Reaction Of Phenyl Radicals Research Articles

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (C6H5) with propargyl radical (H2CCCH) provides an important prototype for radical-radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300-1000 K and pressure range of 4-10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the C9H8 and C9H7 + H product channels and report experimental isomer-resolved product branching fractions for the C9H8 product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These ab initio transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical-radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products C9H7 + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (C9H7) recombination to indene and H-assisted isomerization that converts less stable C9H8 isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons.

The Reaction of o-Benzyne with Vinylacetylene: An Unexplored Way to Produce Naphthalene.

The mechanism and kinetics of the reaction of ortho-benzyne with vinylacetylene have been studied by ab initio and density functional CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the pertinent potential energy surface combined with Rice-Ramsperger-Kassel-Marcus-Master Equation calculations of reaction rate constants at various temperatures and pressures. Under prevailing combustion conditions, the reaction has been shown to predominantly proceed by the biradical acetylenic mechanism initiated by the addition of C4 H4 to one of the C atoms of the triple bond in ortho-benzyne by the acetylenic end, with a significant contribution of the concerted addition mechanism. Following the initial reaction steps, an extra six-membered ring is produced and the rearrangement of H atoms in this new ring leads to the formation of naphthalene, which can further dissociate to 1- or 2-naphthyl radicals. The o-C6 H4 +C4 H4 reaction is highly exothermic, by ∼143 kcal/mol to form naphthalene and by 31-32 kcal mol-1 to produce naphthyl radicals plus H, but features relatively high entrance barriers of 9-11 kcal mol-1 . Although the reaction is rather slow, much slower than the reaction of phenyl radical with vinylacetylene, it forms naphthalene and 1- and 2-naphthyl radicals directly, with their relative yields controlled by the temperature and pressure, and thus represents a viable source of the naphthalene core under conditions where ortho-benzyne and vinylacetylene are available.

Investigation of kinetics of phenyl radicals with ethyl formate in the gas phase using cavity ring-down spectroscopy and theoretical methodologies.

The gas-phase kinetics of phenyl radical (·C6H5) with ethyl formate (HCO2Et, EF) was investigated experimentally using ultrasensitive laser-based cavity ring-down spectroscopy (CRDS). Phenyl radicals were generated by photolyzing nitrosobenzene (C6H5NO) at 248nm and thereby probed at 504.8nm. The rate coefficients for the (phenyl radical + EF) reaction were investigated between the temperatures of 260 and 361K and at a pressure of 61Torr with nitrogen (N2) as diluent. The temperature-dependent Arrhenius expression for the test reaction was obtained as: [Formula: see text]=(1.20 ± 0.16) × 10-13 exp[-(435.6 ± 50.0)/T] cm3 molecule-1s-1 and the rate coefficient at room temperature was measured out to be: [Formula: see text]=(4.54 ± 0.42) × 10-14 cm3 molecule-1s-1. The effects of pressure and laser fluence on the kinetics of the test reaction were found to be negligible within the experimental uncertainties. To complement the experimental findings, kinetics for the reaction of phenyl radicals with EF was investigated theoretically using Canonical Variational Transition State Theory (CVT) with Small Curvature Tunnelling (SCT) at CCSD(T)/cc-pVDZ//B3LYP/6-31 + G(d,p) level of theory in the temperatures between 200 and 400K. The theoretically calculated rate coefficients for the title reaction were expressed in the Arrhenius form as: [Formula: see text]= (1.48 ± 0.56) × 10-38 × T8.47 × exp[(2431.3 ± 322.0)/T] cm3 molecule-1s-1 and the corresponding rate coefficient at room temperature was calculated to be: [Formula: see text]= 4.91 × 10-14 cm3 molecule-1s-1. A very good agreement was observed between the experimentally measured and theoretically calculated rate coefficients at 298K. Thermochemical parameters as well as branching ratios for the reaction of (phenyl radical + EF) are also discussed in this manuscript.

