Decomposition of Picolyl Radicals at High Temperature: A Mass Selective Threshold Photoelectron Spectroscopy Study

The reaction products of the picolyl radicals at high temperature were characterized by mass-selective threshold photoelectron spectroscopy in the gas phase. Aminomethylpyridines were pyrolyzed to initially produce picolyl radicals (m/z=92). At higher temperatures further thermal reaction products are generated in the pyrolysis reactor. All compounds were identified by mass-selected threshold photoelectron spectroscopy and several hitherto unexplored reactive molecules were characterized. The mechanism for several dissociation pathways was outlined in computations. The spectrum of m/z=91, resulting from hydrogen loss of picolyl, shows four isomers, two ethynyl pyrroles with adiabatic ionization energies (IE_ad) of 7.99 eV (2-ethynyl-1H-pyrrole) and 8.12 eV (3-ethynyl-1H-pyrrole), and two cyclopentadiene carbonitriles with IE′s of 9.14 eV (cyclopenta-1,3-diene-1-carbonitrile) and 9.25 eV (cyclopenta-1,4-diene-1-carbonitrile). A second consecutive hydrogen loss forms the cyanocyclopentadienyl radical with IE′s of 9.07 eV (T₀) and 9.21 eV (S₁). This compound dissociates further to acetylene and the cyanopropynyl radical (IE=9.35 eV). Furthermore, the cyclopentadienyl radical, penta-1,3-diyne, cyclopentadiene and propargyl were identified in the spectra. Computations indicate that dissociation of picolyl proceeds initially via a resonance-stabilized seven-membered ring.     

Lutidyl Radical Photoelectron Spectra Reveal Additive Substituent Effects on Benzyl Derivatives’ Ionization Energy

Understanding how isomerism influences photoelectron spectra helps in the assignment and analysis of reactive mixtures, especially for heterocycles with numerous isomers. Threshold photoelectron spectra of lutidyl radical isomers, i. e., benzyl derivatives with a nitrogen heteroatom and a methyl substituent, are recorded using vacuum ultraviolet synchrotron radiation. The radicals are produced by flash pyrolysis from aminomethyl methylpyridine precursors. Experimental ionization energies are determined to be 7.54, 7.50, and 7.45 eV for 2,4-, 2,6- and 3,5-lutidyl, respectively, in excellent agreement with composite method calculations. Franck–Condon simulations aid the TPES assignment but are also shown to exhibit artifacts if large-amplitude motions, notably the methyl internal rotation are assumed to be active in the double harmonic approximation. Based on calculated adiabatic ionization energies (AIE) of benzyl, picolyl, and xylyl radicals, the N and CH₃ substituent effects are found to be additive, position-dependent and decrease in the para>ortho≳meta order in magnitude with the nitrogen heteroatom increasing and the methyl substituent decreasing the AIE. These effects are discussed in light of the charge distribution upon ionization. The additivity of the substituent effects also helps predict the influence of substituents on the binding energy of the unpaired electron in analogous radicals.     

Shock‐tube spectroscopic water measurements and detailed kinetics modeling of 1‐pentene and 3‐methyl‐1‐butene

To understand the effects of the chemical structure of two C₅ alkene isomers on their combustion properties, and to highlight the major chemical reactions occurring during their high‐temperature oxidation, water time histories were measured behind reflected shock waves for the oxidation of 1‐pentene (C₅H₁₀‐1) and 3‐methyl‐1‐butene (3M1B) in 99.5% Ar. The experiments were carried out at three different equivalence ratios (φ = 0.5, 1.0, and 2.0) at pressures and temperatures ranging from 1.29 to 1.47 atm and 1 331 to 1 877 K, respectively. The H₂O quantification extends the database for 1‐pentene and provides new insights for 3M1B. These unique results were used to validate and to develop a new detailed kinetics model. Numerical predictions are presented, and the new model was able to capture the results with suitable accuracy, with 3M1B being notably more reactive than C₅H₁₀‐1. Sensitivity and rate‐of‐production analyses were performed to help explain the results. Under the present conditions, the reactivity is rapidly initiated by molecular dissociation of a fraction of the pentene isomers. The initiation phase then induces H‐atom abstraction by active radicals (H, OH, O, HO₂, and CH₃) to first produce alkenyl C₅H₉ radicals (or an alkyl radical and an alkenyl radical by breaking a C─C bond) and subsequent, smaller fragments. The difference in terms of reactivity between the isomers is essentially due to the fact that 3M1B has one particularly weak tertiary allylic C─H bond, which allows for fast H‐atom abstraction compared with 1‐pentene.     

