Nitroxy Radicals Research Articles

First principle investigations of long‐range magnetic exchange interactions via polyacene couplers

AbstractThe electronic and magnetic properties of polyacenes become quite fascinating as the number of linearly conjugated benzene rings increases. Higher‐order conjugated polyacenes develop radicaloid characters due to the transition of electronic structures from closed‐shell to the open‐shell system. Here we have investigated the role of such polyacenes as the magnetic coupler when placed between the two spin‐sources based on nitroxy radicals. To do so, the magnetic exchange interactions () are computed employing broken‐symmetry (BS) approach within the density functional theory (DFT). In this approach, various genre of exchange‐ correlation (XC) functionals such as generalized gradient approximation (GGA), meta‐GGA, hybrid functional, constrained spin density (i.e. CDFT) and on‐site Coulomb correlation corrected GGA + U functionals are adopted. All DFT based calculations estimate an exponential increase in values with the length of the couplers. This observation has been understood in terms of increase in number of near‐degenerate or quasi‐degenerate molecular orbitals (MOs), reduction of HOMO‐LUMO energy gap and descend of low‐lying excited states as the number of fused benzene rings are increased.

Nano-Magnetic NiFe2O4 and Its Photocatalytic Oxidation of Vanillyl Alcohol-Synthesis, Characterization, and Application in the Valorization of Lignin.

Here, we report on a phyto-mediated bimetallic (NiFe2O4) preparation using a Boswellia carterii extract, which was characterized by XRD, FT-IR, TGA, electron microscopy, magnetic spectroscopy, and Mössbauer spectroscopy measurements. The prepared nano-catalysts were tested for oxidation of lignin monomer molecules—vanillyl alcohol and cinnamyl alcohol. In comparison with previously reported methods, the nano NiFe2O4 catalysts showed high photocatalytic activity and selectivity, under visible light irradiation with a nitroxy radical initiator (2,2,6,6-tetramethylpiperidinyloxy or 2,2,6,6-tetramethylpiperidine 1-oxyl; TEMPO) at room temperature and aerobic conditions. The multifold advantages of the catalyst both in terms of reduced catalyst loading and ambient temperature conditions were manifested by higher conversion of the starting material.

Investigation of the Relationship between Solvation Structure and Battery Performance in Highly Concentrated Aqueous Nitroxy Radical Catholyte

A new battery catholyte material composed of equimolar 4-methoxy-2,2,6,6-tetramethylpiperidine 1-oxyl (MT) and lithium bis(trifluoromethanesulfonyl) imide (LT) and its highly concentrated mixtures with water were studied using experiments and molecular dynamics simulations. It was found that the dynamic properties of the mixture are significantly improved by adding even a small amount of water. Detailed analysis on the solvation structure in the mixtures reveals that water molecules can break the strong interaction between MT and LT, and thus the ions can move more freely. As a result, the ionic conductivity of the catholyte mixtures increases with increasing water molar ratio in the water concentration range covered in the current work. The performance of the catholyte mixture systems was also tested in battery cells. The best utility efficiency of the capacity was found for the mixture of water:MT:LT ratio at 5.3:1:1. Water molar ratios of 5 to 6 were also found to be the lowest concentration at which M...

Electrostatic Self-assembly Deposition of Layered Calcium Niobate Intercalated with Task-specific Ionic Liquid and Its Electrocatalytic Activity

A novel intercalation compound of Dion–Jacobson-type perovskite calcium niobate (KCa2Nb3O10) with the nitroxy radical 2,2,6,6-tetramethylpiperidyl-1-oxy (TEMPO) task-special ionic liquid (TEMPO-IL)...

Redox-Modulated Magnetic Transformations between Ferro- and Antiferromagnetism in Organic Systems: Rational Design of Magnetic Organic Molecular Switches

Organic molecules with switchable magnetic properties have extensively technological applications due to the fact that magnetic conversion can be realized through diverse methods. In particular, the redox-induced magnetic reversal is easy to accomplish and exhibits promising application in the field of magnetic materials, and thus it is an imperative task to find magnetism-switchable systems. Herein, we computationally design two couples of nitroxy–pyrazinyl–nitroxy diradicals in which two nitroxy radical groups are connected to a redox-active pyrazinyl coupler in the para or meta modes. We find that the magnetic conversion can occur from ferromagnetic to antiferromagnetic exchange coupling or vice versa by means of the redox method in these designed magnetic organic molecules, and their magnetic exchange coupling constants are considerably large no matter for ferromagnetic or antiferromagnetic couplings, as evidenced at both the B3LYP and M06-2X levels of theory. Analyses indicate that redox-induced stru...

