Resonance-stabilized Radicals Research Articles

1,2-H Atom Rearrangements in Benzyloxyl Radicals.

The rate constants for solvent-assisted 1,2-H atom rearrangements in para-substituted benzyloxyl radicals were studied with density functional theory. The rate of the radical rearrangement, calculated through transition state theory with Eckhart tunneling corrections, was shown to be drastically impacted by the presence of both implicit and explicit solvent molecules, with a quantitative agreement with laser flash photolysis studies for a variety of electron-donating and -withdrawing substituents. The rate of rearrangement was found to be correlated to the distance between the rearranging hydrogen atom and the α-carbon in the transition state, which could be modified through the para substituent and the type of assisting solvent molecule (e.g., water, ethanol, methanol, acetic acid, or a mixture of the latter). Natural bond orbital analysis showed that the rearrangement does not proceed through a hydrogen radical but through a quasi-proton exchange and charge transfer between the benzyl carbon and the adjacent oxygen atom. Energetic and spin population results indicated that electron-withdrawing groups induce faster rearrangement kinetics. Understanding 1,2-H atom shifts in benzyloxyl radicals are essential for tuning the rate of superoxide production in aqueous systems, as the resonance-stabilized carbon radical produced from the rearrangement can bind oxygen and decompose to produce superoxide radical anion, an important reactive intermediate in environmental and biological systems.

Resonance-stabilized hydrocarbon-radical chain reactions may explain soot inception and growth.

Mystery surrounds the transition from gas-phase hydrocarbon precursors to terrestrial soot and interstellar dust, which are carbonaceous particles formed under similar conditions. Although polycyclic aromatic hydrocarbons (PAHs) are known precursors to high-temperature carbonaceous-particle formation, the molecular pathways that initiate particle formation are unknown. We present experimental and theoretical evidence for rapid molecular clustering-reaction pathways involving radicals with extended conjugation. These radicals react with other hydrocarbon species to form covalently bound complexes that promote further growth and clustering by regenerating resonance-stabilized radicals through low-barrier hydrogen-abstraction and hydrogen-ejection reactions. Such radical-chain reaction pathways may lead to covalently bound clusters of PAHs and other hydrocarbons that would otherwise be too small to condense at high temperatures, thus providing the key mechanistic steps for rapid particle formation and surface growth by hydrocarbon chemisorption.

A radical route to soot

Combustion Chemistry The chemical origin of soot is a persistent puzzle. It is clear that small hydrocarbon fragments formed in flames must aggregate into larger particles, but the initial driving force for aggregation remains a mystery. Johansson et al. combined theory and mass spectrometry to suggest a solution based on resonance-stabilized radicals (see the Perspective by Thomson and Mitra). Aromatics such as cyclopentadiene have a characteristically weak C–H bond because their cleavage produces radicals with extended spans of π-electron conjugation. Clusters thus build up through successive coupling reactions that extend conjugation in stabilized radicals of larger and larger size. Science , this issue p. [997][1]; see also p. [978][2] [1]: /lookup/doi/10.1126/science.aat3417 [2]: /lookup/doi/10.1126/science.aau5941

A radical approach to soot formation.

Chain reactions of resonance-stabilized radicals turn molecules into clusters and nanoparticles

Rethinking sulfate radical-based oxidation of nitrophenols: Formation of toxic polynitrophenols, nitrated biphenyls and diphenyl ethers.

