Xylyl Radicals Research Articles

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.

Insights into pyrolysis kinetics of xylene isomers behind reflected shock waves

This work reports a kinetic study on the pyrolysis of three xylene isomers based on single-pulse shock tube experiments and detailed kinetic modeling. Speciation measurements for the post-shock gas mixtures are obtained via sampling and gas-chromatography-mass spectrometry over a temperature range of 1150−1650 K at a nominal pressure of 20 bar. A sub-mechanism incorporating consumption reactions of xylene isomers as well as subsequent pathways leading to polycyclic aromatic hydrocarbons (PAHs) is developed and integrated into our on-going PAH formation kinetic model. The model can satisfactorily predict the qualitative measurements and accurately characterize both the similarities and fuel-specific features in the pyrolysis of three xylenes. The three isomers decompose at similar rates, though m-xylene exhibits a slightly weaker reactivity. This arises from the fact that m-xylyl, different from its o- and p- counterparts, cannot undergo hydrogen loss to form m-xylylene diradical. Distinct consumption schemes of the three xylyl radicals also result in the different species pools observed at temperatures below 1400 K. A large amount of styrene results from a stepwise isomerization of o-xylylene following dehydrogenation of o-xylyl. An early formation of anthracene is noted as a unique phenomenon in o-xylene pyrolysis, which is attributed to specific reactions of o-xylyl. Due to a relatively high abundance of m-xylyl, the self-recombination product 3,3′-dimethylbibenzyl is among the major species formed at the initial stage of m-xylene pyrolysis. p-Xylyl predominantly converts to p-xylylene which is largely consumed via polymerization processes. Besides, mutual conversions among the three xylyl radicals are found to play an essential role in the pyrolysis of three xylenes. Toluene is measured of significant concentrations in the pyrolysis of all three isomers, which rationalizes that PAH speciation in the pyrolysis of three xylenes and toluene is highly similar at elevated temperatures. Modeling analyses show that apart from the benzyl/toluene chemistry, reactions involving C8 species also have important contributions to PAH formation in xylenes pyrolysis.

The Gas-Phase Infrared Spectra of Xylyl Radicals.

The three isomers of the xylyl radical, C8H9, are possible intermediates in the formation of soot and polycyclic aromatic hydrocarbons (PAH). Their infrared spectra have been recorded by IR/UV ion dip spectroscopy using free electron laser radiation. The radicals were generated by flash pyrolysis from the corresponding nitrites and resonantly ionized via the D3 ← D0 transition around 310 nm. Mid-infrared spectra of the three xylyl isomers were recorded between 550 and 1700 cm-1 and are in excellent agreement with computations, provided that overtones and combination bands are included in the simulation. The results show that the three xylyl isomers can be distinguished by their infrared spectra and that no isomerization occurs in the pyrolysis reactor. The IR spectra obtained at m/z = 208 indicate that dimerization of xylyl radicals leads to substituted stilbenes, which has not been observed for benzyl.

How the methyl group position influences the ultrafast deactivation in aromatic radicals.

Excited xylyl (methyl-benzyl) radical isomers have been studied by femtosecond time-resolved photoelectron spectroscopy and mass spectrometry. Depending on the substitution we find different deactivation channels after excitation into the D3(2A'') state (310 nm, 4 eV). While the ortho and para isomer exhibit deactivation rates similar to the benzyl radical, meta-xylyl sticks out and depletes twice as fast into the vibrationally hot ground state. We found that a ring deformation mode rather than the methyl pseudorotation enables access to a conical intersection, which is responsible for the faster deactivation. Transitions in the photoelectron spectrum can be assigned to several Rydberg series with mostly d angular momentum components. Absorption of two 4 eV photons triggers hydrogen loss reactions on a femtosecond timescale.

Interconversion of Methyltropyl and Xylyl Radicals: A Pathway Unavailable to the Benzyl-Tropyl Rearrangement.

The products of an electrical discharge containing toluene are interrogated using resonance-enhanced multiphoton ionization and laser-induced fluorescence spectroscopies. A previously unreported electronic spectrum recorded at m/z = 105, with a putative origin band at 26053 cm-1, is assigned to methyltropyl radical, which appears to be a major product of the toluene discharge, plausibly arising from CH insertion. All three o-, m-, and p-xylyl isomers are also identified. These isomers are detected in electrical discharges containing various xylenes, where it is also found that interconversion occurs: A discharge of o-xylene produces some m-xylyl; a discharge of m-xylene produces some o-xylyl; and a discharge of p-xylene produces all three isomers. No α-methylbenzyl was detected, but styrene was. These observations are supported by state-of-the-art quantum chemical calculations, which reveal an isomerization pathway between methyltropyl and xylyl radicals for which there is no analogue in the canonical tropyl-benzyl isomerization.