2,2,6,6-Tetramethyl-1-oxopiperidinetribromide and two forms of 1-hydroxy-2,2,6,6-tetramethylpiperidinium bromide salt: Syntheses, crystal structures and theoretical calculations

The reaction of the nitroxy radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) with Br2 has been investigated with CCl4 and hexane to obtain TEMPO-Br salts: 2,2,6,6-tetramethylpiperidine-1-oxopiperidine tribromide [TEMPO][Br3−] (I), and the 1-hydroxy-2,2,6,6-tetramethylpiperidinium bromide salts [TEMPH+OHBr−] (II and III). The salt I was isolated in crystalline form directly from the synthesis and II and III by only changing the solvent. The crystals of I belong to the orthorhombic crystal systems with space group Cmc21, a = 10.5596(4) Å, b = 14.0464(4) Å, c = 9.4202(5), and with asymmetric unit of Z = 4. Crystals II belong to Pnna a = 11.9860(3) Å, b = 23.6720(9) Å, c = 7.7051(3) Å while III belongs to Cmc21 with a = 10.2686(3) Å, b = 10.7661(3) Å, c = 10.0274(2) Å; the asymmetric unit of II and III was Z = 8 and Z = 4, respectively. The crystal structure of I shows the Br3− ion as [Br–Br–Br]− for each molecule of TEMPO. The crystal structure of II shows a weak intermolecular hydrogen bond between –N-H⋯Br(1) and O(1)H⋯Br(2) due to the presence of the –N+HOH– moiety. In contrast, crystal III shows intermolecular hydrogen bonding between O(1)H⋯Br(1)⋯HN(1) due to the –N+HOH– moiety. The resulting compounds were characterized by FT-IR and UV–vis spectroscopy. The structural parameters have been compared with the related hydroxylaminotrichlorosilane known from the literature and with results of DFT calculations.

Reversible exchange of stable nitroxyl radicals on nanosilver particles

Environmental context Nanometre-sized silver particles promote unique chemical reactions on their surface. This work examines the ability of silver nanoparticles to collect and store unpaired electrons, called radicals, on their surface. This capability by silver nanoparticles could potentially serve to drive degradation reactions in the environment. Abstract Radicals drive important chemical reactions in the environment. These unpaired electron species can be generated by energetic inputs, such as electromagnetic radiation, or from ultrasonication processes, whereby oxygen radicals are generated in aqueous solution through a cavitation mechanism. Previous evidence has demonstrated the potential for radicals to be stored on the surface of metallic gold nanoparticles, thus suggesting a potential transference of radical species from the nanoparticle surface for catalytic reactions, particularly during preparations of nanoparticle suspensions through ultrasonication. This work investigates the potential for the nanosilver (nAg) particles to similarly scavenge radicals from solution. nAg suspensions were reacted with 0.3-mM solutions containing the stable nitroxy radicals 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPONE) and 4-amino-2,2,6,6-tetramethylpiperidino-1-oxyl (TEMPAMINE) analysed by quantitative electron spin resonance (ESR) spectroscopy. In ambient air, the addition of a nAg suspension to the 0.3-mM solutions reduced the integrated ESR intensity of the stable radicals by 50–93% depending on radical species and nAg concentration, which we attributed to the sorption of the radicals onto the nAg surface. In separate experiments, the ESR intensities were further decreased under an Ar atmosphere, suggesting potential competition from ambient OH• to the sorption of the stable radicals. To verify this, we observed substantial increases in the integrated ESR intensity when the systems previously equilibrated under Ar atmosphere were exposed to ambient air. These results demonstrated that nAg scavenged the stable radicals from solution and were exchangeable from the metallic conduction band with OH•. Our work represents the first evidence for this mechanism to be demonstrated for nAg.

SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF POLY-[2, 5-DI-N-(2, 2, 6, 6-TETRAMETHYL-4-PIPERIDINE–N-OXYL) BENZAMIDE] ANILINE AS A CATHODE MATERIAL FOR LITHIUM-ION BATTERIES

Novel TEMPO radical grafted PANI [polymer (3)] was synthesized from 2,5-dicarboxyaniline by two-step process, and investigated with Fourier transform infrared spectroscopy, electron spin resonance spectra, thermogravimetric analysis, scanning electron microscopy, cyclic voltammograms, and galvanostatic charge–discharge techniques. The polymer (3) shows six-electron redox reaction process in the voltage ranges of 2.0–4.0 V versus Li +/ Li . A high specific capacity of 156 mA h g−1 is obtained in the experiment, close to half of the theoretical capacity of the polymer based on six-electron redox reaction (330 mA h g−1. The polymer (3) also exhibits good rate capability and cycle life. The excellent electrochemical properties benefit from fast redox reaction of nitroxy radical and conductive polyaniline nanotubes structure.