Double H-bridged and single H-bridged diboryl radicals

The structures of B₂H₅·, B₂H₅CO·, and B₂H₅N₂· radicals are investigated using the 6–31G^* basis set. Both double H-bridged and single H-bridged isomers are found to be local minima on the potential energy surface. The effects of electron correlation are taken into account using single point MP4/6–31G^* calculations and, for the diboryl radicals, complete MP3/6–31G^* optimizations. In all cases the single H-bridged isomers are found to be more stable than the corresponding double H-bridged isomers.The transition state for the double H-bridged to single H-bridged B₂H₅· isomerization reaction is calculated to be 2.54 kcal mol^–1 above the double H-bridged radical at the MP4SDTQ/6-31G^*//UHF/ 6–31 G^* level when corrected for zero point energy. Barrier tunneling increased the reaction rate by a factor of 2.5–3.0, strongly suggesting the system is fluxional at this temperature.The addition of CO and N₂ to the diboryl radicals leads to relocation of the unpaired electron and rehybridization of the C and N atoms adjacent to the boron atoms. The isomers of B₂H₅CO· and B₂H₅N₂· are different and should be distinguishable experimentally. While the CO moiety is bound to the diboryl radicals isomers by over 19 kcal mol^–1, no binding energy is evident for N₂.     

DFT and MP2 molecular orbital determination of OH–toluene–O2 isomeric structures in the atmospheric oxidation of toluene

We have performed an exhaustive theoretical study, using a density functional theory (DFT) and ab initio techniques, of the possible isomers of the OH–toluene–O₂ radical. DFT calculations of the all electron type using the hybrid B3LYP approach and 6‐31G* orbital basis set were employed. In addition to the well‐established ortho position, addition of OH at C₁ on the benzene ring of toluene was also considered for the initial methylhydroxycyclohexadienyl adduct. In all, 28 different intermediate structures of the OH–toluene–O₂ system, consisting of peroxyl radicals, bicyclic structures, and epoxides, have been explored through fully optimized electronic structure calculations. Starting from the 1,3‐O₂‐methylorthohydroxycyclohexadienyl radical, or ortho‐OH adduct, several peroxyl radicals are found to have low‐lying structures contained within a small energy range (about 1 kcal/mol). Only two bicyclic structures are stable with respect to the methylhydroxycyclohexadienyl radical plus O₂, one of them being clearly favored. The four possible epoxy structures are all found to lie more than 15 kcal/mol lower than any of their peroxyl and bicyclic isomers. The preference, first noted by Bartolotti and Edney, for structures in which the OH group lies on the same side of the ring as the O₂ group, is obeyed in all cases. If the 1‐CH₃, 1‐OH cyclohexadienyl radical (or C₁–OH adduct) is used as the initial adduct, three peroxyl radicals are expected to be formed, while two bicyclic structure and three epoxides need to be considered. These structures are found to be, in general, less stable than the ones arising from the ortho adduct. However, the 4‐O, 2,3‐epoxy, 1,1‐methylhydroxycyclohexadienyl radical is found to be the most stable of all the isomers considered, and this, by more than 3 kcal/mol. In this work, most structures were also calculated with the MP2 method with a 6‐31G* basis set. The geometries obtained with the two methods are similar. Contrary to the B3LYP method, MP2 always yields an extra stability to structures in which the C₁ carbon atom has sp³ hybridization. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 716–730, 2000     