Sulfate radical (SO4-)-based oxidation of nitrophenols (NPs) have been widely studied; however, formation of potentially more toxic polynitroaromatic intermediates has been overlooked. In this contribution, we systematically investigated the degradation of four NPs by a SO4--based oxidation process. Degradation efficiency of NPs followed the order: 2-nitrophenol (2-NP) > 4-nitrophenol (4-NP) > 2,4-dinitrophenol (2,4-DNP) > 2,6-dinitrophenol (2,6-DNP). HPLC and LC-MS/MS analysis confirmed the formation of 2,4-DNP, 2,6-DNP and 2,4,6-trinitrophenol (2,4,6-TNP) during NPs transformation by SO4-, suggesting that both denitration and renitration processes occurred. Nitrogen dioxide radicals (NO2) and phenoxy radicals are responsible for the formation of polynitrophenols. Coupling products including nitrated biphenyls and diphenyl ethers were also detected, which were proposed to be formed by combinations of resonance-stabilized radicals. Electron spin density and charge density calculation showed that ortho C-ortho C and ortho C-phenolic O were the most likely combination ways responsible for coupling products formation. ECOSAR program predicted that polynitrated diphenyl ethers and biphenyls had higher ecotoxicological effects on aquatic species such as fish and daphnia. Therefore, the formation of toxic polynitroaromatic intermediates in SO4--based advanced oxidation processes should be scrutinized before this technology can be safely utilized for water and wastewater treatment.

Broadband Microwave Spectroscopy of 2-Furanyloxy Radical: Primary Pyrolysis Product of the Second-Generation Biofuel 2-Methoxyfuran.

Broadband microwave spectra over the 2-18 GHz range have been recorded for the resonance-stabilized 2-furanyloxy radical, formed in the first step of pyrolysis of the second-generation biofuel 2-methoxyfuran by methyl loss. Using a flash pyrolysis source attached to a pulsed valve, a 0.7% mixture of 2-methoxyfuran in argon was pyrolyzed at a series of temperatures ranging from 300 to 1600 K. Subsequent cooling in a supersonic expansion produced rotational temperatures of ∼2 K in the interrogation region. Using chirped-pulse Fourier transform microwave (CP-FTMW) methods, combined with strong-field coherence breaking (SFCB), a set of transitions due to the radical were identified and assigned. The experimental rotational constants ( A = 8897.732(93), B = 4019.946(24), C = 2770.321(84)), centrifugal distortion constants, and spin-rotation coupling constants have been determined for the radical and compared with ab initio predictions at the CCSD(T) level of theory. Compared to the 2-methoxyfuran precursor, the 2-furanyloxy radical has allylic C-C bond lengths intermediate between single and double bonds, a shortened C(5)-O(6) bond characteristic of partial double-bond character, and an O(1)-C(5)-O(6) bond angle of 121°, which resembles the O-C-O angle of an ester. Atomic spin densities extracted from the calculations confirm that the 2-furanyloxy radical is best viewed as a carbon-centered allylic lactone radical, with 80% of the spin density on the two allylic carbons and 20% on the pendant O(6) atom.

Polycyclic aromatic hydrocarbons formation and growth during the supercritical pyrolysis of 1-octene

To better understand the roles of larger 1-alkenes in reactions leading to the production of polycyclic aromatic hydrocarbons (PAH) and solids in the supercritical pre-combustion environments of future-high-speed-aircraft fuels, supercritical pyrolysis experiments have been conducted with the model fuel 1-octene, a representative 1-alkene product of supercritical n-alkane-fuel pyrolysis. The 1-octene pyrolysis experiments are performed in an isothermal, silica-lined stainless-steel flow reactor at 94.6 atm, 133 s, and ten temperatures in the range 440–535 °C. Straight-chain aliphatics—particularly n-alkanes, 1-alkenes, and 2-alkenes—are the highest-yield products at all temperatures. At temperatures ≤500 °C, high yields of the C8 product 2-octene and the C16 product 6-hexadecene evidence 1-octene's facile formation of the resonance-stabilized alkylallylic radical 1-octen-3-yl, whose resonance structure 2-octen-1-yl not only abstracts H to produce 2-octene but also reacts with 1-octene to produce, with high selectivity, 6-hexadecene—establishing that, at low temperatures, a long-chain alkene with an interior CC double bond can result from alkylallyl-radical addition to a 1-alkene. Gas-chromatographic analyses of 1-octene's aromatic products show that one- and two-ring aromatics are produced at temperatures ≥ 475 °C. High-pressure-liquid-chromatographic analyses reveal the identities of 180 three- to nine-ring PAH products—178 of which have never before been identified as pyrolysis products of a large 1-alkene and all of which are produced from supercritical n-decane pyrolysis as well. As in the case for n-decane, the high levels of methyl-substituted PAH in the 1-octene products provide a ready source for resonance-stabilized arylmethyl radicals and (after reaction with ethylene) phenalenyl-type radicals. Reactions of these arylmethyl and phenalenyl-type radicals with C2C4 1-alkenes lead to the sequential PAH growth that accounts for 1-octene's production of PAH as large as nine rings at temperatures as low as 500 °C and the dramatic increases in the PAH-product yields as temperature approaches 535°C, 1-octene's temperature of incipient solids formation.

