Electrochemical and ESR studies of the methane C-H bond activation in the presence of radical cations of pyrazine-di-N-oxide and its substituted derivatives

Activation of the methane C-H bond in the presence of electrochemically generated radical cations of pyrazine-di-N-oxide and also of 2,5-dimethyl- and 2,3,5,6-tetramethyl-pyrazine-di-N-oxides is studied by methods of cyclic voltammetry (CVA), quantum chemical simulations, and ESR electrolysis. The studies are carried out on glassy carbon (GC) and Pt electrodes in 0.1 M LiClO₄ solutions in acetonitrile. ESR spectra of radical cations of aromatic di-N-oxides in the absence and in the presence of methane are recorded. The changes in the shape CVA curves and the intensity of ESR signals of di-N-oxide radical cations observed in the presence of methane point to the activation of the methane C-H bond followed by its oxidation. The reaction of pyrazinedi-N-oxide at the methane C-H bond is simulated by quantum chemical methods. The obtained results are explained within the framework of the mechanism of overall two-electron oxidation of methane within its complex with an aromatic di-N-oxide radical cation.     

Infrared studies of acetophenone and its deuterated derivatives

The i.r. spectra of acetophenone and their deuterated analogues (-d₃, -d₅, -d₈) in the liquid-phase have been recorded and analyzed in the range 4000-130 cm⁻¹. Additional data on band contours in the gas-phase, in conjunction with the deuteration effects, allowed us to assign all the fundamentals for the four isotopic varieties. A valence force field calculation was also used to support the proposed assignment.     

Electrochemical and ESR studies of <Emphasis Type="Italic">tert</Emphasis>-butanol oxidation mechanism in the presence of radical cations pyrazine-di-N-oxide and its substituted derivatives as mediators

The methods of cyclic voltammetry, ESR electrolysis, and quantum chemical simulation were used to study the tert-butanol (tert-BuOH) oxidation mechanism in the presence of mediator cation radicals of pyrazine-di-N-oxide, 2,5-di-Me- and 2,3,5,6-tetra-Me-pyrazine-di-N-oxdides. This study was carried out on carbon glass (CG) and Pt electrodes in 0.1 M LiClO₄ solution in acetonitrile and on Au electrode in tert-butanol containing 0.05 M LiClO₄. The ESR spectra of cation and anion radicals of aromatic di-N-oxides were recorded in tert-BuOH. The quantum chemical simulation of the reaction between pyrazine-di-N-oxide radical cation and C-H bond in tert-BuOH was performed. The results were explained in the terms of the general two-electron oxidation mechanism of tert-BuOH in the complex with aromatic di-N-oxide cation radical as mediator.     

In silico studies of the mechanism of methanol oxidation by quinoprotein methanol dehydrogenase

The mechanism of bacterial methanol dehydrogenase involves hydride equivalent transfer from substrate to the ortho-quinone PQQ to provide a C5-reduced intermediate that subsequently rearranges to the hydroquinone PQQH(2). We have studied the PQQ reduction by molecular dynamic (MD) simulations in aqueous solution. Among the five simulated structures, either Asp297 or Glu171 or both are ionized. Reasonable structures are obtained only when both carboxyl groups are ionized. This is not unexpected since the kinetic pH optimum is 9.0. In the structure of the enzyme.PQQ.HOCH(3) complex, the hydrogen bonded Glu171-CO(2)(-).H-OCH(3) is in a position to act as a general base catalyst for hydride equivalent transfer to C5 of PQQ. We thus suggest that Glu171 plays the role of general base catalyst in PQQ reduction rather than Asp297 as previously suggested. The reduction is assisted by Arg324, which hydrogen bonds to the ortho-quinone moiety of PQQ. The rearrangement of the C5-reduced intermediate to provide hydroquinone PQQH(2) is also assisted by proton abstraction by Glu171-CO(2)(-) and the continuous hydrogen bonding of Arg324 throughout the entire reaction. These features as well as the mapping of the channel for substrate and water into the active site entrance are the observations of major importance.     