Intramolecular hydrogen bond in the hydroxycyclohexadienyl peroxy radicals

The hydroxycyclohexadienyl peroxy radicals (HO? C₆H₆? O₂) produced from the reaction of OH‐benzene adduct with O₂ were studied with density functional theory (DFT) calculations to determine their characteristics. The optimized geometries, vibrational frequencies, and total energies of 2‐hydroxycyclohexadienyl peroxy radical II_s and 4‐hydroxycyclohexadienyl peroxy radical III_s were calculated at the following theoretical levels, B3LYP/6‐31G(d), B3LYP/6‐311G(d,p), and B3LYP/6‐311+G(d,p). Both were shown to contain a red‐shifted intramolecular hydrogen bond (O? H … O? H bond). According to atoms‐in‐molecules (AIM) analysis, the intramolecular hydrogen bond in the 2‐hydroxycyclohexadienyl peroxy radical II_s is stronger than that one in 4‐hydroxycyclohexadienyl peroxy radical III_s, and the former is the most stable conformation among its isomers. Generally speaking, hydrogen bonding in these radicals plays an important role to make them more stable. Based on natural bond orbital (NBO) analysis, the stabilization energy between orbitals is the main factor to produce red‐shifted intramolecular hydrogen bond within these peroxy radicals. The hyperconjugative interactions can promote the transfer of some electron density to the O? H antibonding orbital, while the increased electron density in the O? H antibonding orbital leads to the elongation of the O? H bond and the red shift of the O? H stretching frequency. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Selective Formation of Indene through the Reaction of Benzyl Radicals with Acetylene

The combustion of fossil fuels forms polycyclic aromatic hydrocarbons (PAHs) composed of five‐ and six‐ membered aromatic rings, such as indene (C₉H₈), which are carcinogenic, mutagenic, and deleterious to the environment. Indene, the simplest PAH with single five‐ and six‐membered rings, has been predicted theoretically to be formed through the reaction of benzyl radicals with acetylene. Benzyl radicals are found in significant concentrations in combustion flames, owing to their highly stable aromatic and resonantly stabilized free‐radical character. We provide compelling experimental evidence that indene is synthesized through the reaction of the benzyl radical (C₇H₇) with acetylene (C₂H₂) under combustion‐like conditions at 600 K. The mechanism involves an initial addition step followed by cyclization and aromatization through atomic hydrogen loss. This reaction was found to form the indene isomer exclusively, which, in conjunction with the high concentrations of benzyl and acetylene in combustion environments, indicates that this pathway is the predominant route to synthesize the prototypical five‐ and six‐membered PAH.     

Reaction kinetics and thermochemistry of the chemically activated and stabilized primary ethyl radical of methyl ethyl sulfide,CH3SCH2CH2•, with O2 to CH3SCH2CH2OO•

Oxidation of methyl ethyl sulfide (CH₃SCH₂CH₃, methylthioethane, MES) under atmospheric and combustion conditions is initiated by hydroxyl radicals, MES radicals, generated after loss of a H atom via OH abstraction, will further react with O₂ to form chemically activated and stabilized peroxyl radical adducts. The kinetics of the chemically activated reaction between the CH₃SCH₂CH₂• radical and molecular oxygen are analyzed using quantum Rice-Ramsperger-Kassel theory for k(E) with master equation analysis and a modified strong-collision approach to account for further reactions and collisional deactivation. Thermodynamic properties of reactants, products, and transition states are determined by the B3LYP/6-31+G(2d,p), M062X/6-311+G(2d,p), ωB97XD/6-311+G(2d,p) density functional theory, and CBS-QB3, G3MP2B3, and G4 composite methods. The reaction of CH₃SCH₂CH₂• with O₂ forms an energized peroxy adduct CH₃SCH₂CH₂OO• with a calculated well depth of 34.1 kcal mol⁻¹ at the CBS-QB3 level of theory. Thermochemical properties of reactants, transition states, and products obtained under CBS-QB3 level are used for calculation of kinetic parameters. Reaction enthalpies are compared between the methods. The temperature and pressure-dependent rate coefficients for both the chemically activated reactions of the energized adduct and the thermally activated reactions of the stabilized adducts are presented. Stabilization and isomerization of the CH₃SCH₂CH₂OO• adduct are important under high pressure and low temperature. At higher temperatures and atmospheric pressure, the chemically activated peroxy adduct reacts to new products before stabilization. Addition of the peroxyl oxygen radical to the sulfur atom followed by sulfur-oxygen double bond formation and elimination of the methyl radical to form S(= O)CCO• + CH₃ (branching) is a potentially important new pathway for other alkyl-sulfide peroxy radical systems under thermal or combustion conditions.     