TEMPO-oxidized cellulose hydrogel as a high-capacity and reusable heavy metal ion adsorbent

Nitroxy radical catalyzed oxidation with hypochlorite/bromide (TEMPO-mediated oxidation) was performed on a cellulose hydrogel prepared using LiOH/urea solvent. TEMPO oxidation successfully introduced carboxyl groups onto the surface of the cellulose hydrogel with retention of the gel structure and its nanoporous property. The equilibrium measurement of Cu2+ adsorption showed favorable interaction with Cu2+ and high maximum adsorption capacity. In addition, over 98% of the adsorbed Cu2+ was recovered using acid treatment, and the subsequent washing allowed the TEMPO-oxidized gels to be used repeatedly. Furthermore, the TEMPO-oxidized cellulose hydrogel showed high adsorption capacity for other toxic metal ions such as Zn2+, Fe3+, Cd2+, and Cs+.

Degradation of 2,4,8-trichlorodibenzofuran by a new isolate of Cerrena sp. F0607

Degradation of 2,4,8-trichlorodibenzofuran (2,4,8-TCDF) by the isolate F0607, identified as belonging to the genus Cerrena, was investigated in liquid culture medium. Strain F0607 showed good decolorization and production of ligninolytic enzymes. The degradation of 2,4,8-TCDF per-unit biomass increased during the incubation period in presence of 1 or 10 mg l−1 2,4,8-TCDF and the mycelial biomass increased with the increase in 2,4,8-TCDF concentration. The rate of glucose consumption (k1) also increased rapidly when 2,4,8-TCDF was present in the culture. All three types of ligninolytic enzymes were detected in which laccase had the highest enzyme activity. CuSO4 could induce the activity of laccase and increase the degradation of 2,4,8-TDCF, whereas 1-hydroxybenzotriazole (HBT) also enhanced degradation via nitroxy radical (>N–O) oxidation.

Molecular weight dependence of non-surface activity for ionic amphiphilic diblock copolymers

The ionic amphiphilic diblock copolymers, (n-butyl acrylate)m-b-(styrene sulfonate)n, were synthesized by nitroxy radical mediated living radical polymerization with precise control of degree of polymerization and molecular weight distribution. Several polymers with different m and n values at m : n = 1 : 1 were obtained, and their non-surface activity was examined by surface tension experiments and foam formation observation. All the polymers with m = n = 7–50 were found to form micelles in aqueous solutions, but the polymers with a degree of polymerization larger than 20 showed non-surface activity while those less than 20 were found to be surface active. As with other polymers investigated previously, surface activity was observed when salts were added to the solution, even for polymers of a non-surface active nature in pure water solution. This means that non-surface activity is not only an electrostatic effect (image charge effect at the air/water interface) but also some kind of polymer effect. Non-surface activity is a specific characteristic only for polymers, and the border of “amphiphilic polymer” and “low molecular weight surfactant” is located around the degree of polymerization of about 20. This is proposed as a new definition of “polymer” from the view of surface science, at least, from the view of non-surface activity. The essential of this “polymer effect” can be attributed to the highly stable nature of polymer micelles in solution compared to adsorbed polymer at the air/water interface, which is highly destabilized by image charge repulsion.

A Radical Approach to Hydroxylaminotrichlorosilanes: Synthesis, Reactivity, and Crystal Structure of TEMPO‐SiCl3 (TEMPO = 2,2,6,6‐Tetramethylpiperidine‐N‐oxyl)

AbstractThe reaction of the nitroxy radical 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) with SiCl4 has been investigated in detail. One silicon‐containing product, TEMPO‐SiCl3 (1), could be isolated in crystalline form, and its crystal structure was determined. The structural parameters have been compared with the related hydroxylaminotrichlorosilane known from the literature and with DFT calculations. According to the calculations, the formation of 1 follows an SN2 mechanism, which is thermodynamically favored over an SN1 pathway. Upon hydrolysis, 1 decomposes and yields several TEMPO‐containing species. These were all characterized crystallographically.