Reaction of phenyl radicals towards propionaldehyde and butyraldehyde over the temperature range of 200–2000 K

The reactivity of phenyl radical towards propionaldehyde and butyraldehyde was investigated by the CVT/SCT/ISPE method using the energetics computed at the CCSD(T)//B3LYP/cc-pVTZ level of theory over the temperature range of 200–2000 K. The rate coefficients (k × 1014 cm3 molecule−1 sec−1) for two aldehydes were computed to be 7.7 and 15.2 at 298 K. The branching ratios explained the contribution of H-abstraction reaction channels to the global reactivity. The degradative mechanism explained the formation of acids, aldehydes, and peroxy nitrates. The reactivity of these aldehydes towards phenyl radicals was also compared with their reactivity towards other atmospheric oxidants.

A high temperature shock tube study of phenyl recombination reaction using laser absorption spectroscopy

The chemistry of first aromatic ring, i.e., benzene (C6H6) and phenyl radial (C6H5), plays a key role in the growth of polycyclic aromatic hydrocarbons (PAHs) and ultimately soot formation. In this work, the self-recombination reaction of phenyl radicals was investigated over the temperature range of 950–1300 K and pressures near 1 atm by employing shock tube and laser absorption diagnostics. Phenyl radical was generated by the rapid thermal unimolecular dissociation of nitrosobenzene (C6H5NO), a clean precursor of C6H5 radical. The reaction progress was monitored by detecting C6H5 and NO simultaneously using visible laser absorption near 445 nm and mid-IR laser absorption near 5.517 µm, respectively. For the reaction C6H5NO → C6H5 + NO (R1), our data show an excellent agreement with earlier reports. The high-pressure limiting rate coefficient, by combining all available data, may be expressed as k1∞(T(K))=3.2×1066T−15.2e−37743/T s−1. This work reports the temperature dependence of the absorption cross-section of phenyl radical at 445 nm for first time. Our experiments indicate that the self-reaction of phenyl radicals yielding biphenyl, C6H5 + C6H5 → C6H5C6H5 (R2a), is a major channel. The rate coefficients of reaction (R2a) show a weak temperature dependence with an average value of k2a = (6.91 ± 0.42) × 1012 cm3 mol−1 s−1 in the temperature range of 950–1300 K. Our measured data, k2a(T, P = 1.1–1.5 atm), are found to be close to the high-pressure limiting rate coefficients. Combining with the literature low-temperature data, the self-recombination reaction of phenyl radicals may be expressed as k2a∞(T=300−1450K)=2.8×1017T−1.44e−540/T cm3 mol−1 s−1. The measurements of this study represent the first high-temperature direct experimental determination of the rate coefficients of this important prototype aromatic radical-radical reaction.

Kinetic Investigations of the Reaction of Phenyl Radicals with Ethyl Acetate in the Gas Phase: An Experimental and Computational Study.

Cavity ring-down spectroscopy (CRDS) was employed to investigate the kinetics of the reaction between phenyl radicals (C6H5•) and ethyl acetate (EtOAc) in the gas phase. Nitrosobenzene (C6H5NO) was used as the radical precursor to generate C6H5• at 248 nm, and the generated radicals were subsequently probed at 504.8 nm. The rate coefficients were investigated experimentally in the temperature range of 258-358 K with an interval of 20 K and at a total pressure of 55 Torr in the nitrogen atmosphere. The obtained Arrhenius expression for the title reaction (C6H5• + EtOAc) in the temperature range of 258-358 K was kphenyl + EtOAcExpt - (258 - 358 K) = (9.33 ± 0.11) × 10-16 exp[(883.7 ± 181.0)/T] cm3 molecule-1 s-1, and the rate coefficient at room temperature (298 K) was kphenyl + EtOAcExpt - 298 K = (2.20 ± 0.12) × 10-14 cm3 molecule-1 s-1. Negligible effects of pressure and photolysis laser fluence were found on the experimentally measured rate coefficients. To complement our experimental findings, rate coefficients of the title reaction were computationally investigated employing the canonical variational transition-state theory with small curvature tunnelling (CVT/SCT) at the CCSD(T)/cc-pVDZ//B3LYP/6-31+G(d,p) level of theory in the temperature range of 200-400 K. The temperature-dependent rate coefficient in the studied temperature range was obtained to be kphenyl + EtOAcTheory - (200 - 400K) = (7.68 ± 0.12) × 10-17 exp[(1731.6 ± 216.0)/T] cm3 molecule-1 s-1, and the rate coefficient at 298 K was obtained as kphenyl + EtOAcTheory - 298K = 2.45 × 10-14 cm3 molecule-1 s-1. Both the experimentally measured and computed rate coefficients show good agreement at 298 K. A negative temperature dependency was observed for both the experimentally measured and computed rate coefficients. A detailed discussion of the thermochemical parameters and branching ratios of the title reaction are also presented in this Article.

