Aromatic Radicals Research Articles

Infrared and Electronic Spectroscopy of Benzene−Ammonia Cluster Radical Cations [C6H6(NH3)1,2]+: Observation of Isolated and Microsolvated σ-Complexes

We report infrared (IR) and electronic spectra of benzene-ammonia cluster radical cations [C(6)H(6)(NH(3))(n)](+) (n = 1 and 2) in the gas phase to explore cluster structures and chemical reactivity of the simplest aromatic radical cation with base (nucleophile) molecules. The electronic spectra in the visible region indicate that these cluster cations no longer have the benzene cation chromophore as a result of an intracluster reaction. Analyses of the IR spectra, on the basis quantum chemical calculations and the vibration-internal rotation analysis, reveal that both [C(6)H(6)(NH(3))(1,2)](+) form σ-complex structures, in which the ammonia moiety is covalently bonded to the benzene moiety due to the intracluster nucleophilic addition. For [C(6)H(6)(NH(3))(2)](+), it is also shown that the second ammonia molecule solvates the σ-complex core via a N-H···N hydrogen bond. Such σ-complex structures are generally supposed to be a key intermediate of aromatic substitution reactions. The observed mass spectra and energetics calculations, however, show that [C(6)H(6)(NH(3))(n)](+) systems are inert for aromatic substitutions. The present experimental observations indicate the inherent stability of these σ-complex structures, even though they do not show the aromatic substitution reactivity.

First Principles Quantum Dynamical Investigation Provides Evidence for the Role of Polycyclic Aromatic Hydrocarbon Radical Cations in Interstellar Physics

Inspired by the recent astronomical discovery of new diffused interstellar bands (DIBs) assigned to the electronic transitions in the naphthalene radical cation based on complementary laboratory measurements, we attempt here an ab initio quantum dynamical study to validate this assignment. In addition, the existence and mechanistic details of nonradiative deactivation of electronically excited polycyclic aromatic hydrocarbon (PAH) radical cations in the interstellar medium and their identity as carriers of DIBs are established here focusing on the prototypical naphthalene and anthracene radical cations of the PAH family.

Enhanced Reactivity of [Hydroxy(tosyloxy)iodo]benzene in Fluoroalcohol Media. Efficient Direct Synthesis of Thienyl(aryl)iodonium Salts

In this manuscript, we report clear evidence for the generation of aromatic cation radicals produced by using [hydroxy(tosyloxy)iodo]benzene (HTIB) in fluoroalcohol solvents such as 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The single-electron-transfer (SET) oxidation ability of HTIB to give cation radicals was first established by ESR and UV measurements. The reaction was broadly applied to various thiophenes, and unique thienyliodonium salts were directly synthesized by this method in excellent yields without the production of any harmful byproducts.

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.

Estimation of standard reduction potentials of alkyl radicals involved in atom transfer radical polymerization

The redox properties of some alkyl radicals, which are important in atom transfer radical polymerization both as initiators and mimics of the propagating radical chains, have been investigated in CH 3CN by an indirect electrochemical method based on homogeneous redox catalysis involving alkyl halides (RX) and electrogenerated aromatic or heteroaromatic radical anions (D −). Dissociative electron transfer between RX and D − yields an intermediate radical (R ), which further reacts with D − either by radical coupling or by electron transfer. Examination of the competition between these reactions, which depends on E D/ D − ° , allows determination of the standard reduction potential of R as well as the self-exchange reorganization energy λ R / R − . The standard reduction potentials obtained for the radicals CH 2CN, CH 2CO 2Et and CH(CH 3)CO 2Me are −0.72 ± 0.06, −0.63 ± 0.07 and −0.66 ± 0.07 V vs. SCE, respectively. Quite high values of λ R / R − (from 122 to 164 kJ mol −1) were found for all radicals, indicating that a significant change of structure accompanies electron transfer to R .