Dimerization of the Benzyl Radical in a High-Temperature Pyrolysis Reactor Investigated by IR/UV Ion Dip Spectroscopy.

We investigate the self-reaction of benzyl, C7 H7 , in a high-temperature pyrolysis reactor. The work is motivated by the observation that resonance-stabilized benzyl radicals can accumulate in reactive environments and contribute to the formation of polycyclic aromatic hydrocarbons (PAHs) and soot. Reaction products are detected by IR/UV ion dip spectroscopy, using infrared radiation from the free electron laser FELIX, and are identified by comparison with computed spectra. Among the reaction products identified by their IR absorption are several PAHs linked to toluene combustion such as bibenzyl, phenanthrene, diphenylmethane, and fluorene. The identification of 9,10-dihydrophenanthrene provides evidence for a mechanism of phenanthrene formation from bibenzyl that proceeds by initial cyclization rather than an initial hydrogen loss to stilbene.

Phenyl radical + propene: a prototypical reaction surface for aromatic-catalyzed 1,2-hydrogen-migration and subsequent resonance-stabilized radical formation.

The C9H11 potential energy surface (PES) was experimentally and theoretically explored because it is a relatively simple, prototypical alkylaromatic radical system. Although the C9H11 PES has already been extensively studied both experimentally (under single-collision and thermal conditions) and theoretically, new insights were made in this work by taking a new experimental approach: flash photolysis combined with time-resolved molecular beam mass spectrometry (MBMS) and visible laser absorbance. The C9H11 PES was experimentally accessed by photolytic generation of the phenyl radical and subsequent reaction with excess propene (C6H5 + C3H6). The overall kinetics of C6H5 + C3H6 was measured using laser absorbance with high time-resolution from 300 to 700 K and was found to be in agreement with earlier measurements over a lower temperature range. Five major product channels of C6H5 + C3H6 were observed with MBMS at 600 and 700 K, four of which were expected: hydrogen (H)-abstraction (measured by the stable benzene, C6H6, product), methyl radical (CH3)-loss (styrene detected), H-loss (phenylpropene isomers detected) and radical adduct stabilization. The fifth, unexpected product observed was the benzyl radical, which was rationalized by the inclusion of a previously unreported pathway on the C9H11 PES: aromatic-catalysed 1,2-H-migration and subsequent resonance stabilized radical (RSR, benzyl radical in this case) formation. The current theoretical understanding of the C9H11 PES was supported (including the aromatic-catalyzed pathway) by quantitative comparisons between modelled and experimental MBMS results. At 700 K, the branching to styrene + CH3 was 2-4 times greater than that of any other product channel, while benzyl radical + C2H4 from the aromatic-catalyzed pathway accounted for ∼10% of the branching. Single-collision conditions were also simulated on the updated PES to explain why previous crossed molecular beam experiments did not see evidence of the aromatic-catalyzed pathway. This experimentally validated knowledge of the C9H11 PES was added to the database of the open-source Reaction Mechanism Generator (RMG), which was then used to generalize the findings on the C9H11 PES to a slightly more complicated alkylaromatic system.

Isomer-Selective Generation and Spectroscopic Characterization of Picolyl Radicals.