Comparative studies of oxidation kinetics of benzidine and its derivatives in the presence of various hemoproteines

Kinetic parameters for the oxidation of benzidine, 3,3-dimethoxybenzidine, 3,3-dimethylbenzidine and 3,3,5,5-tetramethylbenzidine by horse-radish peroxidase and hemoglobin in the presence of hydrogen peroxide have been measured in citrate-acetate buffer at 30°C and pH=5.5. The mechanism of peroxidase oxidation of several benzidine derivatives by hemoproteines is discussed with respect to the number of substituents.

---

30°C - 5,5 , 3,3-, 3,3- 3,3,5,5-- . .

     

Electrochemical and ESR study of the mechanism of oxidation of phenazine-di-N-oxide in the presence of cyclohexanol on glassy carbon and single-walled carbon nanotube electrodes

The mechanism of oxidation of phenazine-di-N-oxide in the presence of cyclohexanol was studied by cyclic voltammetry on glassy carbon (GC) and single-walled carbon nanotube (SWCNT) electrodes in 0.1 M LiClO₄ solutions in acetonitrile. The effect of cyclohexanol on the shape of the cyclic voltammograms of phenazine-di-N-oxide and the intensity of the ESR signal of its radical cation was investigated. It was shown by ESR that the products of the one-electron oxidation and reduction of phenazine-di-N-oxide were radical cations and anions. The catalytic currents were recorded during the oxidation of phenazine-di-N-oxide on the SWCNT and GC electrodes in the presence of cyclohexanol. The results were explained in terms of the E₁C₁E₂C₂ mechanism of the two-stage electrode process characterized by the catalytic current recorded at the second electrode stage. The overall two-electron catalytic oxidation of cyclohexanol in the complex with the phenazine-di-N-oxide radical cation was assumed to occur. It was shown that SWCNT electrodes can be used in the electrocatalytic oxidation of organic compounds in the presence of the electrochemically generated phenazine-di-N-oxide radical cation.     

Electrochemical oxidation of some catechol derivatives in the presence of some betadicetone derivatives: mechanistic and thermodynamic study

Electrochemical oxidations of 4-methylcatechol (1), 4-tert-butylcatechol (5) and catechol (7) in the presence of different nucleophiles have been investigated both experimentally and theoretically. Experimental results have been obtained by means of cyclic voltammetry and controlled-potential coulometry. Also the theoretical results were calculated at DFT (B3LYP) level of theory and 6–311+G (p, d) basis set. The calculated results indicate that oxidation potential of catechols (1, 5, 7) and their substituted species are directly dependent on the ?G _tot, and continuance of reactions during electrolysis is dependent on ?G _tot of produced species on the surface of electrode. The current study indicates that theoretical studies along with empirical research can be useful in displaying electrochemical reaction mechanisms.     

Electrochemical oxidation of alkyl-substituted allenes in methanol

Mono-, di- and tri-alkyl-substituted allenes were potentiostatically oxidized in methanol at graphite and Pt anodes. At the former electrode, α-methoxylated ketones (due to 4F/mole electricity consumption) and esters (6F/mole) were the major products. At a Pt anode, intermediate products such as vinyl-ether derivatives (2F/mole) were characterised too. Unlike the anodic oxidation of alkenes and alkynes previously reproted in the literature, dimerisation is not a typical process in the allenes' oxidation, since of all the products obtained only a sole dimer has been observed. The mechanism for the formation of most products is discussed.     

Spectroscopic studies of poly(vinyl chloride) and its deuterated derivatives

Poly(vinyl chloride) (PVC) and all its deuterated derivatives were prepared by variation of the polymerization method and polymerization temperature to study the structure of PVC and the mechanism of addition polymerization by infrared spectroscopy and high resolution NMR spectroscopy. The CH and CH₂ stretching modes of PVC were assigned completely from the infrared spectra of PVC-αd₁, PVC-β,βd₂ and PVC-d₃. The frequencies of the CCI stretching modes of the polymers depended not only on the substituents in the trans position to the Cl atom across both adjacent C? C bonds, but also on the atom attached to the C atom of the C? Cl bond. The frequency shifts were used to assign the CCl stretching modes of PVC-βd₁ and PVC-α,βd₂ and to study the opening of the double bond of VC in the addition polymerization. The differences of the chemical shifts of the α and β protons of PVC due to the tacticity were determined experimentally by PVC-β,βd₂ and PVC-αd₁ without using the spin-decoupling technique. With PVC-α,βd₂, the conception of the tetrad was required to interpret the four observed peaks whose intensities changed with the polymerization temperature and the trans–cis composition of the monomer used.     