Investigating the Mechanisms of Aromatic Amine-Induced Protein Free Radical Formation by Quantitative Structure−Activity Relationships: Implications for Drug-Induced Agranulocytosis

Aromatic amine drugs have been associated with agranulocytosis (neutrophil depletion) for which the mechanism is unknown. We have previously shown that the metabolism of two aromatic amine drugs by human myeloperoxidase (MPO) results in phenyl radical metabolite formation and also in protein free radical formation on MPO. Because the concentration of drug required to produce a maximum signal for MPO protein free radical (MPO*) detection was different for each drug, this prompted us to consider that other aromatic amines may also show varying degrees of ability to induce MPO* formation. Immunoassay experiments using the immuno-spin-trapping technique were performed, which evaluated the potency of different aromatic amines containing the aniline substructure to generate the MPO*. Each reaction contained equal amounts of H(2)O(2), 5,5-dimethyl-1-pyrroline-N-oxide, MPO, and variable concentrations of aniline derivatives. Several physicochemical parameters for aniline derivatives were used to derive quantitative structure-activity relationship equations, which showed that the Hammett constant (sigma) best correlated with the MPO* formation for all aniline derivatives. More statistically robust equations were derived if the anilines were separated into mono- and disubstituted groups. However, some aniline derivatives did not induce MPO* formation. Using electron spin resonance spectroscopy, we evaluated the ability of all aniline derivatives tested to produce phenyl radical metabolites, as previously shown by spin trapping for the aromatic amine drugs. Interestingly, we found that only those aniline derivatives that produced a phenyl radical also formed MPO*. We propose that the phenyl radical is the reactive free radical metabolite responsible for generating the MPO*.

Formation of the Phenyl Radical [C6H5(X2A1)] under Single Collision Conditions: A Crossed Molecular Beam and ab Initio Study

Reactions of dicarbon molecules (C(2)) with C(4)H(6) isomers such as 1,3-butadiene represent a potential, but hitherto unnoticed, route to synthesize the first aromatic C(6) ring in hydrocarbon flames and in the interstellar medium where concentrations of dicarbon transient species are significant. Here, crossed molecular beams experiments of dicarbon molecules in their X(1)Sigma(g)(+) electronic ground state and in the first electronically excited a(3)Pi(u) state have been conducted with 1,3-butadiene and two partially deuterated counterparts (1,1,4,4-D4-1,3-butadiene and 2,3-D2-1,3-butadiene) at two collision energies of 12.7 and 33.7 kJ mol(-1). Combining these scattering experiments with electronic structure and RRKM calculations on the singlet and triplet C(6)H(6) surfaces, our investigation reveals that the aromatic phenyl radical is formed predominantly on the triplet surface via indirect scattering dynamics through a long-lived reaction intermediate. Initiated by a barrierless addition of triplet dicarbon to one of the terminal carbon atoms of 1,3-butadiene, the collision complex undergoes trans-cis isomerization followed by ring closure and hydrogen migration prior to hydrogen atom elimination, ultimately forming the phenyl radical. The latter step emits the hydrogen atom almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. On the singlet surface, smaller contributions of phenyl radical could not be excluded; experiments with partially deuterated 1,3-butadiene indicate the formation of the thermodynamically less stable acyclic H(2)CCHCCCCH(2) isomer. This study presents the very first experimental evidence, contemplated by theoretical studies, that under single collision conditions an aromatic hydrocarbon molecule can be formed in a bimolecular gas-phase reaction via reaction of two acyclic molecules involving cyclization processes at collision energies highly relevant to combustion flames.