Investigation of the Absorption Cross Section of Phenyl Radical and Its Kinetics with Methanol in the Gas Phase Using Cavity Ring-Down Spectroscopy and Theoretical Methodologies.

Cavity ring-down spectroscopy (CRDS) was used to measure the absorption cross section of phenyl radicals (C6H5•) at 504.8 nm (2B1 ← 2A1 transition) in the nitrogen atmosphere at 40 Torr total pressure and 298 K using nitrosobenzene (C6H5NO) as the radical precursor. At 504.8 nm, the absorption cross section was measured to be σphenyl504.8nm = (5.7 ± 1.4) × 10-19 cm2 molecule-1. The absorption cross section was independent of the total pressure range (40-200 Torr) over which it was studied with a precursor concentration of (4-5) × 1013 molecules cm-3. In addition to this, the absolute rate coefficients for the reaction of phenyl radicals with methanol were measured over the temperature range of 263-298 K and at 40 Torr pressure with N2 using CRDS. The temperature-dependent rate coefficient for the title reaction over the studied temperature range was obtained to be k263-298Kexperiment (T) = (1.38 ± 0.60) × 10-11 exp [-(1764 ± 321)/T] cm3 molecule-1 s-1 with a rate coefficient of k(T) = (3.50 ± 0.32) × 10-14 cm3 molecule-1 s-1 at 298 K. The effect of pressure and laser fluence was found to be negligible within the experimental uncertainties in the studied range. In addition, to complement our experimental findings, the T-dependent rate coefficients for the title reaction were investigated using computational methods. The B3LYP/6-311 + G(d,p) level of theory was used in combination with canonical variational transition-state theory with small-curvature tunneling to calculate the rate coefficients. The T-dependent rate coefficient in the range of 200-400 K was obtained as k200-400Ktheory (T) = 2.43 × 10-13 exp[-(478.38/T)] cm3 molecule-1 s-1 with a room-temperature (298 K) rate coefficient of 4.67 × 10-14 cm3 molecule-1 s-1.

Experimental and modeling study of benzaldehyde oxidation

Benzaldehyde is an aromatic aldehyde commonly considered in bio-oil surrogate formulation, and an important intermediate in the oxidation of other aromatic reference fuels such as toluene. However, its oxidation has never been previously investigated experimentally and no product formation profiles were reported in the very limited pyrolysis studies available in the literature. In this study, the gas-phase oxidation of benzaldehyde was investigated in a jet-stirred reactor. 48 species were detected using gas chromatography, mainly CO, CO2 and phenol. The important formation of CO and phenol indicates a rapid formation of phenyl radicals. This was confirmed by a kinetic analysis performed using the current version of the CRECK kinetic model, in which the reactions of phenyl radicals and oxygenated aromatic compounds have been updated.

Formation Mechanisms of Naphthalene and Indene: From the Interstellar Medium to Combustion Flames.