Probing different spin states in xylyl radicals and ions

Resonant one-color two-photon ionization spectroscopy and mass-selected threshold photoelectron spectroscopy were applied to study the electronic doublet states of the three xylyl (methyl-benzyl) radicals above 3.9 eV as well as the singlet and triplet states of the cations up to 10.5 eV. The experiments are complemented by quantum chemical calculations and Franck-Condon simulations to characterize the transitions and to identify the origin bands, allowing a precise determination of singlet-triplet splittings in the cations. Torsional motions of the methyl group notably affect the D0 → D3 transition of m-xylyl. All other investigated transitions either lead to electronic states with very low rotational barriers or suffer from spectral broadening in excess of methyl torsional energy levels. The methyl internal rotational potential is faithfully reproduced with the most basic ab initio methods, yet hyperconjugation could not be identified as a significant force shaping them. Time-dependent density functional theory describes the excited electronic states better than wave function theory approaches, notably EOM-CCSD.

Reaction mechanism and modeling study for the oxidation by SO2 of o‐xylene and p‐xylene in Claus process

AbstractClaus process, comprising of a furnace and a catalytic unit, is used to produce sulfur from H2S. The aromatic contaminants (benzene, toluene, and xylenes) in H2S feed form soot, and clog and deactivate the catalysts. Xylenes are known to be the most damaging ones. Therefore, there is a need to oxidize them in the furnace to enhance catalyst life. This article presents a kinetics study on the oxidation of o‐ and p‐xylene radicals by SO2 (an oxidant that is already present in the furnace) using density functional theory and a composite method. The mechanism begins with H‐abstraction from xylenes to form xylyl radicals, followed by exothermic addition of SO2 to them. The breakage of OS bond in the xylyl‐SO2 adducts leads to the loss of SO molecule, while the remaining O atom on them helps in their oxidation. The isomerization study shows that less‐stable dimethylphenyl radicals have a high tendency to isomerize to resonantly stabilized methylbenzyl radicals. However, methylbenzyl radicals have lower reactivity toward SO2 than dimethylphenyl radicals. The reaction rate constants were found using transition state theory. The reactor simulations reveal that p‐xylene has lower reactivity toward SO2 than o‐xylene, and CO, SO, and CHO are the main by‐products of oxidation.

Photodissociation dynamics of the ortho- and para-xylyl radicals.

The photodissociation dynamics of the C8H9 isomers ortho- and para-xylyl are investigated in a free jet. The xylyl radicals are generated by flash pyrolysis from 2-(2-methylphenyl)- and 2-(4-methylphenyl) ethyl nitrite and are excited into the D3 state. REMPI- spectra show vibronic structure and the origin of the transition is identified at 32 291 cm-1 for the para- and at 32 132 cm-1 for the ortho-isomer. Photofragment H-atom action spectra show bands at the same energy and thus confirm H-atom loss from xylyl radicals. To gain further insight into the photodissociation dynamics, velocity map images of the hydrogen atom photofragments are recorded. Their angular distribution is isotropic and the translational energy release is in agreement with a dissociation to products in their electronic ground state. Photodissociation of para-xylyl leads to the formation of para-xylylene (C8H8), while the data for ortho-xylyl agree much better with the isomer benzocyclobutene as the dominant molecular fragment rather than ortho-xylylene. In computations we identified a new pathway for the reaction ortho-xylyl → benzocyclobutene + H with a barrier of 3.39 eV (27 340 cm-1), which becomes accessible at the employed excitation energy. It proceeds via a combination of scissoring and rotational motion of the -CH2 and -CH3 groups. However, the observed rate constants measured by delaying the excitation and ionization laser with respect to each other are significantly faster than computed ones, indicating intrinsic non-RRKM behaviour. A comparably high value of around 30% of the excess energy is released as translation of the H-atom photofragment.