Nitrogen-containing resonance-stabilized radicals such as the picolyl radical are important in combustion chemistry and astrochemistry. They have only been scarcely studied because an isomer-selective generation is often difficult. Herein, we present threshold photoelectron spectra of the three picolyl radical isomers, C6 H6 N, that were obtained with synchrotron radiation. The radicals were selectively generated by flash pyrolysis from aminomethylpyridine precursors through deamination. Ionization energies of 7.70, 7.59, and 8.01 eV were determined for 2-, 3-, and 4-picolyl, respectively. The observed vibrational structure was assigned to an in-plane deformation mode of the aromatic ring. The spectroscopic insight gained in this study can be used to distinguish different picolyl isomers in on-line combustion analysis, for example.

Directionality and the Role of Polarization in Electric Field Effects on Radical Stability

Accurate quantum-chemical calculations are used to analyze the effects of charges on the kinetics and thermodynamics of radical reactions, with specific attention given to the origin and directionality of the effects. Conventionally, large effects of the charges are expected to occur in systems with pronounced charge-separated resonance contributors. The nature (stabilization or destabilization) and magnitude of these effects thus depend on the orientation of the interacting multipoles. However, we show that a significant component of the stabilizing effects of the external electric field is largely independent of the orientation of external electric field (e.g. a charged functional group, a point charge, or an electrode) and occurs even in the absence of any pre-existing charge separation. This effect arises from polarization of the electron density of the molecule induced by the electric field. This polarization effect is greater for highly delocalized species such as resonance-stabilized radicals and transition states of radical reactions. We show that this effect on the stability of such species is preserved in chemical reaction energies, leading to lower bond-dissociation energies and barrier heights. Finally, our simplified modelling of the diol dehydratase-catalyzed 1,2-hydroxyl shift indicates that such stabilizing polarization is likely to contribute to the catalytic activity of enzymes.

Growth reactions of polycyclic aromatic hydrocarbons during the supercritical pyrolysis of n-decane, as determined from doping experiments with 1-methylphenanthrene and fluorene

To better understand mechanisms for the growth of polycyclic aromatic hydrocarbons (PAH) under conditions relevant to the pre-combustion environments of future-high-speed-aircraft fuels, we have performed supercritical pyrolysis experiments with the model fuel n-decane, to which 1-methylphenanthrene and fluorene, two major three-ring PAH products of n-decane pyrolysis, have each been added as a dopant, at concentrations of 0.73mg/g n-decane and 2.3mg/g n-decane, respectively. The experiments are conducted in an isothermal, silica-lined stainless-steel flow reactor at 570°C, 94.6atm, and 133s, conditions of rapid PAH growth. Analyses of the PAH products by high-pressure liquid chromatography with diode-array ultraviolet–visible absorption and mass-spectrometric detection have exposed the important roles of four resonance-stabilized aromatic radicals—9-fluorenyl, 1-phenanthrylmethyl, 9-phenanthrylmethyl, and benz[de]anthracenyl—and revealed the channels through which these radicals react with the principal aliphatic (methyl, ethylene, propene, and 1-butene) and aromatic (benzyl, the two naphthylmethyls, and phenalenyl) reactants in the supercritical n-decane pyrolysis environment to effect PAH growth. 9-Fluorenyl's reactions with these principal reactants—in some cases involving “ring expansion” of fluorene's five-membered ring—result in the formation of phenanthrene, 9-methylphenanthrene, 9-ethylphenanthrene, fluoranthene, and eleven fluoranthene benzologues of five to seven rings. 1-phenanthrylmethyl, stemming from the 1-methylphenanthrene dopant, and 9-phenanthrylmethyl, stemming from the fluorene dopant's production of 9-methylphenanthrene and 9-ethylphenanthrene, react with n-decane's principal reactant species to form chiefly four- to six-ring cata-condensed benzenoid PAH. Their reaction with ethylene leads to formation of the four-ring phenalenyl-type radical benz[de]anthracenyl, which reacts with the principal alkenes and arylmethyl radicals in the system to produce several five- to seven-ring peri-condensed benzenoid PAH. Since the three-ring structures of 1-methylphenanthrene and fluorene are embedded in the structures of many of n-decane's higher-ring-number PAH products, we would expect the PAH-growth pathways outlined here to have widespread applicability to larger PAH in the reaction environment as well.

