Aromatic Free Radicals Research Articles

The behavior of free radicals during the carbonization of anthracite briquette

The variation in behavior of free radicals during the pyrolysis and co-pyrolysis of anthracite and coal-based binder (AS) was determined by Electron paramagnetic resonance (EPR). Besides, Thermogravimetry (TG) and Carbon-13 nuclear magnetic resonance (13C NMR) were also applied to investigate the nature of pyrolysis and structural characterization of the coal samples. The results indicated that the induced effect of free radical existed in the co-pyrolysis of anthracite and AS, such that the free radical concentration (Ng) of AS carbonized briquette (ASB) at 300–500 °C is far higher than the additive value of Ng when anthracite and AS are separately carbonized. When the temperature exceeds 500 °C, the timely release of AS free radicals could combine with the aromatic free radicals of anthracite, leading to a slight decrease in the Ng value of ASB. Due to the limited combination, the rapid polycondensation of aromatic free radicals in anthracite was inhibited and more number of short side-chains and cross-links were generated in ASB. These effects delay the major polycondensation process of ASB to a higher temperature compared with other samples. As a result, the Ng value of ASB drops significantly above 700 °C and the line shape of it transforms closer to the Lorentz line shape. The lapping of free radical variation in anthracite and binder would broaden the temperature range for bonding-polymerization, which makes the free radicals of the blend components to rearrange and to form strong carbon–carbon bonds. Thus, the mechanical strength (M40) of carbonized briquette would be improved.

Thermal degradation of azobenzene dyes

Ten azobenzene dyes were tested for their thermal degradation products from ambient temperature to 800 °C using a thermogravimetric technique. Degradation products were analyzed at each 100 °C increment using gas chromatography-mass spectrometry. Products were identified based on their mass spectral data and their retention times. Thermal degradation from all dyes resulted in loss of the azo group as nitrogen gas and aromatic free radicals. Electron withdrawing groups enhance the dissociation of the phenyl nitrogen bond at the benzene moiety carrying the electron withdrawing group. On the other hand, electron donating groups inhibited the dissociation of the phenyl-N bond of the benzene moiety carrying the electron donating groups. The loss of nitrogen molecule from the dyes appeared to be taken place in a two-step rather than a one-step reaction in which the phenyl nitrogen bond of the phenyl group carrying the electron withdrawing group splits to form phenyl and azo phenyl radicals following by the loss of a nitrogen molecule and a new phenyl radical. This was also supported by calculating degradation reaction enthalpies and bond dissociation energies using density functional theory quantum mechanics.

Electron transfer in biologically important systems: Polycyclic aromatic hydrocarbons, DNA bases and free radicals

Occurrence of electron transfer was studied for different combinations of polycyclic aromatic hydrocarbons (PAHs) and DNA bases as electron donors or acceptors and free radicals only as electron acceptors. Geometries of all the molecules and radicals were optimized in aqueous medium employing the polarizable continuum model. Single electron transfer (SET) and sequential proton loss electron transfer mechanisms were investigated employing Gibbs free energies of the appropriate neutral, anionic and cationic species. Barrier energies involved in these phenomena were calculated using the Marcus theory. The SET barrier energies were found to be linearly correlated with [Formula: see text] (Electron affinities of acceptors – Ionization potentials of donors). SET barrier energies from the DNA bases to the PAHs follow the order Cy [Formula: see text] Th [Formula: see text] Ad [Formula: see text] Gu, whereas SET barrier energies from the PAHs to the DNA bases follow the order Gu [Formula: see text] Ad [Formula: see text] Th [Formula: see text] Cy. Thus, guanine, among the DNA bases, is the best electron donor to the PAHs and worst electron acceptor from the same.

BDPA-Doped Polystyrene Beads as Polarization Agents for DNP-NMR.