Electrochemical and ESR studies on the reduction of some indolinone derivatives and of their N-oxides in DMF

2-Phenyl-3-oxo-3H-indole 4 and 3-imino-, 3-iminoxy-, derivatives, the corresponding N-oxide 3, and 2-phenyl- 5 and 2-(2'-pyridyl-) isatogen 6 have been reduced in DMF and in DMF in presence of proton donors, and followed by polarography, controlled potential coulometry, cyclic voltammetry, UV, and ESR. In aprotic media, compounds 1, 4, 5 and 6, are reduced via anion radical intermediates.     

Electrochemical studies of the oxidation mechanism of polyamide on glassy carbon electrode

Polyamides containing N-methyl pyrroles and N-methyl imidazoles are a type of small molecule that can bind and recognize the bases of DNA with high affinity and specificity. Five polyamides were studied at glassy carbon electrode in acetate buffer by cyclic and differential pulse voltammetry to clarify their redox pathways. The polyamide electrochemical responses are compared by peak currents and peak potentials. The slopes of the three anodic E_p vs. pH plots of a typical polyamide are linear and show 0.059, 0.057, 0.056 V per pH in acid media, respectively, which correspond to a mechanism involving the equal number of electrons and protons. A possible mechanism for the redox pathway of various polyamides is proposed: the oxidation product of imidazole ring is acylamide and the results of in situ UV–Vis spectroscopy at Pt web electrode support the proposed mechanism. electrospray ionization mass spectroscopy (ESI-MS) indicates that one or two oxygen atoms are added into polyamide molecule after electrochemical oxidation.     

Vibrational analysis of tellurophene and its deuterated derivatives

The i.r. and Raman spectra of tellurophene, tellurophene-2-d₁, tellurophene-2-5-d₂, tellurophene-3-4-d₂, tellurophene-2-3-4-d₃ and tellurophene-2-3-4-5-d₄ have been studied. A vibrational assignment of the fundamental frequencies of all these molecules has been carried out on the basis of i.r. vapor band contours and depolarization states of Raman lines. Such an assignment conveniently fits the isotopic “sum” and “product” rules. A comparison of the vibrational behaviour of tellurophene and that of the other congeners is made and a rationalization of some frequency shifts is attempted.     

Electrochemical and ESR studies of au-protein from Micrococcus luteus

Au-protein from Micrococcus luteus, with and without Au inactive center, and chloroauric acid (HAu ^IIICl₄·4H₂O) with the addition of rutin, catechol, and riboflavin have been studied by means of electrochemistry and ESR. The redox potentials for Au-protein, as well as for the complexes Au-rutin and Au-catechol, have been measured, and ESR spectra of complexes Au-rutin and Au-catechol have been recorded. It has been shown that the Au atom binds to Au-protein via OH-groups of rutin. Flavin does not participatein gold binding. Au-protein is characterized by two peaks of cyclic voltammogram, −0.37 and −0.54 V. Au-protein with these potentials is able to function in the electron-transport chain of membranes between flavoproteins and quinones.     

Electrochemical and ESR studies of aromatic nitroso spin traps

The oxidation and reduction potentials of 23 nitroso compounds among which were included the most important spin traps have been determined by means of cyclic voltammetry and voltammetry at the rotating electrode. The electron transfer generally involved one electron, leading to the corresponding ion radicals. When persistent species resulted from the electron transfer, their ESR spectra were recorded and analyzed with the help of EHT calculations. The observed anion radicals are typical delocalized Π radicals, while the cation radicals are Σ radicals with a SOMO orthogonal to the Π system. Secondary species formed from these primary ion radicals were also observed and identified by e.s.r.     