Theoretical investigation on the detailed mechanism of the OH‐initiated atmospheric photooxidation of o‐xylene

The reaction mechanism for o‐xylene with OH radical and O₂ was studied by density functional theory (DFT) method. The geometries of the reactants, intermediates, transition states, and products were optimized at B3LYP/6‐31G(d,p) level. The corresponding vibration frequencies were calculated at the same level. The single‐point calculations for all the stationary points were carried out at the B3LYP/6‐311++G(2df,2pd) level using the B3LYP/6‐31G(d,p) optimized geometries. Reaction energies for the formation of the aromatic intermediate radicals have been obtained to determine their relative stability and reversibility, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the o‐xylene oxidation. The results of the theoretical study indicate that OH addition to o‐xylene forms ipso, meta, and para isomers of o‐xylene‐OH adducts, and the ipso o‐xylene adduct is the most stable among these isomers. Oxygen is expected to add to the o‐xylene‐OH adducts forming o‐xylene peroxy radicals. And subsequent ring closure of the peroxyl radicals to form bicyclic radicals. With relatively low barriers, isomerization of the o‐xylene bicyclic radicals to more stable epoxide radicals likely occurs, competing with O₂ addition to form bicyclic peroxy radicals. The study provides thermochemical data for assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from o‐xylene photooxidation. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Isomerization and fragmentation of aliphatic ether radical cations: Interconversion of distonic ions and cyclopropane intermediates

The reactions of propyl ether radical cations close to threshold are initiated by (reversible) formation of γ-disitonic isomers, R$ \mathop {\rm O}\limits^ + $ (H)CH₂CH₂CH₂^·. The three methylene groups in these ions lose their positional identity by ring closure/ring opening via [cyclopropane + alcohol]^+· intermediates. Extensive hydrogen exchange occurs within the C₃-chain. When R is not methyl the γ-distonic isomer undergoes further intramolecular hydrogen atom transfer reactions that lead to formation of α- and β-distonic ions. The α-distonic isomers expel ethyl and propyl radicals by C? O bond cleavage.     

Copper‐Catalyzed Radical/Radical CH/PH Cross‐Coupling: α‐Phosphorylation of Aryl Ketone O‐Acetyloximes

The selective radical/radical cross‐coupling of two different organic radicals is a great challenge due to the inherent activity of radicals. In this paper, a copper‐catalyzed radical/radical C? H/P? H cross‐coupling has been developed. It provides a radical/radical cross‐coupling in a selective manner. This work offers a simple way toward β‐ketophosphonates by oxidative coupling of aryl ketone o‐acetyloximes with phosphine oxides using CuCl as catalyst and PCy₃ as ligand in dioxane under N₂ atmosphere at 130 °C for 5 h, and yields ranging from 47 % to 86 %. The preliminary mechanistic studies by electron paramagnetic resonance (EPR) showed that, 1) the reduction of ketone o‐acetyloximes generates iminium radicals, which could isomerize to α‐sp³‐carbon radical species; 2) phosphorus radicals were generated from the oxidation of phosphine oxides. Various aryl ketone o‐acetyloximes and phosphine oxides were suitable for this transformation.     