The aromatic free radical BDPA (α,γ-bisdiphenylene-β-phenylallyl), which has been widely used as a polarizing agent for Dynamic Nuclear Polarization (DNP) of hydrophobic analytes, has been incorporated into nanometer-scale polystyrene latex beads. We have shown that the resulting BDPA-doped beads can be used to hyperpolarize (13)C and (7)Li nuclei in aqueous environments, without the need for a glassing cosolvent. DNP enhancement factors of between 20 and 100 were achieved with overall BDPA concentrations of 2 mM or less. These Highly-Effective Polymer/Radical Beads (HYPR-beads) have potential use as an inexpensive polarizing agent for water-soluble analytes, and also have applications as model nanoparticles in DNP studies.

Solid-state NMR and ESR studies of activated carbons produced from pecan shells

A large number of solid-state NMR and ESR experiments were explored as potential tools to study chemical structure, mobility, and pore volume of activated carbon. We used a model system where pecan shells were activated with phosphoric acid, and carbonized at 450°C for 4h with varying amounts of air flow. Through the use of different NMR experiments (e.g., CP-MAS, SPE-MAS, and DD-MAS) several structural parameters were calculated such as mole fraction of bridgehead aromatic carbons, number of carbons per aromatic ring system, and number of phenolic carbons per aromatic ring system. The relaxation time measurements (T1, TCH, and T1ρH) were indicative of the relative mobility of different structural units. ESR spectra showed the presence of π-type aromatic free radicals in the carbonized samples with a slight shift in g value with increasing oxidation. The combined NMR and ESR data give a consistent picture of the carbon structure and the carbonization process, which is not easily available otherwise. In addition, the 1H NMR data on adsorbed water are shown to be consistent with the trends in the amount of pore volumes for different samples of activated carbons.

Vibronic Emission Spectra of Jet-Cooled o-Cyanobenzyl Radical

Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Pusan 609-735, KoreaReceived April 1, 2004Key Words : Spectroscopy, Benzyl-type radicals, Electronic tran sition, Corona discharg e, Supersonic expansionBenzyl radical is a prototypical aromatic free radical thathas been the subject of numerous spectroscopic studies.

The Synthesis of Aminobenzothiazoles from 2,3-Biaryl-5-anilino-Δ3-1,2,4-thiadiazolines

2-Amidinobenzothiazoles 4 can be prepared in high yields by a thermally-promoted rearrangement of thiadiazolium salts 1 or thiadiazolines 2. Addition of base to the rearrangement of the thiadiazolium salts can improve the yields by the prior conversion of the thiadiazolium salts to the corresponding thiadiazoline free bases. The rearrangement of the thiadiazolines may go through an electrophilic aromatic substitution or free radical pathway.

Flash photolysis and triplet states and free radicals in solution.

A personal account is given of the development of microsecond flash photolysis in George Porter's laboratory at Cambridge in the early 1950s. This made possible, for the first time, the observation of the absorption spectra of the lowest triplet states of many polycyclic aromatic hydrocarbons in fluid solutions. The T-T transitions were found to be very intense with oscillator strengths approaching unity in many cases. Detailed kinetic studies showed that triplet state decay was first order and viscosity dependent and strongly quenched by dissolved oxygen. Several aromatic free radicals such as benzyl were also observed in liquid solution for the first time. The advent of the laser in the early 1960s made it possible to extend flash photolysis studies to the ns and ps and eventually the fs time range. Such studies have revolutionized the study of excited states and free radicals and have made possible advances in the understanding of the dynamics of chemical and biological systems that would have been unimaginable 60 years ago.

The mechanism of spin polarization in aromatic free radicals

The spin-polarization mechanism in aromatic systems is analyzed with reference to the prototypical phenoxyl, cyclohexadienyl and benzyl radicals. In particular, a decomposition into “first-order” and “second-order” contributions is proposed, which helps to rationalize the different nature of the spin density for atoms in α or in β positions with respect to the radical center. The different weights of the two contributions are discussed on the basis of Hartree–Fock and density functional computations.

