Methyl Radicals Research Articles

New tetrapropylammonium tetrabromozincate complex [(C3H7)4N]2ZnBr4(II) - synthesis, spectral, thermal characterization and antioxidant activity

ABSTRACT. The importance of the A2BX4 complex (A = bivalent ion, NH4+ and its alkyl radical derivatives; B = bivalent transition metal ion and X = halogen) has increased in recent years due to fascinating physical properties such as ferro elastic, ferroelectric, and corresponding – unequal phases at low temperatures. In modern technologies the use of transistors, ferromagnetic garnets, sensor systems, softener templates, and ultraviolet light, as well as IR solid-state lasers, efficient crystalline materials are expected. A slow evaporation approach at room temperature was used to grow the TPAPBr-Zn [(CH3CH2CH2)4N]2ZnBr4(II) challenging kind A2BX4. Elemental analysis, powder diffraction, differential thermal analysis, FT-IR, proton and carbon-13 NMR spectroscopic techniques are used to characterize the synthesized compound. The presence of chemical groups and methylene radical groups in the compound are confirmed by the FT-IR spectra. The presence of an N-propyl group in the molecule is indicated by the nuclear magnetic resonance spectra. Thermogram of the compound reveals two-stage decomposition showing endo followed by exothermic decomposition resulting zinc oxide as an end product. As synthesised compound shown significant inhibitory activity towards free radical.
 
 KEY WORDS: Synthesis, Elemental analysis, Spectroscopic studies, TG analysis, Antioxidant, Tetrapropylammonium tetrabromozincate(II)
 
 Bull. Chem. Soc. Ethiop. 2023, 37(3), 623-631. 
 DOI: https://dx.doi.org/10.4314/bcse.v37i3.7

One-step synthesis of methanol and hydrogen from methane and water using non-thermal plasma and Cu-Mordenite catalyst

In this study, the copper-mordenite zeolite (Cu-MOR) and non-thermal plasma were used to produce methanol and hydrogen from methane and water at low temperature (120 °C) without CO2 formation. Energy efficiency for methanol and hydrogen were 68.8 mmol·kJ−1 and 562.3 mmol·kJ−1, respectively, and the selectivity of methanol in liquid products reached 86%. The reaction mechanism likely involves methane being excited to methyl (·CH3) and hydrogen radicals (·H) under DBD plasma conditions, which can then adsorb on the catalyst surface at the Cu2-(μ-O)2+ active sites, accompanied by water co-adsorption to facilitate formation and desorption of methanol. XPS analysis showed that with the prolongation of reaction time, Cu2-(μ-O)2+ was reduced to Cu+ and Cu(II) hydroxide, and the catalyst had carbon deposition, resulting in the decrease of catalytic activity. It is also worth noting that in an oxygen atmosphere, Cu2-(μ-O)2+ regenerates rapidly and eliminates catalyst carbon deposits. Trace oxygen was introduced into the reactor, and the reaction was carried out for 30 h, and the catalytic activity did not decrease.

Red-Fluorescing Paramagnetic Conjugated Polymer Nanoparticles─Triphenyl Methyl Radicals as Monomers in C–C Cross-Coupling Dispersion Polymerization

We report the synthesis of conjugated polymer nanoparticles carrying stable luminescent radical units. These monodisperse conjugated radical nanoparticles can be tuned in their diameter over several hundred nanometers. They are stable in aqueous medium and highly luminescent in the red and near-infrared spectra, representing a powerful future tool for bioimaging. Moreover, the polymer nanoparticles exhibit paramagnetic properties, making them highly suitable for dual-mode optical and magnetic resonance imaging. In this study, we investigate their synthesis as well as optical and magnetic properties, and use quantum mechanical calculations to elucidate the effect of the conjugated polymer backbone and electron-withdrawing substituents on the electronic properties of the open-shell molecule in the polymer network of the particles.

A Combined Experimental and Modeling Study on Isopropyl Nitrate Pyrolysis.