The reactivity of antioxidants with peroxy and alkoxy radicals in the presence of transition metals. II. Radical processes in multicomponent antioxidant systems

In nonpolar solvents the mechanism of bimolecular substitution reactions between t-butyl-peroxy and t-butoxy radicals with different phenols and diarylamines was quantitatively studied by means of electron paramagnetic resonance (EPR) spectroscopy in the temperature range of +70 to −140°C. The selective reactivity, in the ligand field, of cobalt complex-bonded radicals [Co] BuO2, [Co] BuO., and the free continuously generated noncomplex-bonded BuO2 and BuO radicals to antioxidants are compared. The antioxidant ArH forms with cobalt (II)-acetylacetonate a complex [Co(acac)2]2 [Arh]n = 1-4 and prevents the subsequent reactions of the primarily generated RO. radicals with the transition metal (RO. + Co(II) RO′ + Co(III)) and with the excess of hydroperoxides (RO. + ROOH ROH + RO2). The concentration and the ratio of RO. and RO.2 radicals in a chain oxidation process in the presence of phenyl β- or α-napthylamine (Ar1HNAr2) can be determined from the different EPR spectra of fixed [Co] [Ar1 NAr2]. and free stable nitroxy (Ar1HNAr2) radicals.

Chemiluminescence of polyamides III. Luminescence accompanying thermooxidation of lactam-based polyamides stabilized by antioxidants

Chemiluminescence accompanying oxidation of lactam-based polyamides stabilized with phenols and secondary aromatic amines as well as with a hindered nitroxy radical was measured. The yields of chemiluminescence per mol of oxygen absorbed were found to be higher for oxidation of polyamide in the presence of antioxidants than in their absence depending on the content of amino and carboxylic end-groups. Because antioxidants react with peroxy radicals by several consecutive inhibition reactions with different yields of chemiluminescence, the intensity of chemiluminiscence also depends on the rate of initiation of the oxidation. The relation between the intensity of chemiluminescence and the rate of oxygen consumption during oxidation of stabilized polyamides differed profoundly for various polyamides depending on their end-group content, on the concentration of antioxidant and also on the conditions of oxidation, especially on the rate of initiation determining the rate of antioxidant consumption. Redox inhibition reactions were found to be the predominant source of chemiluminescence accompanying the oxidation of N-alkylamides in the presence of antioxidants.

Photooxidation of fire retarded polypropylene: III. Mechanism of HAS inactivation

This paper, which is the third of a series dealing with the behaviour of fire retarded polypropylene on exposure to UV light, is devoted to the photostabilisation aspects. The influence of various Hindered Amine Stabilisers (HAS) on the stability of fire retarded polypropylene samples, exposed to an irradiation in artificial accelerated media, is reported. The kinetic curves of oxidation emphasise the poor efficiency of the studied stabilisers in fire retarded media. The antagonisms between fire retardant and stabilisers are discussed. The FTIR analysis indicate that the structure and the distribution of the stabiliser in the irradiated sample are subject to severe modifications upon irradiation. Particularly, the formation of amine salts can be observed at the surface of thick samples. This phenomenon is related to the reaction of the amine groups of the stabiliser with hydrobromic acid generated by the halogenated fire retardant. The migration of the HAS, from the core of the sample towards the surface, is also observed. The reduced efficiency of the studied HAS to stabilise polypropylene is attributed to the formation of amine salt, that prevents the stabiliser from being converted into the active forms (i.e. nitroxy radical and alkoxyamine). An improved stability can be obtained by varying the substitution on the nitrogen atom of the HAS molecule, particularly in the case of the N-oxyl substituted HAS.