The article addresses the formation mechanisms of naphthalene and indene, which represent prototype polycyclic aromatic hydrocarbons (PAH) carrying two six-membered and one five- plus a six-membered ring. Theoretical studies of the relevant chemical reactions are overviewed in terms of their potential energy surfaces, rate constants, and product branching ratios; these data are compared with experimental measurements in crossed molecular beams and the pyrolytic chemical reactor emulating the extreme conditions in the interstellar medium (ISM) and the combustion-like environment, respectively. The outcome of the reactions potentially producing naphthalene and indene is shown to critically depend on temperature and pressure or collision energy and hence the reaction mechanisms and their contributions to the PAH growth can be rather different in the ISM, planetary atmospheres, and in combustion flames at different temperatures and pressures. Specifically, this paradigm is illustrated with new theoretical results for rate constants and product branching ratios for the reaction of phenyl radical with vinylacetylene. The analysis of the formation mechanisms of naphthalene and its derivatives shows that in combustion they can be produced via hydrogen-abstraction-acetylene-addition (HACA) routes, recombination of cyclopentadienyl radical with itself and with cyclopentadiene, the reaction of benzyl radical with propargyl, methylation of indenyl radical, and the reactions of phenyl radical with vinylacetylene and 1,3-butadiene. In extreme astrochemical conditions, naphthalene and dihydronaphthalene can be formed in the C6H5 + vinylacetylene and C6H5 + 1,3-butadiene reactions, respectively. Ethynyl-substituted naphthalenes can be produced via the ethynyl addition mechanism beginning with benzene (in dehydrogenated forms) or with styrene. The formation mechanisms of indene in combustion include the reactions of the phenyl radical with C3H4 isomers allene and propyne, reaction of the benzyl radical with acetylene, and unimolecular decomposition of the 1-phenylallyl radical originating from 3-phenylpropene, a product of the C6H5 + propene reaction, or from C6H5 + C3H5.

Surface Modification of Carbon Black Using Polymer Resin Synthesized by a Phenyl Radical Reaction

Surface Modification of Carbon Black Using Polymer Resin Synthesized by a Phenyl Radical Reaction

Formation of polycyclic aromatic hydrocarbons from bimolecular reactions of phenyl radicals at high temperatures.

The self-reaction of the phenyl radical is one of the key reactions in combustion chemistry. Here we study this reaction in a high-temperature flow reactor by IR/UV ion dip spectroscopy, using free electron laser radiation as mid-infrared source. We identified several major reaction products based on their infrared spectra, among them indene, 1,2-dihydronaphthalene, naphthalene, biphenyl and para-terphenyl. Due to the structural sensitivity of the method, the reaction products were identified isomer-selectively. The work shows that the formation of indene and naphthalene, which was previously considered to be evidence for the HACA (hydrogen abstraction C2H2 addition) mechanism in the formation of polycyclic aromatic hydrocarbons and soot can also be understood in a phenyl addition model.

Formation of 2- and 1-methyl-1,4-dihydronaphthalene isomers via the crossed beam reactions of phenyl radicals (C6H5) with isoprene (CH2C(CH3)CHCH2) and 1,3-pentadiene (CH2CHCHCHCH3).

Crossed molecular beam reactions were exploited to elucidate the chemical dynamics of the reactions of phenyl radicals with isoprene and with 1,3-pentadiene at a collision energy of 55 ± 4 kJ mol(-1). Both reactions were found to proceed via indirect scattering dynamics and involve the formation of a van-der-Waals complex in the entrance channel. The latter isomerized via the addition of the phenyl radical to the terminal C1/C4 carbon atoms through submerged barriers forming resonantly stabilized free radicals C11H13, which then underwent cis-trans isomerization followed by ring closure. The resulting bicyclic intermediates fragmented via unimolecular decomposition though the atomic hydrogen loss via tight exit transition states located 30 kJ mol(-1) above the separated reactants in overall exoergic reactions forming 2- and 1-methyl-1,4-dihydronaphthalene isomers. The hydrogen atoms are emitted almost perpendicularly to the plane of the decomposing complex and almost parallel to the total angular momentum vector ('sideways scattering') which is in strong analogy to the phenyl-1,3-butadiene system studied earlier. RRKM calculations confirm that 2- and 1-methyl-1,4-dihydronaphthalene are the dominating reaction products formed at levels of 97% and 80% in the reactions of the phenyl radical with isoprene and 1,3-pentadiene, respectively. This barrier-less formation of methyl-substituted, hydrogenated PAH molecules further supports our understanding of the formation of aromatic molecules in extreme environments holding temperatures as low as 10 K.