Alkyl nitrates thermally decompose by homolytic cleavage of the weak nitrate bond at very low temperatures (e.g., around 500 K at reaction times of a few seconds). This provides the opportunity to study the subsequent chemistry of the initially formed radical (or its subsequent pyrolysis products, if unstable) and nitrogen dioxide at such mild conditions. In this work this idea is applied to isopropyl nitrate (iPN) pyrolysis, which is studied in a tubular reactor at atmospheric pressure, temperatures ranging from 373 to 773 K, and residence times of around 2 s. At the experimental conditions, iPN decomposition starts at 473 K with O-N bond fission producing isopropoxy radical (i-C3H7O) and NO2. i-C3H7O is rapidly converted to acetaldehyde (CH3CHO), which is the most abundant product detected, and methyl radicals. Other major products detected are formaldehyde (CH2O), methanol (CH3OH), nitromethane (CH3NO2), NO, methane, formamide (CHONH2), and methyl nitrite (CH3ONO). Four literature nitrogen chemistry models─three of those augmented with iPN specific reactions─have been tested for their ability to predict the iPN decomposition and product profiles. The mechanism by the Curran group performs best, but it still underpredicts the observed high formaldehyde and methanol yields. A rate analysis indicates that the branching ratio of the reaction between methyl radicals and nitrogen dioxide is of significant importance. Based on recent theoretical and experimental data, new rate expressions for the two reactions CH3 + NO2 → CH3O + NO and CH3 + NO2 + He → CH3ONO2 + He are calculated and incorporated in the kinetic models. It is shown that this change clearly improves the predictions, although additional work is needed to achieve good agreement between calculated and measured species profiles.

A DFT study on the scavenging activity of curcumin toward methyl and ethyl radicals

The curcumin is a well-known antioxidant that can scavenge free radicals efficiently. The methyl free radicals, generated by the metabolism of various genotoxic compounds such as hydrazines and peroxides, can methylate various sites in DNA. Herein, we have carried out density functional theory calculations to investigate the scavenging activity of curcumin toward the methyl and ethyl radicals through radical adduct formation (RAF), hydrogen atom transfer (HAT) and single electron transfer (SET) mechanisms. The SET mechanism is found to be highly endergonic and so not viable. Our calculations show that the curcumin can scavenge methyl radicals through both RAF and HAT mechanisms but RAF would be preferred over the HAT. Further, it is found that the curcumin can scavenge methyl radicals more efficiently as compared to ethyl radicals through RAF mechanism.

Fe-Catalyzed Aliphatic C-H Methylation of Glycine Derivatives and Peptides.

Direct and selective C-H methylation is a powerful tool with which to install methyl groups into organic molecules, and is particularly useful in pharmaceutical chemistry. However, practical methods for such modification of biologically interesting targets have been rarely developed. We here report an iron-catalyzed C(sp3 )-H methylation reaction of glycine derivatives, peptides and drug-like molecules in an alcohol in the presence of di-tert-butyl peroxide. A readily available iron catalyst plays multiple roles in the transformation, which accelerates oxidation of C-N bonds to C=N double bonds, activates imine intermediates as Lewis acids by bidentate chelation, and at the same time facilitates cleavage of the peroxide to generate methyl radicals. A variety of methylated N-aryl glycine derivatives and peptides were obtained in good yield and with excellent chemo- and site-selectivity. This reaction is scalable, easily managed, and can be completed within 1-2 h. It features an economic, bio-friendly catalyst, a green solvent and low toxic reagents, and will provide effective access to precise C-H modification of biomolecules and natural products.

Thermal Methane Conversion to Formaldehyde Mediated by NiAlO3+ in the Gas Phase.

Understanding the active sites and reaction mechanisms of Ni-based catalysts, such as Ni/Al2O3, toward methane is a prerequisite for improving their rational design. Here, the gas-phase reactivity of NiAlO3+ cations toward CH4 is studied using mass spectrometry combined with density functional theory. Similar to our previous study on NiAl2O4+, we find evidence for the formation of both the methyl radical (CH3•) and formaldehyde (CH2O). The first step for methane activation is hydrogen atom abstraction by the terminal oxygen radical Ni(O)2AlO• from methane forming a [Ni(O)2AlOH+, •CH3] complex and leaving the Ni-oxidation state unchanged. The second C-H bond is subsequently activated by the association of a bridged Ni-O2--Al. The oxidation state of the Ni atom is reduced from +3 to +1 during the formation of formaldehyde. Compared to Al2O3+/CH4 and YAlO3+/CH4 systems, the Ni-atom substitution increases the overall reaction rate by roughly an order of magnitude and yields a CH3•/CH2O branching ratio of 0.62/0.38. The present study provides molecular-level insights into the highly efficient gas-phase reaction mechanism contributing to an improved understanding of methane conversion by Ni/Al2O3 catalysts.

Radical Bicyclization of 2‐Cyanoaryl Acrylamides with Dicumyl Peroxide

AbstractA radical bicyclization of 2‐cyanoaryl acrylamides with dicumyl peroxide has been developed for constructing a broad spectrum of benzo[h]naphtho[1,8‐bc][1,6]naphthyridine. This reaction allows the formation of three new chemical bonds through addition of methyl radical across alkene double bond, intramolecular addition of carbon‐centered radical to cyano group and cyclization of the iminyl radical cascade, and features simple reaction system and wide substrate scope.magnified image

Theory-Based Mechanism for Fluoromethane Combustion I: Thermochemistry and Abstraction Reactions