OES and GC/MS Study of RF Plasma of Xylenes

The radio-frequency discharge of xylene isomers was monitored with optical emission spectroscopy (OES). It was found that the meta isomer showed relatively stronger excimer to monomer intensity ratio than the other two isomers. OES also indicates the formation of xylyl (methylbenzyl) radicals. The reaction products of low pressure xylene plasmas were analyzed by gas-chromatography mass spectrometry (GC/MS). It showed that the main composition of the reaction products was 1,2-di-p-tolylethane (DPTE), regardless the types of xylene isomers used. It is known that o- and m-xylyl radicals can undergo rearrangement and convert to p-xylyl radicals. Similar to the cases in benzene and toluene plasmas, the recombination reaction between two p-xylyl radicals is believed to be responsible for the formation of DPTE. Density functional theory calculations suggest that the direct conversion of xylene excimers to DPTE is unlikely.

Isomer-Specific Product Detection of Gas-Phase Xylyl Radical Rearrangement and Decomposition Using VUV Synchrotron Photoionization

Xylyl radicals are intermediates in combustion processes since their parent molecules, xylenes, are present as fuel additives. In this study we report on the photoelectron spectra of the three isomeric xylyl radicals and the subsequent decomposition reactions of the o-xylyl radical, generated in a tubular reactor and probed by mass selected threshold photoelectron spectroscopy and VUV synchrotron radiation. Franck-Condon simulations are applied to augment the assignment of elusive species. Below 1000 K, o-xylyl radicals decompose by hydrogen atom loss to form closed-shell o-xylylene, which equilibrates with benzocyclobutene. At higher temperatures relevant to combustion engines, o-xylylene generates styrene in a multistep rearrangement, whereas the p-xylylene isomer is thermally stable, a key point of difference in the combustion of these two isomeric fuels. Another striking result is that all three xylyl isomers can generate p-xylylene upon decomposition. In addition to C8H8 isomers, phenylacetylene and traces of benzocyclobutadiene are observed and identified as further reaction products of o-xylylene, while there is also some preliminary evidence for benzene and benzyne formation. The experimental results reported here are complemented by a comprehensive theoretical C8H8 potential energy surface, which together with the spectroscopic assignments can explain the complex high-temperature chemistry of o-xylyl radicals.

Direct Observation of para-Xylylene as the Decomposition Product of the meta-Xylyl Radical Using VUV Synchrotron Radiation

Xylyl (methylbenzyl) radicals are important combustion intermediates, formed in the pyrolysis and oxidation of xylenes and other substituted aromatic fuel additives. We have used VUV synchrotron radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy techniques to identify para-xylylene as the dominant stable C8H8 product arising from thermal decomposition of the meta-xylyl radical. A complex rearrangement from a meta- to a para-substituted aromatic, supported by quantum chemical calculations, can rationalize the observed reaction products. This work provides the first experimental evidence for the pyrolysis products of the meta-xylyl radical and can explain why the decomposition of this radical is considerably slower than that of the ortho and para isomers. This study emphasizes the utility of VUV synchrotron radiation and iPEPICO spectroscopy to tackle the reaction mechanism of combustion-relevant processes.

Spectroscopic observation of jet-cooled 2-fluoro-m-xylyl radical

The jet-cooled 2-fluoro-m-xylyl radical was generated and vibronically excited in a corona excited supersonic expansion from precursor 2-fluoro-m-xylene seeded in a large amount of carrier gas helium. The well-resolved visible vibronic emission spectrum of the jet-cooled 2-fluoro-m-xylyl radical was recorded using a long-path monochromator. From the analysis of the spectrum, we determine an accurate electronic energy of the D(1) → D(0) transition and the frequencies of vibrational modes in the ground electronic state by comparison with those of ab initio calculations and the known spectroscopic data of 2-fluoro-m-xylene for the first time.

Observation of Jet-Cooled 2,6-Dichlorobenzyl Radical in a Corona Excited Supersonic Expansion