Molecular and electronic structures of bis(dipicolylamine)copper(II) perchlorate and bis[2-(2-pyridylethyl)picolylamine]copper(II) perchlorate

The geometric isomers of bis(dipicolylamine)copper(II) perchlorate, [Cu(dipica)₂](ClO₄)₂, and bis[2-(2-pyridylethyl)picolylamine]copper(II) perchlorate, [Cu(pepica)₂](ClO₄)₂, have been prepared and their molecular structures determined by X-ray diffraction methods. The copper atom of cis-fac-[Cu(dipica)₂](ClO₄)₂, is six coordinate with an amine nitrogen and a pyridyl group of each facial dipica ligand forming a cis coordination plane, and the remaining pyridyl nuclei on the axial sites completing a distorted octahedral structure. The mixed trans-fac- & square-pyramidal-[Cu(dipica)₂](ClO₄)₂ comprises discrete hexacoordinate and pentacoordinate cations. The distorted trans-facial octahedral cation has two picolyl chelates in the equatorial plane and two slightly longer axial pyridyl groups. In the square-pyramidal cation, the basal plane is formed by a meridional tridentate dipicia and a pyridyl of another bidentate dipica ligand, of which the amine group is bound at the apex. The copper ion of trans-fac-[Cu(pepica)₂](ClO₄)₂ is bound by two picolyl chelates in the equatorial plane and two elongated axial pyridyl groups. The electronic structures of these complexes are deduced based on their electronic and e.p.r. spectra. The bonding properties and the formation of the geometric isomers are elucidated.     

Mobile protons versus mobile radicals: Gas-phase unimolecular chemistry of radical cations of cysteine-containing peptides

A combination of electrospray ionization (ESI), multistage, and high-resolution mass spectrometry experiments are used to examine the gas-phase fragmentation reactions of radical cations of cysteine containing di- and tripeptides. Two different chemical methods were used to form initial populations of radical cations in which the radical sites were located at different positions: (1) sulfur-centered cysteinyl radicals via bond homolysis of protonated S-nitrosocysteine containing peptides; and (2) α-carbon backbone-centered radicals via Siu’s sequence of reactions (J. Am. Chem. Soc. 2008, 130, 7862). Comparison of the fragmentation reactions of these regiospecifically generated radicals suggests that hydrogen atom transfer (HAT) between the α C-H of adjacent residues and the cysteinyl radical can occur. In addition, using accurate mass measurements, deuterium labeling, and comparison with an authentic sample, a novel loss of part of the N-terminal cysteine residue was shown to give rise to the protonated, truncated N-formyl peptide (an even-electron x_n ion). DFT calculations were performed on the radical cation [GCG]^.+ to examine: the relative stabilities of isomers with different radical and protonation sites; the barriers associated with radical migration between four possible radical sites, [G.CG]⁺, [GC.G]⁺, [GCG.]⁺, and [GC(S.)G]⁺; and for dissociation from these sites to yield b₂-type ions.     

Potentiometric method for determination of kinetic characteristics of radical reactions in aqueous media

Kinetic features of the reactions of K₄[Fe(CN)₆] with radicals initiated by water-soluble azo-initiator 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) at 37 °C were studied using the potentiometric method. Potassium ferrocyanide was shown to be a radical acceptor, whereas K₃[Fe(CN)₆] formed by the oxidation with the radicals in combination with K₄[Fe(CN)₆] is an electrochemical system, the study of which makes it possible to determine kinetic characteristics of radical reactions. The rate constants for the reactions of peroxide radicals RO₂ · with K₄[Fe(CN)₆] were calculated.     