A new detailed chemical kinetic mechanism is presented for small fluorinated hydrocarbons. Ab initio electronic structure theory is used to provide heats of formation with subchemical accuracy. The ANL0 method is extended to include fluorine. The resulting heats of formation at 0 K are in excellent agreement with 36 benchmark species in the Active Thermochemical Tables, with a mean error of μ = -0.02 kJ/mol and a standard deviation of σ = 0.91 kJ/mol. The thermophysical properties for 92 small-molecule H/C/O/F species are computed. The rate coefficients for 40+ H-abstraction reactions involving H, O, F, OH, OF, HO2, and various methyl radicals with CH4, CH3F, CH2F2, CHF3, CH2O, and CHFO are discussed. The computed rate constants are in excellent agreement with the available literature. Additionally, 30+ rate constants are provided for F abstraction, which are several orders of magnitude smaller than H abstraction. The thermophysical properties and rate constants are provided in a mechanism. This mechanism is the first in a series of theory-based investigations into the thermal destruction of per- and polyfluorinated species.

Five-membered ring compounds from the ortho-benzyne + methyl radical reaction under interstellar conditions

Five-membered ring compounds from the ortho-benzyne + methyl radical reaction under interstellar conditions

In-Situ Electrochemical Exfoliation and Methylation of Black Phosphorus into Functionalized Phosphorene Nanosheets.

Two-dimensional black phosphorus (BP) has attracted great attention as a perspective material for various applications. The chemical functionalization of BP is an important pathway for the preparation of materials with improved stability and enhanced intrinsic electronic properties. Currently, most of the methods for BP functionalization with organic substrates require either the use of low-stable precursors of highly reactive intermediates or the use of difficult-to-manufacture and flammable BP intercalates. Herein we report a facile route for simultaneous electrochemical exfoliation and methylation of BP. Conducting the cathodic exfoliation of BP in the presence of iodomethane makes it possible to generate highly active methyl radicals, which readily react with the electrode's surface yielding the functionalized material. The covalent functionalization of BP nanosheets with the P-C bond formation has been proven by various microscopic and spectroscopic methods. The functionalization degree estimated by solid-state 31P NMR spectroscopy analysis reached 9.7%.

NaCl-Promoted Cobalt-Catalyzed Dioxygen-Mediated Methane Oxidation to Methylene Bis(trifluoroacetate) with a Dramatic Salt Effect

Although O2 is an ideal oxidant in the transformation of methane, very limited studies realize this process in homogeneous solvent systems with high efficiency under mild conditions. Herein, we present a NaCl-promoted cobalt-catalyzed O2-mediated methane oxidation with turnovers over 700 at 120 °C with a dramatic salt effect. The unexpected methylene bis(trifluoroacetate) can be produced with a selectivity up to 90%. This product is used in the reaction with TsNH2 and 1,2-diimine to efficiently form triazinane and imidazole salt, respectively. The Co-NaCl catalytic system is also applied in the oxidation of other hydrocarbons such as adamantane, 9,10-dihydrophenanthrene, 9,10-dihydroanthracene, and 9,10-dihydroacridine. Mechanistic studies indicate that NaCl can promote the decomposition of trifluoroacetic anhydride (TFAA) in the presence of Co(II), and a highly reactive fluorine radical might be formed to react with methane, generating a methyl radical, which will further react with O2 and then with trifluoroacetic anhydride to form methylene bis(trifluoroacetate).

Theoretical study on the kinetics of hydrogen cyanide and hydrogen isocyanide reactions with the methyl radical.

The detailed kinetic mechanisms for the reactions of hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) with the methyl radical (CH3) are discussed. These are important reactions in combustion and Titan's atmosphere chemistry and were investigated at the CCSD(T)/cc-pVQZ//M06-2X/6-311++G(2df,2p) level of theory. The multiwell and multichannel potential energy surface (PES) was constructed. The rate constants were determined by using variational transition state theory (VTST) and Master Equation/Rice-Ramsperger-Kassell-Marcus (ME/RRKM) method over a temperature range of 300-2000 K and a pressure range of 1-10 000 torr. Corrections of the Eckart tunneling effect were included and the calculated results were in good agreement with the literature. A clear dependence of the reaction mechanism on temperature and pressure was revealed via detailed kinetic and species analysis. For the HCN reaction, the channel of C-addition forms an intermediate that is dominant at low temperatures and high pressures, leading to the total rate constant exhibiting a pressure dependence, but this dependence disappears at high temperatures. The H-abstraction channel is more competitive with increasing temperatures, but it is still not dominant. For the HNC reaction, the C-addition channel is dominant, and CH3CN and H constitute almost all the products. The proposed temperature and pressure-dependent rate constants can be used in the combustion and atmospheric model development for related systems.