Toward the Oxidation of the Phenyl Radical and Prevention of PAH Formation in Combustion Systems.

The reaction of the phenyl radical (C6H5) with molecular oxygen (O2) plays a central role in the degradation of poly- and monocyclic aromatic radicals in combustion systems which would otherwise react with fuel components to form polycyclic aromatic hydrocarbons (PAHs) and eventually soot. Despite intense theoretical and experimental scrutiny over half a century, the overall reaction channels have not all been experimentally identified. Tunable vacuum ultraviolet photoionization in conjunction with a combustion simulating chemical reactor uniquely provides the complete isomer specific product spectrum and branching ratios of this prototype reaction. In the reaction of phenyl radicals and molecular oxygen at 873 K and 1003 K, ortho-benzoquinone (o-C6H4O2), the phenoxy radical (C6H5O), and cyclopentadienyl radical (C5H5) were identified as primary products formed through emission of atomic hydrogen, atomic oxygen and carbon dioxide. Furan (C4H4O), acrolein (C3H4O), and ketene (C2H2O) were also identified as primary products formed through ring opening and fragmentation of the 7-membered ring 2-oxepinoxy radical. Secondary reaction products para-benzoquinone (p-C6H4O2), phenol (C6H5OH), cyclopentadiene (C5H6), 2,4-cyclopentadienone (C5H4O), vinylacetylene (C4H4), and acetylene (C2H2) were also identified. The pyranyl radical (C5H5O) was not detected; however, electronic structure calculations show that it is formed and isomerizes to 2,4-cyclopentadienone through atomic hydrogen emission. In combustion systems, barrierless phenyl-type radical oxidation reactions could even degrade more complex aromatic radicals. An understanding of these elementary processes is expected to lead to a better understanding toward the elimination of carcinogenic, mutagenic, and environmentally hazardous byproducts of combustion systems such as PAHs.

Acid generation mechanism in anion-bound chemically amplified resists used for extreme ultraviolet lithography

Extreme ultraviolet (EUV) lithography is the most promising candidate for the high-volume production of semiconductor devices with half-pitches of sub-10 nm. An anion-bound polymer (ABP), in which the anion part of onium salts is polymerized, has attracted much attention from the viewpoint of the control of acid diffusion. In this study, the acid generation mechanism in ABP films was investigated using electron (pulse), γ, and EUV radiolyses. On the basis of experimental results, the acid generation mechanism in anion-bound chemically amplified resists was proposed. The major path for proton generation in the absence of effective proton sources is considered to be the reaction of phenyl radicals with diphenylsulfide radical cations that are produced through hole transfer to the decomposition products of onium salts.

Crossed Molecular Beams Studies of Phenyl Radical Reactions with Propene and trans-2-Butene

The reactions of phenyl radicals with propene have been studied at collision energies of 84 and 108 kJ/mol using the crossed molecular beams technique. The branching ratios between methyl radical elimination forming C8H8 and H-atom elimination forming C9H10 were found to be 10 ± 1:1 at 84 kJ/mol and 3 ± 1:1 at 108 kJ/mol. By using "soft" 9.9 eV vacuum ultraviolet photoionization for product detection, we were able to observe both product channels with negligible fragmentation of C9H10 to C8H8(+). Our finding that CH3 elimination is dominant is consistent with conclusions from a recent study employing a pyrolysis molecular beam reactor using photoionization detection. However, our C8H8/C9H10 branching ratios are significantly larger than inferred from previous CMB experiments and RRKM calculations. For comparison, we have also studied the reactions of phenyl radicals with trans-2-butene at Ecoll = 97 kJ/mol. In this case, the symmetry of trans-2-butene makes both alkene addition sites chemically equivalent. The intermediate formed in the reaction with trans-2-butene is similar to the 2-carbon addition intermediate in the reaction with propene. We observed only methyl elimination in the reaction with trans-2-butene, with no evidence for H-atom elimination, consistent with conclusions that C-C bond fission is the most favorable channel in these systems. Analogies between phenyl radical reactions with propene and trans-2-butene are used to provide insight into the mechanisms in the propene reaction.