Numerical Simulation of the Partial Oxidation of Hot Coke Oven Gas with a Detailed Chemical Kinetic Model†

The non-catalytic partial oxidation of hot coke oven gas (HCOG) was numerically simulated using a detailed chemical kinetic model and a completely mixed batch reactor model. The kinetic model was primarily based on that developed by Richter and Howard [Phys. Chem. Chem. Phys. 2002, 4 (11), 2038−2055], including more than 200 chemical species and more than 2000 elementary-step-like reactions. The HCOG was modeled as a multi-component gas mixture involving H2, CO, CO2, CH4, C2 hydrocarbons, H2O, and 31 aromatic hydrocarbons, such as benzene and toluene, as well as polycyclic aromatic hydrocarbons up to coronene, to represent the HCOG tar. The effect of oxygen addition in the inlet gas mixture was investigated at oxygen concentrations from 0 to 15 vol %. The simulations indicated that oxygen was consumed almost completely for the combustion of reactive light gases, such as H2 and CO, and light hydrocarbons, such as CH4 and C2H6, within a reaction time of a few tens of milliseconds when the inlet gas temperature was 1173 K. In reforming the tar involved in HCOG, the primary role of oxygen should be to induce temperature increases of the reacting gas by combustion, thereby accelerating the subsequent reforming of the tar by steam. A reaction pathway analysis suggested that the co-existence of oxygen and aromatic hydrocarbon radicals was necessary to decompose aromatic compounds into compounds of lower molecular mass.

Photolysis of o-iodo- and o-bromophenols in aqueous solution of sodium sulfite and organic solvents

The photochemical reaction of o-iodo- and o-bromophenol in an aqueous sodium sulfite solution proceeds via both nonchain and chain mechanisms. The formation of the intermediate product, aromatic radical anion, was observed. The quantum yield of the photochemical reaction of o-iodophenol increases, when the electron donor diphenylamine is irradiated. In the photolysis of o-halophenols in organic solvents, free iodine is evolved in addition to aromatic products. The products of the photolysis of o-iodophenol in ethanol and carbon tetrachloride were identified by gas chromatography-mass spectrometry. These are phenol in the case of ethanol and a mixture of o-chlorophenol and hydroxychloro-substituted biphenyls in the case of carbon tetrachloride. The quantum yields were determined for all photochemical reactions studied.

Effects in 94 GHz Orientation-Selected PELDOR on a Rigid Pair of Radicals with Non-Collinear Axes

For aromatic organic radicals, pulsed electron-electron double resonance (PELDOR) experiments at high magnetic fields provide information not only about the distance between the paramagnetic species but also about their relative orientation. However, the three-dimensional biradical structure is encoded in a complex pattern of orientation-selected PELDOR traces and the execution of the experiment is generally aggravated by constraints posed by the available hardware and the intrinsically low modulation depth observed. We present a 94 GHz PELDOR experiment performed with a commercial spectrometer and probe heads that permit separation of pump and detection frequencies up to 150 MHz. The setup is employed to examine the orientation selections on a general case of rigid biradicals with non-collinear g axes. The interacting radicals, a tyrosyl radical (Y122·) located in the β2 subunit and an 3-aminotyrosyl radical (NH2Y731·) located in the α2 subunit, are generated by Escherichia coli ribonucleotide reductase with a 3-aminotyrosine (NH2Y) site specifically incorporated into α2 in the presence of cytidine 5′-diphosphate and adenosine 5′-triphosphate. The experimental designs as well as some characteristic features of the observed modulation pattern are discussed.

Impact of Chlorine Substitution on Spin–Lattice Relaxation of Triarylmethyl and 1,4-Benzosemiquinone Radicals in Glass-Forming Solvents Between 25 and 295 K

Spin-lattice relaxation rates measured by long-pulse saturation recovery in glassy solvents for chlorinated aromatic radicals: perchlorotriphenylmethyl radical, 2,5-dichloro-3,6-dihydroxy-1,4-benzosemiquinone, and tetrachloro-1,4-benzosemiquinone, were compared with relaxation rates for non-chlorinated analogs. The impact of the quadrupolar chlorines is small, and less than the effects of changing the rigidity of the glass. The temperature dependence of relaxation rates below the glass transition temperature could be modeled as the sum of contributions from the direct, Raman, and local mode processes.