Stereochemical Selection in Phosphoranyl Radical Formation Using Ionizing Radiation

Abstract

Recent electron spin resonance (ESR) experiments on phosphorus-centered radicals generated by ionizing radiation demonstrate that stereochemical aspects act strongly on the rate of radical formation and can be decisive in the selection between the possible resulting radical structures. This phenomenon was first established in a single crystal ESR study on radiogenic electron-capture phosphorus-centered radicals of the racemic and meso stereoisomers of 1.2-dimethyl-1,2-diphenyldiphosphine disulfide (1). The radiation process of the racemic form involves the formation of a symmetric species with a threeelectron bond in an overall low yield. The meso isomer, on the other hand, yields exclusively asymmetric radical configurations in which the unpaired electmn resides on one of the two phosphorus nuclei. The high intensity of the ESR spectra for the meso compound indicate a more efficient electron-capture process. A similar pronounced difference in radiosensitivity was observed for the R_p (1 and S_p (2) isomers of (4S,5R)-2-chloro-3,4-dimethyl-5-phenyl-1,3,2-Oxazaphospholidine 2-sulfide. Upon X irradiation, 1 readily results in an electron-capture phosphorus centered radical, whereas the concurrent process in 2 is almost completely absent. Since the geometric parameters of the atoms directly linked to phosphorus are very much alike for 1 and 2 il can be concluded that the efficiency of electron-capture at phosphorus strongly depends on the relative configuration of the distant chiral centers at C₄ and C₅.     

碳氢化合物燃烧中间体Lennard-Jones系数的计算

在已有的基团贡献法公式的基础上,提出了一种新的基团贡献法公式,并通过拟合250种化合物(包括185种稳定化合物临界性质的实验值和65种自由基临界性质的计算值)的临界性质得到了40种基团的贡献值,并用于预测未知化合物的临界性质.选取了训练集以外的、有临界性质实验值的30种化合物作为独立测试集,用于验证所建模型对临界性质的预测能力,T_C和P_C平均绝对偏差分别为8.52%和16.83%.结果表明,预测结果和实验值相吻合,该模型可以用于大分子化合物及自由基的临界性质预测.根据临界性质与Lennard-Jones(L-J)系数的经验关系式,预测了碳氢化合物燃烧中间体的L-J系数,得到独立测试集46种碳氢化合物的L-J系数,与文献值接近,T_C和P_C的平均绝对偏差分别为9.88%和9.96%.比较了训练集中烷烃自由基·C_6H_(13)、烯烃自由基·C_5H_9和炔烃自由基·C_5H_7同分异构体的L-J系数,同时,将己烷自由基·C_6H_(13)与相似的邻近烷烃C_6H_(14)的L-J系数进行比较,发现同分异构体之间或相似化合物之间L-J系数有较大偏差.此外,对缺少L-J系数的114种常见碳氢化合物自由基进行了预测.这对于碳氢化合物的燃烧模拟及基元反应中压强相关的速率常数计算有重要意义.     

Reaction mechanisms of a cyclic ether intermediate: Ethyloxirane

Oxiranes are a class of cyclic ethers formed in abundance during low‐temperature combustion of hydrocarbons and biofuels, either via chain‐propagating steps that occur from unimolecular decomposition of β‐hydroperoxyalkyl radicals (β‐?QOOH) or from reactions of HO? with alkenes. Ethyloxirane is one of four alkyl‐substituted cyclic ether isomers produced as an intermediate from n‐butane oxidation. While rate coefficients for β‐?QOOH → ethyloxirane + ?H are reported extensively, subsequent reaction mechanisms of the cyclic ether are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe ethyloxirane consumption by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error—uncertainty derived from missing or incomplete chemistry. The present work provides fundamental insight on reaction mechanisms of ethyloxirane in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred from the detection of products during chlorine atom‐initiated oxidation experiments using multiplexed photoionization mass spectrometry conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, calculations of stationary point energies were conducted using the ccCA‐PS3 composite method on ?R + O₂ potential energy surfaces for the four ethyloxiranyl radical isomers, which produced barrier heights for 24 reaction pathways. In addition to products from ?QOOH → cyclic ether + ?H and ?R + O₂ → conjugate alkene + HO?, both of which were significant pathways and are prototypical to alkane oxidation, other species were identified from ring‐opening of both ethyloxiranyl and ?QOOH radicals. The latter occurs when the unpaired electron is localized on the ether group, causing the initial ?QOOH structure to ring‐open and form a resonance‐stabilized ketohydroperoxide‐type radical. The present work provides the first analysis of ethyloxirane oxidation chemistry, which reveals that consumption pathways are complex and may require an expansion of submechanisms to increase the fidelity of chemical kinetics mechanisms.     

