Combined Quantum Chemical/RRKM-ME Computational Study of the Phenyl + Ethylene, Vinyl + Benzene, and H + Styrene Reactions

Gas-Phase Formation of 1,3,5,7-Cyclooctatetraene (C8H8) through Ring Expansion via the Aromatic 1,3,5-Cyclooctatrien-7-yl Radical (C8H9•) Transient

Gas-phase 1,3,5,7-cyclooctatetraene (C₈H₈) and triplet aromatic 1,3,5,7-cyclooctatetraene (C₈H₈) were formed for the first time through bimolecular methylidyne radical (CH)-1,3,5-cycloheptatriene (C₇H₈) reactions under single-collision conditions on a doublet surface. The reaction involves methylidyne radical addition to the olefinic π electron system of 1,3,5-cycloheptatriene followed by isomerization and ring expansion to an aromatic 1,3,5-cyclooctatrien-7-yl radical (C₈H₉^•). The chemically activated doublet radical intermediate undergoes unimolecular decomposition to 1,3,5,7-cyclooctatetraene. Substituted 1,3,5,7-cyclooctatetraene molecules can be prepared in the gas phase with hydrogen atom(s) in the 1,3,5-cycloheptatriene reactant being replaced by organic side groups. These findings are also of potential interest to organometallic chemists by expanding the synthesis of exotic transition-metal complexes incorporating substituted 1,3,5,7-cyclooctatetraene dianion (C₈H₈^2-) ligands and to untangle the unimolecular decomposition of chemically activated and substituted 1,3,5-cyclooctatrien-7-yl radical, eventually gaining a fundamental insight of their bonding chemistry, electronic structures, and stabilities.

Chemical insights into the atmospheric oxidation of thiophene by hydroperoxyl radical

The reaction mechanisms and kinetics of thiophene oxidation reactions initiated by hydroperoxyl radical, and decomposition of the related intermediates and complexes, have been considered herein by using high-level DFT and ab initio calculations. The main energetic parameters of all stationary points of the suggested potential energy surfaces have been computed at the BD(T) and CCSD(T) methods, based on the geometries optimized at the B3LYP/6-311 + g(d,p) level of theory. Rate constants of bimolecular reactions (high-pressure limit rate constants) at temperatures from 300 to 3000 K for the first steps of the title reaction have been obtained through the conventional transition state theory (TST), while the pressure dependent rate constants and the rate constants of the second and other steps have been calculated employing the Rice-Ramsperger-Kassel-Marcus/Master equation (RRKM/ME). The results show that the rate constants of addition to α and β carbons have positive temperature dependence and negative pressure dependence. It is found that the additions of HO₂ to the α and β carbons of thiophene in the initial steps of the title reaction are the most favored pathways. Also, the addition to the sulfur atom has a minor contribution. But, all efforts for simulating hydrogen abstraction reactions have been unsuccessful. In this complex oxidation reaction, about 12 different products are obtained, including important isomers such as thiophene-epoxide, thiophene-ol, thiophene-oxide, oxathiane, and thiophenone. The calculated total rate constants for generation of all minimum stationary points show that the addition reactions to the α and β carbons are the fastest among all at temperatures below 1000 K, while the proposed multi-step parallel reactions are more competitive at temperatures above 1200 K. Furthermore, important inter-and intra-molecular interactions for some species have been investigated by two well-known quantum chemistry method, the NBO and AIM analyses. Thermochemical properties such as free energy, enthalpy, internal energy, and entropy for thiophene and hydroperoxyl radical and related species in the simulated reactions have been predicted using a combination of the B3LYP and BD(T) methods.