Photoelectrochemical C–H activation of methane to methyl radical at room temperature

Visible light illumination induced CH4oxidation over a WO3photoanode. The production of C2H6implies the formation of methyl radicals through photoelectrochemical activation of methane.

A rapid electrochemical method to recycle carbon fiber composites using methyl radicals.

We introduce an electrochemical approach to recycle carbon fiber (CF) fabrics from amine-epoxy carbon fiber-reinforced polymers (CFRPs). Our novel method utilizes a Kolbe-like mechanism to generate methyl radicals from CH3COOH to cleave C-N bonds within epoxy matrices via hydrogen atom abstraction. Recovered CFs are then remanufactured into CFRPs without resizing.

Donor engineering of diphenylamine-substituted tris(2,4,6-trichlorophenyl)methyl radicals for controlling the intramolecular charge transfer and near-infrared photothermal conversion

Donor engineering of diphenylamine-substituted tris(2,4,6-trichlorophenyl)methyl (TTM) radicals triggers the switching of the TTM-based radicals from the photoluminescent to NIR photothermal mode.

Crafting mono- and novel bis-methylated pyrroloquinoxaline derivatives from a shared precursor and its application in the total synthesis of marinoquinoline A.

The synthesis of mono- and novel bis-methylated pyrrolo[1,2-a]quinoxalines through the addition of unstable methyl radicals to aryl isocyanides is described contingent upon the reaction conditions employed. The strategy has been effectively employed in the total synthesis of the natural product marinoquinoline A.

Methylation Profile of 18S rDNA Gene in Brain and Muscle of Tambaqui Exposed to Parasiticide Trichlorfon

Until recently, there were no reports on the methylation profile of Amazonian fish specimens employing restriction enzymes. The 18S rDNA methylation status of samples of captive tambaqui (Colossoma macropomum) exposed to the antiparasitic Trichlorfon was examined in the current study. The method performed makes use of the CCGG site's sensitivity to the presence of methylation in palindrome cytosines for the restriction enzymes HpaII and MspI. The cleavage occurs by the detection or non-detection of the methyl radical in the inner (C5mCGG) or outer (5mCCGG) cytosine, which makes it possible to compare the DNA methylation profile. When compared to the specimens examined as the control group, the results obtained revealed an increase in methylated fragments of the 18S rDNA gene at LC concentrations of 30% and 50%.

Role and dynamics of transition metal carbides in methane coupling.

Transition metal carbides have numerous applications and are known to excel in terms of hardness, thermal stability and conductivity. In particular, the Pt-like behavior of Mo and W carbides has led to the popularization of metal carbides in catalysis, ranging from electrochemically-driven reactions to thermal methane coupling. Herein, we show the active participation of carbidic carbon in the formation of C2 products during methane coupling at high temperature that is associated with the dynamics of Mo and W carbides. A detailed mechanistic study reveals that the catalyst performance of these metal carbides can be traced back to its carbon diffusivity and exchange capability upon interaction with methane (gas phase carbon). A stable C2 selectivity over time on stream for Mo carbide (Mo2C) can be rationalized by fast carbon diffusion dynamics, while W carbide (WC) shows loss of selectivity due to slow diffusion leading to surface carbon depletion. This finding showcases that the bulk carbidic carbon of the catalyst plays a crucial role and that the metal carbide is not only responsible for methyl radical formation. Overall, this study evidences the presence of a carbon equivalent to the Mars-Van Krevelen type mechanism for non-oxidative coupling of methane.

Structural Factors That Determine the Activity of the Xenobiotic Reductase B Enzyme from Pseudomonas putida on Nitroaromatic Compounds

Xenobiotic reductase B (XenB) catalyzes the reduction of the aromatic ring or nitro groups of nitroaromatic compounds with methyl, amino or hydroxyl radicals. This reaction is of biotechnological interest for bioremediation, the reuse of industrial waste or the activation of prodrugs. However, the structural factors that explain the binding of XenB to different substrates are unknown. Molecular dynamics simulations and quantum mechanical calculations were performed to identify the residues involved in the formation and stabilization of the enzyme/substrate complex and to explain the use of different substrates by this enzyme. Our results show that Tyr65 and Tyr335 residues stabilize the ligands through hydrophobic interactions mediated by the aromatic rings of these aminoacids. The higher XenB activity determined with the substrates 1,3,5-trinitrobenzene and 2,4,6-trinitrotoluene is consistent with the lower energy of the highest occupied molecular orbital (LUMO) orbitals and a lower energy of the homo orbital (LUMO), which favors electrophile and nucleophilic activity, respectively. The electrostatic potential maps of these compounds suggest that the bonding requires a large hydrophobic region in the aromatic ring, which is promoted by substituents in ortho and para positions. These results are consistent with experimental data and could be used to propose point mutations that allow this enzyme to process new molecules of biotechnological interest.