Hypervalent iodine(III): selective and efficient single-electron-transfer (SET) oxidizing agent

In 1994, we first determined the single-electron-transfer (SET) oxidation ability of phenyliodine(III) bis(trifluoroacetate) (PIFA) toward phenyl ethers, affording the corresponding aromatic cation radicals. Since then, hypervalent iodine(III) has been utilized as a selective and efficient SET oxidizing agent that enables a variety of direct C–H functionalizations of aromatic rings in electron-rich arenes under mild conditions. We have now extended the original method to work in a series of heteroaromatic compounds such as thiophenes, pyrroles, and indoles. The investigations and results obtained since the start of this century are summarized in this article.

Interpreting the Physical Background of Empirical Solvent Polarity via Photodetachment Spectroscopy of Microsolvated Aromatic Ketyl Anions

The physical background of empirical solvent polarity is explored in regard to trends in solute-solvent intermolecular potential energy functions. Aromatic ketyl anions, benzophenone, and 9-fluorenone radical anions, are chosen for a model solute molecule showing solvatochromic behavior similar to betaine-30 dye, which provides the most established solvent polarity scale, E(T)(30). Common features among the ketyl anions and betaine-30 were examined with quantum chemical calculations for the electronic states and solvation structure. Vertical photodetachment and photoabsorption energies were determined for the ketyl anions microsolvated with a single solvent molecule by measuring photoelectron spectra as well as photodetachment excitation spectra for several aprotic and protic solvents. The spectroscopic data were analyzed through quantum chemical calculations based on density functional theory, and their relationship with the characteristics of intermolecular potential energies was considered. As a result, the typical solvent polarity parameter can be interpreted to reflect essentially the gradient of a potential energy function (namely, the strength of force) between a negative charge and the solvent molecules in the attractive region. A large polarity for protic solvents is attributed to an effective interaction of a proton-like hydrogen atom with the negative charge in a short-range.

Pseudorotation as a possible origin of fast paramagnetic relaxation in radical ions with a degenerate or quasi-degenerate ground state

In early 1960s, aromatic radical ions with a three-fold or higher symmetry axis were shown to haveanomalously high spin–lattice relaxation rates [1, 2].For example, the paramagnetic relaxation rate in ben-zene and coronene radical anions in solution turned outto be one to two orders of magnitude higher than in rad-icals of lower symmetry. An even higher relaxation ratewas later revealed in the fullerene radical anion [3] andin radical cations of some cycloalkanes [4–6].The observed anomalously fast spin–lattice relax-ation in radical ions of high-symmetry molecules insolution cannot be explained by common relaxationmechanisms, such as modulation of anisotropic hyper-fine coupling (HFC) and g tensor by chaotic molecularmotion or modulation of isotropic HFC due to transi-tions between distorted Jahn–Teller structures, as wellas by spin–rotation interaction [2, 7]. At present, thehypothesis that spin–orbit coupling in such radicals isresponsible for the anomalous relaxation is believed tobe the most probable [2–7]. However, despite numer-ous attempts, no concrete theoretical model of thisrelaxation has been suggestedOne of the factors complicating the creation of thetheory of paramagnetic relaxation of high-symmetryradicals is the absence of experimental information onthe nature of the state in which the unpaired electronspin is efficiently coupled to the other degrees of free-dom. In particular, for the best studied benzene radicalanion, the doubly degenerate ground vibronic state [2],in which the average projection of the orbital momentonto the symmetry axis can be nonzero, is suggested assuch a state. These concepts make it possible to realis-tically estimate the relaxation rate of the benzene radi-cal anion [7]; however, such arguments are untenablefor asymmetric particles, lacking symmetry-relateddegeneracy, with fast paramagnetic relaxation [4–6].It is worth noting that studying the paramagneticrelaxation in high-symmetry molecular structures is notonly of theoretical significance. Symmetric structures(fullerenes, fullerites, nanotubes) are candidates fordesign of nanosized devices of molecular electronicsand spintronics [8]. Therefore, it should be possible tocontrol the states of separate spins in them, and, hence,the paramagnetic relaxation of these states should berather slow.Radical ions exhibiting anomalous paramagneticrelaxation are structurally nonrigid [9], and it is quitelikely that there is a correlation between this relaxationand intramolecular dynamics of such particles. Theo-retical analysis of the adiabatic potential energy surface(PES) of the ground state of radical ions is required tostudy this correlation in detail.In this paper, we briefly describe the results of suchanalysis for radical cations (RCs) of a series of alkyl-substituted cyclohexanes ( R–C