Key role of the medium in methemalbumin-catalyzed peroxidation of aromatic free radical scavengers and antioxidants

Participation of the complexes of hemin and albumins (or delipidated albumins) in peroxidation of aromatic free radical scavengers and antioxidants was studied at varying hemin/albumin ratios. The radicalscavenging amines includedo-phenylenediamine (OPD) and tetramethylbenzidine (TMB); the antioxidants were gallic acid (GA) and GA polydisulfide (GAPD). Peroxidation reactions were carried out in buffered physiological saline (BPS) supplemented with 2% dimethylsulfoxide (DMSO), pH 7.4 (medium A), or in 40% aqueous dimethylformamide (DMF), pH 7.4 (medium B). In all systems involving methemalbumins, kinetic constants (kcat), Michaelis constants(K M), and the ratios thereof(k cat/KM) were determined for OPD oxidation in medium A and TMB oxidation in medium B. Oxidation of OPD, GA, and GAPD in medium A was characterized by a decrease in the catalytic activity of hemin after the formation of hemin-albumin complexes. Conversely, oxidation of TMB and OPD in medium B was distinguished by pronounced activation of hemin present within methemalbumins.

A tyrosine-derived free radical in apogalactose oxidase.

Oxidation of apogalactose oxidase with ferricyanide leads to the formation of a stable free radical exhibiting distinctive optical absorption and EPR spectral features. The radical is associated with absorption in both near-UV and near-IR spectral regions, and its EPR spectrum is characteristic of an aromatic free radical with gav = 2.005. Reconstitution of both the apoenzyme and the free radical-containing form with copper substantially restores both the absorption spectra and the catalytic activity of the active enzyme, indicating that the preparation of the radical species does not significantly damage the protein. The absence of a free radical EPR signal in reconstituted and activated galactose oxidase containing nearly stoichiometric copper suggests the radical is an active site species relating to the free radical-coupled copper site previously proposed for this enzyme. Isotopic labeling experiments demonstrate that the radical derives from a tyrosine residue. The distinctive spectra associated with this radical indicate an environment which is different from that associated with the tyrosyl phenoxyl sites in other free radical enzymes.

Ab Initio Calculations of Effective Exchange Integrals. Possibilities of Superparamagnetic, Mictomagnetic and Amorphous Feromagnetic States for Aggregates of Aromatic Free Radicals and Polymer Radicals

Abstract The intermolecular effective exchange integrals (Jab) for clusters of aromatic free radicals were calculated by the ab initio spin-projected unrestricted Hartree-Fock (UHF) Moller-Plesset (APUMP) method. It was found that the sign and magnitude of the calculated Jab-values vary with the stacking modes of the species. Implications of the ab initio results were discussed in relation to possibilities of spin glass, superparamagnetic, micto-magnetic and amorphous ferromagnetic states for aggregates of aromatic free radicals and polymer radicals.

Resonance Raman evidence for tyrosine involvement in the radical site of galactose oxidase

Resonance Raman data are reported for the redox-activated form of galactose oxidase from Dactylium dendroides. Excitation within the red (659 nm) and blue (457.9 nm) absorption bands leads to strong resonance enhancement of ligated tyrosine vibrational modes at 550, 1170, 1247, 1484, and 1595 cm-1. The ring mode frequencies are unusually low, indicating a decreased bond order in the ring. The spectra clearly differ in both frequencies and relative intensities from those characteristic of known aromatic pi-radicals. Enhancement of tyrosine ring modes on excitation within absorption bands previously associated with the presence of the radical in the active site suggests that the ligated tyrosine residue is present in the radical site and may stabilize this radical species through formation of a charge transfer complex. A dramatically different Raman spectrum is observed for the N3- adduct of galactose oxidase, exhibiting a single strong 1483 cm-1 feature. The intense visible-near IR absorption bands for galactose oxidase may derive from transitions within a charge transfer complex between an aromatic free radical and a tyrosine-copper complex.