Resonance Stabilization Effects on Ketone Autoxidation: Isomer-Specific Cyclic Ether and Ketohydroperoxide Formation in the Low-Temperature (400-625 K) Oxidation of Diethyl Ketone.

The pulsed photolytic chlorine-initiated oxidation of diethyl ketone [DEK; (CH3CH2)2C═O], 2,2,4,4-d4-DEK [d4-DEK; (CH3CD2)2C═O], and 1,1,1,5,5,5-d6-DEK [d6-DEK; (CD3CH2)2C═O] is studied at 8 torr and 1-2 atm and from 400-625 K. Cl atoms produced by laser photolysis react with diethyl ketone to form either primary (3-pentan-on-1-yl, RP) or secondary (3-pentan-on-2-yl, RS) radicals, which in turn react with O2. Multiplexed time-of-flight mass spectrometry, coupled to either a hydrogen discharge lamp or tunable synchrotron photoionizing radiation, is used to detect products as a function of mass, time, and photon energy. At 8 torr, the nature of the chain propagating cyclic ether + OH channel changes as a function of temperature. At 450 K, the production of OH is mainly in conjunction with formation of 2,4-dimethyloxetan-3-one, resulting from reaction of the resonance-stabilized secondary RS with O2. In contrast, at 550 K and 8 torr, 2-methyl-tetrahydrofuran-3-one, originating from oxidation of the primary radical (RP), is observed as the dominant cyclic ether product. Formation of both of these cyclic ether production channels proceeds via a resonance-stabilized hydroperoxy alkyl (QOOH) intermediate. Little or no ketohydroperoxide (KHP) is observed under the low-pressure conditions. At higher O2 concentrations and higher pressures (1-2 atm), a strong KHP signal appears as the temperature is increased above 450 K. Definitive isomeric identification from measurements on the deuterated DEK isotopologues indicates the favored pathway produces a γ-KHP via resonance-stabilized alkyl, QOOH, and HOOPOOH radicals. Time-resolved measurements reveal the KHP formation becomes faster and signal more intense upon increasing temperature from 450 to 575 K before intensity drops significantly at 625 K. The KHP time profile also shows a peak followed by a gradual depletion for the extent of experiment. Several tertiary products exhibit a slow accumulation in coincidence with the observed KHP decay. These products can be associated with decomposition of KHP by β-scission pathways or via isomerization of a γ-KHP into a cyclic peroxide intermediate (Korcek mechanism). The oxidation of d4-DEK, where kinetic isotope effects disfavor γ-KHP formation, shows greatly reduced KHP formation and associated signatures from KHP decomposition products.

VISIBLE ABSORPTIONS OF POTENTIAL DIFFUSE ISM HYDROCARBONS: C9H9 AND C9H5 RADICALS

ABSTRACT The laboratory detection of previously unobserved resonance-stabilized C9H5 and C9H9 radicals in the supersonic expansion of a hydrocarbon discharge source is reported. The radicals are tentatively assigned as acetylenic-substituted cyclopentadienyl C9H5 and vinyl-substituted benzyl C9H9 species. They are found to feature visible absorption bands that coincide with a few very weak diffuse interstellar bands toward HD183143 and HD204827.

Conformation-specific spectroscopy of alkyl benzyl radicals: Effects of a radical center on the CH stretch infrared spectrum of an alkyl chain.