Theoretical study of intramolecular anion radical cycloaddition of the phenyl-substituted bis(enone)

DFT (U)B3LYP calculations with the 6-31G(d,p), 6-31+G(d,p), and 6-311+G(d,p) basis sets were carried out to investigate the mechanisms of the three reactions from the phenyl-substituted bis(enone) anion radical to the cis-bicyclobutane anion radical (cis-P −) [ R - → cis- P - (1)], to the trans-bicyclobutane anion radical (trans-P −) [ R - → trans- P - (2)], and to the Diels–Alder cycloaddition anion radical (da-P −) [ R - → da- P - (3)], respectively, in both the vacuum and the tetrahydrofuran (THF) solvent. The calculations indicate that the reactions occurring in both phases have the same skeleton of the energy profile for each of the three channels. The three reaction processes all consist of two reaction steps. The first step for reaction (1) is to form a trans-intermediate having a five-atom ring via a trans-transition state and the second step is to form the cis-P − product via a cis-transition state. The first steps of reactions (2) and (3) are both to form a cis-intermediate having a five-atom ring via a cis-transition state and the second steps are to form the trans-P − product via a trans-transition state for reaction (2) and to form the da-P − product via a transition state for reaction (3). The B3LYP calculations indicate that trans-P − is the dominant product in both the vacuum and the THF solvent. The possibilities of the electron transfers from the three kinds of aromatic anion radicals (the chrysene, anthracene, and naphthalene anion radicals) to the neutral reactant and from the trans-P −, cis-P −, and da-P − products to the neutral reactant were explored using the B3LYP/6-311+G(d,p) method by means of calculating the electron affinities of the respective neutral species. The calculations indicate that the chain reaction mechanism for the anion radical cyclobutanation of the phenyl-substituted bis(enone) suggested by experimentalists is feasible thermodynamically.

EPR and DFT Study of the Polycyclic Aromatic Radical Cations from Friedel-Crafts Alkylation Reactions

Electron paramagnetic resonance and electron-nuclear double resonance methods were used to study the polycyclic aromatic radical cations produced in a Friedel-Crafts alkylating system, with m-xylene, or p-xylene and alkyl chloride. The results indicate that the observed electron paramagnetic resonance spectra are due to polycyclic aromatic radicals formed from the parent hydrocarbons. It is suggested that benzyl halides produced in the Friedel-Crafts alkylation reactions undergo Scholl self-condensation to give polycyclic aromatic hydrocarbons, which are converted into corresponding polycyclic aromatic radical cations in the presence of AlCl3. The identification of observed two radicals 2,6-dimethylanthracene and 1,4,5,8-tetramethylanthracene were supported by density functional theory calculations using the B3LYP/631G(d,p)//B3LYP/631G(d) approach. The theoretical coupling constants support the experimental assignment of the observed radicals.