A Combined Crossed Beam and Ab Initio Investigation of the Gas Phase Reaction of Dicarbon Molecules (C2; X1Σg+/a3Πu) with Propene (C3H6; X1A′): Identification of the Resonantly Stabilized Free Radicals 1- and 3-Vinylpropargyl

The crossed molecular beam reactions of dicarbon, C2(X(1)Σg(+), a(3)Πu), with propene (C3H6; X(1)A') and with the partially deuterated D3 counterparts (CD3CHCH2, CH3CDCD2) were conducted at collision energies of about 21 kJ mol(-1) under single collision conditions. The experimental data were combined with ab initio and statistical (RRKM) calculations to reveal the underlying reaction mechanisms. Both on the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the addition of the dicarbon reactant to the carbon-carbon double bond of propene. These initial addition complexes rearrange via multiple isomerization steps leading ultimately via atomic hydrogen elimination from the former methyl and vinyl groups to the formation of 1-vinylpropargyl and 3-vinylpropargyl. Both triplet and singlet methylbutatriene species were identified as important reaction intermediates. On the singlet surface, the unimolecular decomposition of the reaction intermediates was found to be barrier-less, whereas on the triplet surface, tight exit transition states were involved. In combustion flames, both radicals can undergo a hydrogen-atom assisted isomerization leading ultimately to the thermodynamically most stable cyclopentadienyl isomer. Alternatively, in a third body process, a subsequent reaction of 1-vinylpropargyl or 3-vinylpropargyl radicals with the propargyl radical might yield to the formation of styrene (C6H5C2H3) in an entrance barrier-less reaction under combustion-like conditions. This presents a strong alternative to the formation of styrene via the reaction of phenyl radicals with ethylene, which is affiliated with an entrance barrier of about 10 kJ mol(-1).

Trialkyl Phosphites and Diaryliodonium Salts as Co-initiators in a System for Radical-Promoted Visible-Light-Induced Cationic Polymerization

Trialkyl phosphites ((RO)3P) can act as co-initiators for the diaryliodonium-induced cationic polymerization of cyclohexene oxide (CHO) or THF. A radical initiation step is also required, consistent with the essential role of a radical chain reaction of the phosphite with the iodonium salt to form polymerization-starting aryltrialkoxyphosphonium salts (ArP(+)(OR)3). We used the visible photolysis of phenylazoisobutyronitrile (PAIBN) as the radical initiation step. The presence of multiple fluorine substituents on the phosphite, as in tris(2,2,2-trifluoroethyl) phosphite (TFP), allows polymerization to proceed with a minimal amount of chain transfer from nucleophilic attack by the phosphite. In a typical experiment, a CHO solution of bis(4-tert-butylphenyl)iodonium hexafluorophosphate (0.05 M), TFP (0.1 M), and PAIBN (0.02 M) was illuminated with a 65-W compact fluorescent bulb for 1 h, resulting in a 78% conversion to poly(cyclohexene oxide) with an average molecular weight (MW) of 25000. We also used competition experiments to determine approximate rate constants for reactions of phenyl radicals with CHO (k = 2 × 10(6) M(-1) s(-1)) and with TFP (k = 2 × 10(8) M(-1) s(-1)).

A VUV photoionization study of the multichannel reaction of phenyl radicals with 1,3-butadiene under combustion relevant conditions