An important initial step in the combustion of gasoline and diesel fuels is the abstraction of hydrogen from alkylbenzenes to form resonance-stabilized alkyl benzyl radicals. This work uses, for the first time, double resonance spectroscopy methods to explore the conformation-specific vibronic and infrared spectroscopy of the α-ethylbenzyl (αEtBz) and α-propylbenzyl (αPrBz) radicals. Local mode Hamiltonian modeling enables assignment of the alkyl CH stretch IR spectra, accounting for Fermi resonance that complicates aliphatic alkyl CH stretch IR spectroscopy. The ground state conformational preferences of the ethyl and propyl chains are changed from those in the alkylbenzenes themselves, with global minima occurring for an in-plane orientation of the alkyl chain (trans) about its first dihedral angle (ϕf123, numbers are alkyl C atoms. C1 is CH radical site). This in-plane structure is the only observed conformer for the α-EtBz radical, while two conformers, tt and tg' share this orientation at the first dihedral, but differ in the second (ϕ1234) for the αPrBz radical. The in-plane orientation lowers the local site frequencies of the CH2 group stretches immediately adjacent to the benzylic radical site by about 50 cm-1 relative to those in pure alkyl chains or alkylbenzenes. This effect of the radical site is localized on the first CH2 group, with little effect on subsequent members of the alkyl chain. In the D1 excited electronic state, an out-of-plane orientation is preferred for the alkyl chains, leading to torsional mode Franck-Condon activity in the D0-D1 spectra that is both conformer-specific and diagnostic of the conformational change.

Infrared and Electronic Spectroscopy of the Jet-Cooled 5-Methyl-2-furanylmethyl Radical Derived from the Biofuel 2,5-Dimethylfuran.

The electronic and infrared spectra of the 5-methyl-2-furanylmethyl (MFM) radical have been characterized under jet-cooled conditions in the gas phase. This resonance-stabilized radical is formed by H atom loss from one of the methyl groups of 2,5-dimethylfuran (DMF), a promising second-generation biofuel. As a resonance-stabilized radical, it plays an important role in the flame chemistry of DMF. The D0-D1 transition was studied using two-color resonant two-photon ionization (2C-R2PI) spectroscopy. The electronic origin is in the middle of the visible spectrum (21934 cm(-1) = 455.9 nm) and is accompanied by Franck-Condon activity involving the hindered methyl rotor. The frequencies and intensities are fit to a one-dimensional methyl rotor potential, using the calculated form of the ground state potential. The methyl rotor reports sensitively on the local electronic environment and how it changes with electronic excitation, shifting from a preferred ground state orientation with one CH in-plane and anti to the furan oxygen, to an orientation in the excited state in which one CH group is axial to the plane of the furan ring. Ground and excited state alkyl CH stretch infrared spectra are recorded using resonant ion-dip infrared (RIDIR) spectroscopy, offering a complementary view of the methyl group and its response to electronic excitation. Dramatic changes in the CH stretch transitions with electronic state reflect the changing preference for the methyl group orientation.

Atmospheric oxidation intermediates: Laser spectroscopy of resonance-stabilized radicals from p-cymene

Several resonance-stabilized radicals are observed as products of an electrical discharge containing p-cymene, a biogenic monoterpene that contributes to secondary organic aerosols after atmospheric oxidation. Three radicals are identified by resonant 2-color 2-photon ionization mass spectrometry, corresponding to loss of a methyl group or hydrogen atom from the parent p-cymene structure. The radicals are found to absorb in the spectral region consistent with a benzylic chromophore. The spectrum observed on m/z 119 is assigned to α,4-dimethylbenzyl radical, and those on m/z 133 to the α,α,4-trimethylbenzyl and p-isopropylbenzyl radicals, the latter two being intermediates in the atmospheric oxidation of p-cymene.

Ionization energies of three resonance-stabilized radicals: cyclohexadienyl (dn, n = 0, 1, 6, 7), 1-phenylpropargyl, and methylcyclohexadienyl.