Comprehensive Theoretical and Experimental Studies on the CF3H Fire‐extinguishing Mechanism

In order to clarify the chemical suppression mechanisms of CF₃H, experimental and theoretical studies were conducted respectively in this paper. Firstly, the combustion species in low pressure laminar premixed flat methane flames with CF₃H addition is measured by synchrotron radiation molecular beam mass spectrometry (SR‐MBMS) experimentally. Fire suppression chemistry of CF₃H is investigated by selective detection of combustion radicals and intermediates in experimental process. Secondly, quantum chemistry calculations are performed to calculate the potential energy surfaces (PES) for the CF₃H unimolecular dissociation reaction and reactions of CF₃H with free radical OH and H at the B3LYP/6‐311++G** and QCISD(T)/6‐311++G** levels. Finally, the chemical suppression mechanism of CF₃H was discussed by comparing the theoretical calculation with experimental measurement.     

Using Distonic Radical Ions to Probe the Chemistry of Key Combustion Intermediates: The Case of the Benzoxyl Radical Anion

The benzoxyl radical is a key intermediate in the combustion of toluene and other aromatic hydrocarbons, yet relatively little experimental work has been performed on this species. Here, a combination of electrospray ionization (ESI), multistage mass spectrometry experiments, and density functional theory (DFT) calculations are used to examine the formation and fragmentation of a benzoxyl (benzyloxyl) distonic radical anion. Excited 4-carboxylatobenzoxyl radical anions were produced via two methods: (1) collision induced dissociation (CID) of the nitrate ester 4-(nitrooxymethyl)benzoate, ^–O₂CC₆H₄CH₂ONO₂, and (2) reaction of ozone with the 4-carboxylatobenzyl radical anion, ^–O₂CC₆H₄CH₂ ^?. In neither case was the stabilized ^–O₂CC₆H₄CH₂O^? radical anion intermediate detected. Instead, dissociation products at m/z 121 and 149 were observed. These products are attributed to benzaldehyde (O₂ ^-CC₆H₄CHO) and benzene (^–O₂CC₆H₅) products from respective loss of H and HCO radicals in the vibrationally excited benzoxyl intermediate. In no experiments was a product at m/z 120 (i.e., ^–O₂CC₆H₄ ^?) detected, corresponding to absence of the commonly assumed phenyl radical + CH₂=O channel. The results reported suggest that distonic ions are useful surrogates for reactive intermediates formed in combustion chemistry.

Esr and structure of sulfur-centered radicals and radical ions. γ-Irradiated dimethyl disulfide and methane thiol single crystals at 77°k

Single crystals of dimethyl disulfide and methane thiol were irradiated at 77°K. CH₃S radicals were produced in both compounds and measurement of the isotropic coupling constant from the methyl protons gave a value of 7.6 G. In the dimethyl disulfide crystal both the anion, CH₃SSCH₃, and the cation, CH₃SSCH⁺₃, radicals were observed. The disulfide anion radical exhibited an isotropic septet of lines with a = 5.0 G. Comparison with measurements on a polycrystalline sample gave g⊥ = 2.020 and g6 = 2.000 for this radical. The disulfide cation radical exhibited an evenly spaced septet of lines with a = 9.1 G and a maximum value for the g factor of 2.032.On illumination with IR radiation (λ > 590 mm) the disulfide cation radicals were easily bleached together with about 50% of the disulfide anion radicals suggesting a photoinduced neutralization process. The presence of weak ³³S satellite lines in the anion radical spectrum indicates that 12% of the unpaired spin is localized to the two sulfur 3s orbitals. The structure of the disulfide cation radical is discussed in relation to earlier studies and a dihedral angle of 180° is proposed. The mechanisms for radical formation and decay in dimethyl disulfide and methane thiol are also discussed.