We studied the reaction of phenyl radicals (C(6)H(5)) with 1,3-butadiene (H(2)CCHCHCH(2)) exploiting a high temperature chemical reactor under combustion-like conditions (300 Torr, 873 K). The reaction products were probed in a supersonic beam by utilizing VUV radiation from the Advanced Light Source and by recording the experimental PIE curves at mass-to-charge ratios of m/z = 130 (C(10)H(10)(+)), 116 (C(9)H(8)(+)), and 104 (C(8)H(8)(+)). Our data suggest that the atomic hydrogen (H), methyl (CH(3)), and vinyl (C(2)H(3)) losses are open with estimated branching ratios of about 86 ± 4%, 8 ± 2%, and 6 ± 2%, respectively. The isomer distributions were probed further by fitting the experimentally recorded PIE curves with a linear combination of the PIE curves of individual C(10)H(10), C(9)H(8), and C(8)H(8) isomers. These fits indicate the formation of three C(10)H(10) isomers (trans-1,3-butadienylbenzene, 1,4-dihydronaphthalene, 1-methylindene), three C(9)H(8) isomers (indene, phenylallene, 1-phenyl-1-methylacetylene), and a C(8)H(8) isomer (styrene). A comparison with results from recent crossed molecular beam studies of the 1,3-butadiene-phenyl radical reaction and electronic structure calculations suggests that trans-1,3-butadienylbenzene (130 amu), 1,4-dihydronaphthalene (130 amu), and styrene (104 amu) are reaction products formed as a consequence of a bimolecular reaction between the phenyl radical and 1,3-butadiene. 1-Methylindene (130 amu), indene (116 amu), phenylallene (116 amu), and 1-phenyl-1-methylacetylene (116 amu) are synthesized upon reaction of the phenyl radical with three C(4)H(6) isomers: 1,2-butadiene (H(2)CCCH(CH(3))), 1-butyne (HCCC(2)H(5)), and 2-butyne (CH(3)CCCH(3)); these C(4)H(6) isomers can be formed from 1,3-butadiene via hydrogen atom assisted isomerization reactions or via thermal rearrangements of 1,3-butadiene involving hydrogen shifts in the high temperature chemical reactor.

Studies of bimolecular reaction dynamics using pulsed high-intensity vacuum-ultraviolet lasers for photoionization detection

This article describes recent progress on the development and application of pulsed high-intensity (~0.1 mJ per pulse) vacuum-ultraviolet (VUV) radiation produced by commercial tabletop lasers for studies of gas phase chemical reaction dynamics involving polyatomic free radicals. Our approach employs near-triply resonant four-wave mixing of unfocussed nanosecond dye lasers in an atomic gas as an alternative to the use of synchrotron light sources for sensitive universal soft photoionization detection of reaction products using a rotatable source crossed molecular beams apparatus with fixed detector. We illustrate this approach in studies of the reactions of phenyl radicals with molecular oxygen and with propene. Future prospects for the use of tabletop laser-based VUV sources for studies of chemical reaction dynamics are discussed.

PAH Formation under Single Collision Conditions: Reaction of Phenyl Radical and 1,3-Butadiene to Form 1,4-Dihydronaphthalene

The crossed beam reactions of the phenyl radical (C(6)H(5), X(2)A(1)) with 1,3-butadiene (C(4)H(6), X(1)A(g)) and D6-1,3-butadiene (C(4)D(6), X(1)A(g)) as well as of the D5-phenyl radical (C(6)D(5), X(2)A(1)) with 2,3-D2-1,3-butadiene and 1,1,4,4-D4-1,3-butadiene were carried out under single collision conditions at collision energies of about 55 kJ mol(-1). Experimentally, the bicyclic 1,4-dihydronaphthalene molecule was identified as a major product of this reaction (58 ± 15%) with the 1-phenyl-1,3-butadiene contributing 34 ± 10%. The reaction is initiated by a barrierless addition of the phenyl radical to the terminal carbon atom of the 1,3-butadiene (C1/C4) to form a bound intermediate; the latter underwent hydrogen elimination from the terminal CH(2) group of the 1,3-butadiene molecule leading to 1-phenyl-trans-1,3-butadiene through a submerged barrier. The dominant product, 1,4-dihydronaphthalene, is formed via an isomerization of the adduct by ring closure and emission of the hydrogen atom from the phenyl moiety at the bridging carbon atom through a tight exit transition state located about 31 kJ mol(-1) above the separated products. The hydrogen atom was found to leave the decomposing complex almost parallel to the total angular momentum vector and perpendicularly to the rotation plane of the decomposing intermediate. The defacto barrierless formation of the 1,4-dihydronaphthalene molecule involving a single collision between a phenyl radical and 1,3-butadiene represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated counterparts in combustion and interstellar chemistry.