The ionization energies for three resonance-stabilized radicals are determined: cyclohexadienyl, 1-phenylpropargyl, and methylcyclohexadienyl. The recommended ionization energies are, respectively, 6.820(1), 6.585(1), and 7.232(1) eV. That of cyclohexadienyl is found to be just 0.02 eV above a high level ab initio calculation [Bargholz, A.; Oswald, R.; Botschwina, P. J. Chem. Phys. 2013, 138, 014307], and that of 1-phenylpropargyl is found within the stated error of a recent experimental determination [Holzmeier, F.; Lang, M.; Hemberger, P.; Fischer, I. ChemPhysChem 2014, DOI: 10.1002/cphc.201402446]. The ionization energy of the methylcyclohexadienyl radical is consistent with the ortho isomer. Ionization energies of a range of isotopologues of cyclohexadienyl radical are given, along with their D1 ← D0 origin band positions, which indicate a blue shift of 18 cm(-1) per deuterium atom substituted. The ionization energy of cyclohexadienyl, along with the calculated bond dissociation energy of Bargholz et al., affords a new estimate of the 0 K proton affinity of benzene: 739.7 ± 2.0 kJ/mol. The ionization energies are discussed in terms of the interplay between radical and cation stabilization energies.

Reaction pathways for the growth of polycyclic aromatic hydrocarbons during the supercritical pyrolysis of n-decane, as determined from doping experiments with 1- and 2-methylnaphthalene

In order to investigate growth mechanisms of polycyclic aromatic hydrocarbons (PAH) that can lead to solids formation in the pre-combustion environments of future high-speed aircraft, we have performed supercritical pyrolysis experiments with the model alkane fuel n-decane (critical temperature, 344.5°C; critical pressure, 20.7atm), to which 2-methylnaphthalene and 1-methylnaphthalene, two two-ring products of n-decane pyrolysis, have each been added as a dopant (each dopant concentration, 8.8mg/g n-decane). The experiments are conducted in an isothermal, silica-lined stainless-steel flow reactor at 570°C, 94.6atm, and 133s, conditions of incipient solids formation. Analyses of the PAH products by high-pressure liquid chromatography with diode-array ultraviolet–visible absorbance and mass-spectrometric detection reveal vast differences in the growth behavior of the two dopants. In the case of 2-methylnaphthalene, growth is virtually limited to the two- and three-ring PAH that result from 2-naphthylmethyl’s reactions with methyl, ethylene, propene, and 1-butene—all abundant in the n-decane pyrolysis environment. In the case of 1-methylnaphthalene, the methyl group’s attachment to a carbon that is just-adjacent to a “valley” carbon of the naphthalene structure permits 1-naphthylmethyl’s reaction with ethylene to form phenalene, an unstable C13H10 PAH that readily loses hydrogen to form the resonance-stabilized phenalenyl radical. This radical sets in motion a sequence for PAH growth that involves reactions of the C2–C4 1-alkenes with both phenalenyl-type radicals and arylmethyl radicals (whose methyl group is attached to a carbon that is just-adjacent to a valley carbon)—producing unsubstituted PAH and methylated PAH of successively higher ring number. As a consequence of these reactions, even though the dopant 1-methylnaphthalene undergoes only 4.5% conversion, it selectively increases (by up to a factor of four) the yields of many of n-decane’s favored four- to eight-ring PAH products—demonstrating the prevalence of the arylmethyl/alkene/phenalenyl reaction mechanism in the supercritical n-decane pyrolysis environment.

The formation of resonance-stabilized sulfur-based radical cations and their gas-phase reactivity

Gas-phase reactivity of several sulfur-based radical cations with various volatile neutrals was studied in a modified quadrupole ion trap mass spectrometer. The radical cations were formed by two methods. The first method (introduced by Siu and co-workers) involved electron transfer from the sulfur-containing ligand of interest (all with a CS double bond) to Cu2+ ion in a ternary complex upon collision-induced dissociation. The second method utilized aromatic/heterocyclic thiol compounds which upon oxidation formed disulfides. Collision-induced dissociation of protonated disulfides lead to homolytic cleavage of SS bond (often among other products). The radical cations produced by these two methods displayed very similar reactivity toward the five neutrals used in this study. This was explained by the resonance stabilization of these species within the NCS motif leading to similar structures. Density functional theory calculations showed that in all of these systems independent of the method of production most of the spin density resided on the sulfur atom and the CS bond held a partially double character.