Intramolecular Through-Space Antiferromagnetic Interactions of Cross-Conjugated Aromatic Polyaminium Radical Gels

Highly networked, cross-conjugated aromatic polyamine gels were prepared by palladium-catalyzed polycondensation. The gels were characterized by the cross-polarization magic angle spinning (CP-MAS) 13C NMR and IR spectroscopies, thermogravimetric analysis, and X-ray diffraction. The data confirm that the polymerization proceeded without any side reactions and that the gels possessing the significant thermal stability are composed of the mixtures of polycrystalline and amorphous moieties. Chemical oxidation of the gels with NOPF6 solubilized with 18-crown-6 in CH2Cl2 gave the corresponding aminium polyradicals. Appearance of the ESR signals at g = 2.003 and the new IR peaks supported the aminium radical generation. The p-methoxy-substituted aromatic polyaminium radical, the polyradical of poly(2), realized the higher spin concentration of 65% and the enhanced chemical stability than the polyradical of poly(1) without any protecting groups. However, the magnetic measurements of the polyradical of poly(2) did not show any intramolecular ferromagnetic interactions, but indicated very strong intramolecular through-space antiferromagnetic interactions.

Bond Dissociation Enthalpies of Large Aromatic Carbon-Centered Radicals

Carbon-hydrogen bond dissociation enthalpy (BDE) values are computed for the class of benzylic radicals. An extended and representative set of large methylated polyaromatics has been submitted to an accurate computational study using various levels of theory. The hybrid B3P86 as well as two contemporary functionals (BMK and M05-2X) are applied. For a selection of species, the suitability of the DFT methods is validated through comparison with high-level G3(MP2)-RAD and SCS-ROMP2 results. The influence of the polyaromatic environment on the BDE results is thoroughly discussed. The results are compared with other hydrocarbon radical types in order to obtain a generalized radical stability scale. In order to complete this investigation, also carbon-carbon BDE values have been calculated, giving information about the influence of the local environment on removing the methyl group from the polyaromatic.

Radical cation/radical reactions: A Fourier transform ion cyclotron resonance study of allyl radical reacting with aromatic radical cations

A method for the study of reactions of open-shell neutrals (radicals) and radical cations is described. Pyrolysis (25–1500 °C) of thermally labile compounds, such as, 1,5-hexadiene via a Chen nozzle yields a seeded beam of reactive species in helium. The pyrolysis products are then analyzed by electron ionization (EI) or reacted with stored ions. Electron ionization of the pyrolysis products of 1,5-hexadiene shows that both the allyl radical and allene are generated. Reactions of benzene radical cations and the pyrolysis products of 1,5-hexadiene result in carbon–carbon bond formation. Those reactions of allyl radical with the benzene radical cation yield the C 7H 7 + ion of m/ z 91, permitting an unusual entry into arenium ions. The reaction of allene with benzene radical cation in contrast yields C 9H 10 + and C 9H 9 +.

Particle formation in opposed-flow diffusion flames of ethylene: An experimental and numerical study

An experimental and numerical study on particles inception and growth is performed in opposed-flow diffusion flames of ethylene and air characterized by different sooting tendencies. Spectrally resolved UV-visible laser induced fluorescence, laser induced incandescence and laser light scattering measurements are used to characterize different classes of combustion-generated compounds. A detailed kinetic model accounting for both gas-phase and particle formation is used. Comparison between experimental results and numerical predictions gives a qualitative view of the mechanism of particle formation in opposed-flow flames.Particle inception is the result of both chemical growth and coagulation of aromatic compounds. In the region close to the flame front where the temperature is relatively high and radicals are abundant, the particle inception is due to a chemical growth mechanism by which aromatic molecules add aromatic radicals leading to the formation of biphenyl-like structures. The growth process continues as high-molecular mass aromatics are moved away from the flame zone towards the stagnation plane by the addition of acetylene and other aromatics forming particles of increasing sizes. Graphitization of these particles and thermal annealing lead to the formation of soot particles. At relatively lower temperatures, found across the stagnation plane, particles growth still occurs and it is mainly due to a process of physical coagulation of PAHs.The experimental and numerical results obtained in this work demonstrate and explain the sensibility of inception and growth of particles to radical concentration and temperature in opposed-flow flame configurations.