Electronic structure of benzyl-type radicals with 7 delocalized π electrons has been the subject of numerous spectroscopic works. Substitution in the benzene ring is also expected to affect the electronic energies and vibrational mode frequencies. Among mono-substituted benzyl-type radicals, the substitution at the para position has been examined more than the other isomers, since the transition dipole moments are parallel to the molecular symmetry axes, resulting in the characteristic bandshapes of the vibrational modes. Of many benzyl-type radicals, xylyl radicals and fluorobenzyl radicals have been extensively examined because they emit strong fluorescence during the electronic transition, from which electronic energies and vibrational modes have been determined by using a variety of spectroscopic techniques. Recently, the visible vibronic emission spectra of the dimethyland difluorobenzyl radicals were observed for the first time in this laboratory using a technique of a corona-excited supersonic expansion (CESE). The 2,6-dichlorobenzyl radical is supposed to play an important role as a reaction intermediate in the formation of the most toxic chemical, dioxin, which is produced in the combustion process of chlorinated aromatic compounds in incinerators. No spectroscopic work has been reported on this compound, probably due to the very weak fluorescence intensity in the visible region. In this work, we report the formation of the jet-cooled 2,6dichlorobenzyl radical for the first time, which was identified through the visible vibronic emission spectra. The origin band in the D1 → D0 transition of the radical was compared with those of chlorobenzyl radicals as well as other types of benzyl radicals of similar structure.. Combination of jet expansion technique with emission spectroscopy has significantly contributed to the spectroscopic development of molecular species that could not otherwise carried out. Among many emission sources developed for these purposes, the only one providing enough continuous photon intensity for high-resolution studies of the weak transition is a pinhole-type nozzle, particularly for benzyl-type radicals in the gas phase. The technique of corona excited supersonic expansion has been employed to generate the 2,6-dichlorobenzyl radical that was produced in a jet from 2,6-dichlorotoluene (reagent grade, Aldrich) and vibronically excited with a large amount of inert carrier gas He of 2.0 atm during the expansion through the pinhole-type glass nozzle (φ = 0.3 mm). The concentration of vapor in the carrier gas was adjusted for the maximum emission intensity monitored from the strongest origin band by controlling the sample temperature and opening the by-pass valve of the carrier gas. The excitation was obtained using a high-voltage dc power supply in the negative polarity, in which the axial discharge current was 35 mA at 2000 V dc potential and stabilized using a 150 kΩ current limiting ballast resister. A green-colored jet was the evidence of the presence of the 2,6-dichlorobenzyl radicals in the jet. It has been known that the benzyl-type radicals generated in a corona excitation will experience stepwise collisional relaxation during the expansion process, from which rovibrationally cold but electronically excited molecules will be formed. Then, the molecules in the jet lose its electronic energy by emitting fluorescence corresponding to the D1 → D0 transition. Thus, the vibronic spectra observed with CESE scheme is similar to the dispersed fluorescence (DF) spectra obtained by exciting the origin band of the electronic transition, in which the spacing of the vibronic bands from the origin band represents the vibrational mode frequencies in the ground electronic state, as shown in Figure 1. In chlorobenzyl radicals, the electronic interaction between the Cl atom and the aromatic ring is undoubtedly of the second-order compared to the interaction between the methylene group and the ring, since the molecule has a planar structure with 7 delocalized π electrons. Thus, the 2,6-dichlorobenzyl radical should exhibit a close relation to those of the benzyl radical of 7 delocalized π electrons, and one might be able to relate the two lowest lying electronic states of the 2,6-dichlorobenzyl radical to the parent benzyl 2B2(D2) and 1 A2(D1) states. 20

Jet-Cooled Emission Spectra of the Xylyl Radicals

Jet-cooled electronic emission spectra from ortho-, meta-, and para-xylyl (methylbenzyl) radicals have been recorded with a corona excited supersonic expansion (CESE) apparatus. A full vibronic analysis of the D1 → D0 transitions for all three isomers has been carried out, allowing for unambiguous assignments of the gas-phase ground state vibrational frequencies. For modes exhibiting progressions (numbering according to Green and Wilson) (11 (18a), 29 (6b), 10 (6a), 9 (1), 25 (3) and 5 (14) in ortho-xylyl; 10 (6b) in meta-xylyl; and 5 (1), 6 (6a), 3 (7a), and 17 (10a) in para-xylyl), anharmonicity constants are calculated and reported. Although CESE excitation of the xylenes (used in this study as precursors) did not result in the interconversion of isomers, it does occur along with homolytic methyl C-H bond dissociation during the formation of the radicals.

Free Radicals by Mass Spectrometry. VIII. The Ionization Potentials of Para-, Ortho-, and Meta-Xylyl Radicals

The vertical ionization potentials of the para-, ortho- and meta-xylyl radicals as measured by electron impact are found to be 7.46±0.03, 7.61±0.05, and 7.65±0.03 v, respectively. The thermal dissociation of the para- and ortho-xylyl radicals to form the quinodimethanes has been observed. The meta-xylyl radical was found to have a considerably greater thermal stability.

Thermal Stability of Ortho-, Para-, and Meta-Xylyl Radicals and the Formation of Quinodimethanes

Thermal Stability of <i>Ortho-, Para-,</i> and <i>Meta</i>-Xylyl Radicals and the Formation of Quinodimethanes