Neoplastic and other diseases in fish in relation to toxic chemicals: an overview

Field studies were conducted over a 5-year period in Puget Sound to investigate etiological relationships between prevalences of hepatic neoplasms in bottom-dwelling marine fish, with emphasis on English sole ( Parophrys vetulus), and concentrations of toxic chemicals in sediments and bottom fish. Statistically significant correlations were found between prevalences of hepatic neoplasms and (1) sediment concentrations of aromatic hydrocarbons, and (2) concentrations of metabolites of aromatic compounds in the bile. A significant difference was also found between the concentrations of N-oxyl derivatives of nitrogen heterocycles (free radicals) in liver microsomes of English sole with liver lesions compared to sole without liver lesions. These N-oxyl free radicals were apparently metabolically derived from complex suites of nitrogen heterocycles present in Puget Sound environments. Recent evidence suggests that aromatic free radicals (possibly derivatives of aromatic hydrocarbons) may also be bound to hepatic DNA isolated from English sole with liver neoplasms collected in several polluted areas of Puget Sound. Such evidence for xenobiotic-DNA interactions has thus far not been obtained for fish from reference areas and for lesion-free fish from polluted areas. Various laboratory studies designed to evaluate the etiology of the liver neoplasms have also yielded evidence that is consistent with the view that high molecular weight aromatic hydrocarbons play a significant role in the etiology of hepatic tumors in the bottom-dwelling fish. Associations between chemical exposures and diseases in other marine species from southern California and Boston Harbor are also discussed. Similar relationships between liver and skin neoplasms in selected bottom feeding fresh water species and sediment-associated chemicals are also presented.

Laser spectroscopy of fast processes in vapors of organic compounds

A study was made of the dynamics and mechanisms of fast bimolecular processes of formation of intermolecular exciplexes and free radicals as a result of laser photolysis of molecules. Luminescence was observed and lasing was achieved in the case of aromatic free radicals in the gaseous phase.

Field and laboratory studies of the etiology of liver neoplasms in marine fish from Puget Sound.

A series of field studies was conducted between 1979 and 1985 in Puget Sound, Washington State, to investigate etiological relationships between prevalences of hepatic neoplasms in bottom-dwelling marine fish species, with emphasis on English sole (Parophrys vetulus), and concentrations of toxic chemicals in sediments and affected fish. Statistically significant (p less than or equal to 0.05) correlations have been found between the prevalences of hepatic neoplasms in English sole and the following parameters: sediment concentrations of aromatic hydrocarbons, and concentrations of the metabolites of aromatic compounds in the bile of affected sole. A significant difference (p less than 0.001) was also found between the relative concentrations of aromatic free radicals in the liver microsomes of English sole with liver lesions compared to sole without liver lesions. Laboratory studies designed to evaluate the etiology of the liver neoplasms in English sole have also yielded evidence that is consistent with the view that high molecular weight aromatic hydrocarbons, e.g., benzo[a]pyrene (BaP), are hepatocarcinogens in English sole. The current status of a series of long-term (up to 18 months) exposures of English sole and rainbow trout (Salmo gairdneri) to selected fractions of Puget Sound sediment extracts, enriched with aromatic hydrocarbons and nitrogen-containing aromatic compounds, and to individual carcinogens (e.g., BaP) is discussed.