Electrochemical oxidation of tertiary phosphines in the presence of camphene

Anodic oxidation of tertiary phosphines (tripropyl-, tributyl-, and triphenylphosphine) in the presence of a bicyclic alkene (camphene) on a platinum and a glassy carbon electrodes was studied. For the first time the voltammetric characteristics of the process of camphene anodic oxidation were obtained. The electrochemical reactions with alkyl and aromatic phosphine were found to be dissimilar. The results of preparative electrooxidation of trialkylphosphines showed that in the course of electrolysis the tertiary phosphine cation-radicals generated on the anode enter into two concurrent reactions: (1) with the parent phosphine to form eventually trialkylphosphonium salts and trialkylphosphine oxides presumably as complex compounds and (2) with camphene to form trialkylcamphenylphosphonium salts and probably phosphonium salts with a monocyclic substituent. Preparative electrochemical oxidation of triphenylphosphine in the presence of camphene affords almost exclusively either triphenylphosphine oxide (in the experiment with platinum anode) or the triphenylphosphine oxide complex with perchloric acid (at the electrolysis on a glassy carbon anode).     

Electrochemical oxidation of sulfodifluoroacetic acid in the presence of halogens

The electrochemical oxidation of sulfodifluoroacetic acid (HOSO₂CF₂CO₂H) in the presence of Cl₂ or Br₂ gives halodifluoromethanesulfonic acid. The anodic oxidation of sulfodifluoromethanecarboxylate ion to form the sulfodifluoromethyl radical as an intermediate is proposed as the rate-determining step. Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanskaya ul., Kiev 253094, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 4, pp. 246–248, July–August, 1999.     

Vibrational analysis of polyethylene terephthalate and its deuterated derivatives

The Vibrational analysis of polyethylene terephthalate, polyethylene-d₄ terephthalate, and polyethylene terephthalate-d₄ has been carried out using a valence force field calculated from the infrared and Raman spectra of a series of low molecular weight aromatic esters. The Raman spectra for polyethylene-d₄ terephthalate and polyethylene terephthalate-d₄ are presented and band assignments for these compounds and polyethylene terephthalate are discussed.     

Electrochemical reduction and oxidation of fullerenopyrrolidines and the ESR spectra of paramagnetic intermediates

Electroreduction and electrooxidation of monosubstituted N-methyl[60]fullerenopyrrolidines were studied by cyclic voltammetry and potentiostatic microelectrolysis in the cavity of an ESR spectrometer. Stepwise reversible transfer of three electrons to the fullerenopyrrolidine molecule results in the formation of stable radical anions (according to ESR, g = 2.0000, H = 0.8 G), dianions, and radical trianions (according to ESR, g = 2.0015, H = 1.5 G). The reduction potentials vary over narrow limits depending on the nature of the substituents in the pyrrolidine fragment of the compounds. Electrooxidation is irreversible and occurs in either one or two steps. For compounds containing the aniline, indole, or phenol fragment, the first step is associated with oxidation of these fragments and only after that, is the fullerenopyrrolidine core oxidized. Oxidation of the pyrrolidine fragment is substantially more difficult than that of tertiary amines.     

Substituted pyrazine-di-N-oxides as the mediators of catalytic oxidation of organic compounds

The mechanism of oxidation of 2,5-dimethyl-, 2,3,5,6-tetramethyl-, 2,3-dimethyl-5,6-cyclohexa, and 3-phenyl-5,6-cyclohexapyrazine-di-N-oxides is studied by cyclic voltammetry, quantum chemical simulations, and ESR electrolysis. The studies are carried out on electrodes of glassy carbon and Pt in 0.1 M LiClO₄ solutions in acetonitrile. ESR spectra of radical cations of substituted pyrazine-di-N-oxides are recorded. The effects of the temperature, oxygen, and the additions of water, pyridine, and acid on the shape of cyclic voltamograms and the intensity of ESR signals of pyrazine-di-N-oxides are studied. A quantum-chemical simulation of the reaction of pyrazine-di-N-oxide radical cations with acetonitrile is carried out. The oxidation of substituted pyrazine-di-N-oxides is described by the E₁C₁E₂C₂ mechanism, which includes the stage of the formation of a complex between the di-N-oxide radical cation and acetonitrile.