidin, which is formed upon the potentials of the first peak. The ESR spectrum of the radical dianion was recorded upon electroreduction of Triazid in the presence of Bu4NOH. The effect of the formation of ion pairs on the reversibility of the second peak of the 1a transformation is shown. A change in the rate and regioselectivity of the protonation of the dianion radical in the presence of Na+ and Li+ ions is assumed. The results of studying the electroreduction of 1a by ESR spectroscopy with a TEMPO trap make it possible to assume the simultaneous formation of both a nitroxyl radical and a radical with the spin density localized on the nitrogen at the 4 position of the six-membered ring. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Funding: This work was supported by the Russian Foundation for Basic Research (RFBR, project No. 19-29-08015 mk).

Experimental facts on the mechanism of the direct oxidative photocatalytic C H/C H coupling of azines with (hetero)aromatic nucleophiles, proceeding in the presence of TiO2 as a photocatalyst, have been obtained by using EPR spectroscopy and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), N-(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl)-2-methylpropanamide (TMT-H), as spin trap reagent and spin probe, respectively. The formation of the superoxide radical anion (O2 −) and N-centered radicals has been detected for the first time for the SNH transformations.

Chemical and physical processes occurring within the nanochannels of mesoporous materials are known to be determined by both the chemical nature of the solution inside the pores/channels as well as the channel surface properties, including surface electrostatic potential. Such properties are important for numerous practical applications such as heterogeneous catalysis and chemical adsorption including chromatography. However, for solute molecules diffusing inside the pores, the surface potential is expected to be effectively screened by counter ions for the distances exceeding the Debye length. Here, we employed electron paramagnetic resonance spectroscopy of ionizable nitroxide spin probes to experimentally examine the conditions for the efficient electrostatic surface potential screening inside the nanochannels of chemically similar silica-based mesoporous molecular sieves (MMS) filled with water at ambient conditions and a moderate ionic strength of 0.1 M. Three silica MMS having average channel diameters of D = 2.3, 3.2, and 8.1 nm (C 12 MCM-41, C 16 MCM-14, and SBA-15, respectively) were chosen to investigate effects of the channel diameter at the nanoscale. The results are compared with the classical Poisson-Boltzmann (PB) double layer theory developed for diluted electrolytes and applied to a cylindrical capillary of infinite extent. While the surface electrostatic potential was effectively screened by the counter ions inside the largest channels of 8.1 nm in diameter (SBA-15), the effect of the surface electrostatic potential on local effective pH was significant for the 3.2 nm channels (C 16 MCM-14). The smaller channels of C 12 MCM-41 (2.3 nm in diameter) provided the most critical test for the PB equation that is based on a continuum electrostatic model and demonstrated its inapplicability likely due to the discrete nature of molecular systems at the nanoscale and nanoconfinement effects, leading to larger spatial heterogeneity. © 2018 American Chemical Society. *E-mail: gek1969@bk.ru (E.G.K.). *E-mail: Alex_Smirnov@ncsu.edu (A.I.S.). ORCID Alex I. Smirnov: 0000-0002-0037-2555 Author Contributions The manuscript was written through contributions from all the authors. All authors have given approval to the final version of the manuscript. Funding E.G.K., L.S.M., D.O.A., D.P.T., and A.N.T. acknowledge the financial support of the Program 211 of the Government of the Russian Federation no. 02.A03.21.0006, RFBR grants 17-03-00641 and 18-29-12129mk, and the State Task from the Ministry of the Education and Science of the Russian Federation nos. 4.9514.2017/8.9 and 4.7772.2017/8.9. A.M. and A.I.S. acknowledge the financial support of the U.S. DOE Contract DE-FG02-02ER15354 (modeling of electrostatic phenomena in the nanochannels and the final preparation of the manuscript). Notes The authors declare no competing financial interest.