High‐spin behavior of molecular crystals and extended π systems

The qualitative rules for the existence of high-spin ground states in extended systems and molecular crystals are examined here on a firmer theoretical footing. Extended systems have been categorized into three groups, namely, type I, type II, and type III, depending on the type of bonding interactions. The general form of the spin Hamiltonian operators have been written down. The active spaces have been restricted to the minimum size for each of these three types of spin systems. The zeroth-order state vectors and the Hartree–Fock ground-state energies have been identified for unit species of each type. The extended system Hamiltonian operators are further truncated in such a way that only the nearest-neighbor interactions are retained. Expressions have been derived for the energy gap from a molecular orbital approach. The relatively small effects of electron correlation on the energy gaps have been estimated for the type I systems, which belong to the systems of solid-state physics. In particular, it has been shown that for the type I systems the singlet–triplet gap, and hence the ferromagnetic coupling constant, primarily depends upon the difference of one-electron kinetic energies and not on the two-electron exchange integrals. This result agrees with the concept of kinetic exchange that was introduced in the context of a resonating valence-bond formalism. Type II systems are exemplified by extended systems that can be prepared from conjugated molecules while organic molecular crystals form examples of type III species. For these systems, however, the Coulomb exchange interaction has been shown to dominate the energy gap. A quick review of the Heisenberg spin Hamiltonian for the H2 molecule is sufficient to point out that the sign of the calculated ferromagnetic coupling constant depends on the method of calculation, the nature of the basis set, and the bond length. This is amply supported by ab initio calculations on this species. Numerical data have also been obtained from computations on m-phenylene-coupled nitroxy radicals and stacks of α-nitronyl nitroxide, but these calculations have been based on a semiempirical quantum chemical methodology (INDO) since some of the species involved are exceedingly large. Computed energy gaps are in good agreement with experimental and other theoretical (AM1, PM3) results. Nevertheless, for the dimer, trimer, tetramer, and pentamer of the type II specimen, the important π orbitals are far from being degenerate. The quantitative results clearly deviate from the criterion of degeneracy that was suggested from qualitative theories for the existence of a high-spin ground state. Therefore, the criteria for the existence of high spins have been reformulated in terms of the monomer orbitals. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 308–324, 2000

Study of some parameters responsible for the efficiency of hindered amine light stabilizers

The photooxidation of a styrene-butadiene-styrene block copolymer previously hydroperoxidized by reaction with singlet oxygen has been studied by measuring the increase of the carbonyl absorbance at 1720 cm−1. Five different hindered amine light stabilizers (HALS) have been introduced into the polymer by diffusion from methanolic solutions, and their efficiencies as inhibitors of the photooxidation have been compared. The sequence of efficiency observed is: tert. amine > sec. amine > amino ether Under the experimental conditions used HALS are localized preferentially in oxidized regions of the polymer, and their efficiency can be correlated with their ability to catalyze the decomposition of clustered hydroperoxides. As a consequence, HALS are oxidized to nitroxy radicals that are the actual inhibitors of the photooxidation. A high local concentration of HALS in oxidized regions of the polymer also contributes to their efficiency, and this effect justifies the observation that some tertiary amines are more efficient than others.

Synergy Effects of Binary and Ternary Mixtures of Inhibitors in the Process of Polypropylene Autooxidation

The autooxidation of the solid isotactic polypropylene, inhibited with three different additives-phenol, amine and nitroxy radical as well as their binary and tertiary mixtures was investigated. Noticeable synergy effects were observed for inhibiting systems: phenol-amine and phenol - nitroxy radical; these effects were especially high in the region of low concentrations of inhibitors, in the so called subcritical region. Mathematical model was proposed for the evaluation of contribution of each inhibitor to the induction period. Possibility of such description was confirmed by comparison of the experimental and calculated data.

The oxidation of hindered amine light stabilisers to nitroxy radicals in solution and in polymers. Part II

Different types of hindered amine light stabilisers (HALS) have been oxidised by peroxy radicals generated by gamma irradiation of air saturated cyclohexane solutions. In all cases the concentration of nitroxy radicals, measured by EPR spectroscopy, increases linearly with absorbed dose. The sequence of reactivity observed is: amino ether > sec. amine > tert. amine The same HALS have been oxidised in a hydroperoxidised styrenebutadiene-styrene block copolymer by peroxy radicals resulting from the decomposition of clustered hydroperoxides. At room temperature the nitroxy radical concentration slowly increases with time and tends to a limit corresponding to the fraction of HALS molecules that are in a suitable environment to be oxidised. The sequence of reactivity observed is the reverse of that mentioned above. In this case, it reflects the ability of the different HALS to catalyse the decomposition of clustered hydroperoxides.

Stabilization of polyamides—II. Reactions of antioxidants with peroxy radicals of 6-hexanelactam

The termination reactions in the oxidation of 6-hexanelactam were studied in the presence of antioxidants near 100 °C under conditions where oxygen diffusion does not control the oxidation. Phenols, secondary aromatic amines and also nitroxy radicals of piperidine derivatives used as antioxidants react in inhibition reactions with peroxy radicals of 6-hexanelactam. With the representatives of the given types of antioxidants, rate constants of these reactions and the limit concentrations required for breaking the kinetic chain in the lactam oxidation were determined and the oxidation products of lactam molecules were identified for the individual stages of oxidation. Under given conditions, one molecule of phenol, piperidine N-oxyl and secondary aromatic amine are able to break two, three and ten oxidation chains of 6-hexanelactam, respectively.