Field and Laboratory Studies of the Etiology of Liver Neoplasms in Marine Fish from Puget Sound

A series of field studies was conducted between 1979 and 1985 in Puget Sound, Washington State, to investigate etiological relationships between prevalences of hepatic neoplasms in bottom-dwelling marine fish species, with emphasis on English sole (Parophrys vetulus), and concentrations of toxic chemicals in sediments and affected fish. Statistically significant (p less than or equal to 0.05) correlations have been found between the prevalences of hepatic neoplasms in English sole and the following parameters: sediment concentrations of aromatic hydrocarbons, and concentrations of the metabolites of aromatic compounds in the bile of affected sole. A significant difference (p less than 0.001) was also found between the relative concentrations of aromatic free radicals in the liver microsomes of English sole with liver lesions compared to sole without liver lesions. Laboratory studies designed to evaluate the etiology of the liver neoplasms in English sole have also yielded evidence that is consistent with the view that high molecular weight aromatic hydrocarbons, e.g., benzo[a]pyrene (BaP), are hepatocarcinogens in English sole. The current status of a series of long-term (up to 18 months) exposures of English sole and rainbow trout (Salmo gairdneri) to selected fractions of Puget Sound sediment extracts, enriched with aromatic hydrocarbons and nitrogen-containing aromatic compounds, and to individual carcinogens (e.g., BaP) is discussed.

The pyrolysis of acenaphthylene studied by proton nuclear magnetic resonance

The thermally induced transformations of acenaphthylene during pyrolysis at temperatures up to 800 K have been monitored dynamically by pulsed-proton NMR thermal analysis techniques. Measurements have been made of both the transverse and the spin-lattice relaxation properties of the protons during heating at 2, 4 and 8 K/min and subsequent isothermal treatment for up to 300 min. As well as clearly delineating the melting of acenaphthylene and the formation and decomposition of polyacenaphthylene, these two NMR properties have been used to follow the transformation of the high-temperature pitch to produce anisotropic mesophase and semicoke. It is concluded from these NMR results, together with supplementary information obtained from differential scanning calorimetry (DSC) experiments and from examination of the pyrolysis residues by ESR, NMR and polarized light microscopy, that 1. (a) the free radical concentration attained by the intermediate pitch during uniform heating is critical in determining the transformation rate of the pitch, which is found to be greater both for pitches prepared by slower heating and for those treated at higher temperatures 2. (b) the preparation of the intermediate pitch by more rapid heating is conducive to mesophase formation during subsequent isothermal treatment because it extends the duration of high molecular mobility, and 3. (c) the appreciable concentration of aromatic free radicals in the residue decreases significantly with time after cooling to room temperature if condensation and cross-linking reactions have not progressed sufficiently during the high-temperature treatment of the pitch.

Fast liquid-phase bimolecular reactions of aromatic free radicals

Fast liquid-phase bimolecular reactions of aromatic free radicals

Solvent effect on reversible self‐termination reactions of aromatic free radicals

AbstractRates and thermodynamic data have been obtained for the reversible self‐termination reaction: Involving aromatic 2‐(4′dimethylaminophenyl)indandione‐1,3‐yl (I), 2‐(4′diphenylaminophenyl)indandione‐1,3‐yl (II), and 2,6 di‐tert‐butyl‐4‐(β‐phthalylvinyl)‐phenoxyl (III) radicals in different solvents. The type of solvent does not tangibly affect the 2k1 of Radical(I), obviously due to a compensation effect. The log(2k1) versus solvent parameter ET(30) curves for the recombination of radicals (II) and (III) have been found to be V shaped, the minimum corresponding to chloroform. The intensive solvation of Radical (II) by chloroform converts the initially diffusion‐controlled recombination of the radical into an activated reaction. The log (2k−1) of the dimer of Radical (I) has been found to be a linear function of the Kirkwood parameter (ε ‐ 1)/(2ε + 1), the dissociation rate increasing with the dielectic constant of the solvent. The investigation revealed an isokinetic relationship for the decay of the dimer of Radical (I), an isokinetic temperature β = 408 K and isoequilibrium relationship for the reversible recombination of Radical (I) with β° = 651 K. For Radical (I) dimer decay In(2k−1) = const + 0.8 In K, where K is the equilibrium constant of this reversible reaction. The transition state of Radical (I) dimer dissociation reaction looks more like a pair of radicals than the initial dimer. The role of specific solvation in radical self‐termination reactions is discussed.