Direct Kinetics and Product Measurement of Phenyl Radical + Ethylene

The phenyl + ethylene (C₆H₅ + C₂H₄) reaction network was explored experimentally and theoretically to understand the temperature dependence of the reaction kinetics and product distribution under various temperature and pressure conditions. The flash photolysis apparatus combining laser absorbance spectroscopy (LAS) and time-resolved molecular beam mass spectrometry (MBMS) was used to study reactions on the C₈H₉ potential energy surface (PES). In LAS experiments, 505.3 nm laser light selectively probed C₆H₅ decay, and we measured the total C₆H₅ consumption rate coefficients in the intermediate temperature region (400–800 K), which connects previous experiments performed in high-temperature (pyrolysis) and low-temperature (cavity-ring-down methods) regions. From the quantum chemistry calculations by Tokmakov and Lin using the G2M(RCC5)//B3LYP method, we constructed a kinetic model and estimated phenomenological pressure-dependent rate coefficients, k(T, P), with the Arkane package in the reaction mechanism generator. The MBMS experiments, performed at 600–800 K and 10–50 Torr, revealed three major product peaks: m/z = 105 (adducts, mostly 2-phenylethyl radical, but also 1-phenylethyl radical, ortho-ethyl phenyl radical, and a spiro-fused ring radical), 104 (styrene, co-product with a H atom), and 78 (benzene, co-product with C₂H₃ radical). Product branching ratios were predicted by the model and validated by experiments for the first time. At 600 K and 10 Torr, the yield ratio of the H-abstraction reaction (forming benzene + C₂H₃) is measured to be 1.1% and the H-loss channel (styrene + H) has a 2.5% yield ratio. The model predicts 1.0% for H-abstraction and 2.3% for H-loss, which is within the experimental error bars. The branching ratio and formation of styrene increase at high temperature due to the favored formally direct channel (1.0% at 600 K and 10 Torr, 5.8% at 800 K and 10 Torr in the model prediction) and the faster β-scission reactions of C₈H₉ isomers. The importance of pressure dependence in kinetics is verified by the increase in the yield of the stabilized adduct from radical addition from 80.2% (800 K, 10 Torr) to 88.9% (800 K, 50 Torr), at the expense of styrene + H. The pressure-dependent model developed in this work is well validated by the LAS and MBMS measurements and gives a complete picture of the C₆H₅ + C₂H₄ reaction.

β-Bond Scission and the Yields of H and CH3 in the Decomposition of Isobutyl Radicals

The relative rates of C-C and C-H β-scission reactions of isobutyl radicals (2-methylprop-1-yl, C₄H₉) were investigated with shock tube experiments at temperatures of (950 to 1250) K and pressures of (200 to 400) kPa. We produced isobutyl radicals from the decomposition of dilute mixtures of isopentylbenzene and observed the stable decomposition products, propene and isobutene. These alkenes are characteristic of C-C and C-H bond scission, respectively. Propene was the main product, approximately 30 times more abundant than isobutene, indicating that C-C β-scission is the primary pathway. Uncertainty in the ratio of [isobutene]/[propene] from isobutyl decomposition is mainly due to a small amount of side chemistry, which we account for using a kinetics model based on JetSurF 2.0. Our data are well-described after adding chemistry specific to our system and adjusting some rate constants. We compare our data to other commonly used kinetics models: JetSurF 2.0, AramcoMech 2.0, and multiple models from Lawrence Livermore National Laboratory (LLNL). With the kinetics model, we have determined an upper limit of 3.0% on the branching fraction for C-H β-scission in the isobutyl radical for the temperatures and pressures of our experiments. While this agrees with previous high quality experimental results, many combustion kinetics models assume C-H branching values above this upper limit, possibly leading to large systematic inaccuracies in model predictions. Some kinetics models additionally assume contributions from 1,2-H shift reactions-which for isobutyl would produce the same products as C-H β-scission-and our upper limit includes possible involvement of such reactions. We suggest kinetics models should be updated to better reflect current experimental measurements.

Computational Study on the Unimolecular Decomposition of JP-8 Jet Fuel Surrogates III: Butylbenzene Isomers ( n-, s-, and t-C14H10)

Ab initio G3(CCSD,MP2)//B3LYP/6-311G(d,p) calculations of potential energy surfaces have been carried out to unravel the mechanism of the initial stages of pyrolysis of three C₁₀H₁₄ isomers: n-, s-, and t-butylbenzenes. The computed energy and molecular parameters have been utilized in RRKM-master equation calculations to predict temperature- and pressure-dependent rate constants and product branching ratios for the primary unimolecular decomposition of these molecules and for the secondary decomposition of their radical fragments. The results showed that the primary dissociation of n-butylbenzene produces mostly benzyl (C₇H₇) + propyl (C₃H₇) and 1-phenyl-2-ethyl (C₆H₅C₂H₄) + ethyl (C₂H₅), with their relative yields strongly dependent on temperature and pressure, together with a minor amount of 1-phenyl-prop-3-yl (C₉H₁₁) + methyl (CH₃). Secondary decomposition reactions that are anticipated to occur on a nanosecond scale under typical combustion conditions split propyl (C₃H₇) into ethylene (C₂H₄) + methyl (CH₃), ethyl (C₂H₅) into ethylene (C₂H₄) + hydrogen (H), 1-phenyl-2-ethyl (C₆H₅C₂H₄) into mostly styrene (C₈H₈) + hydrogen (H) and to a lesser extent phenyl (C₆H₅) + ethylene (C₂H₄), and 1-phenyl-prop-3-yl (C₉H₁₁) into predominantly benzyl (C₇H₇) + ethylene (C₂H₄). The primary decomposition of s-butylbenzene is predicted to produce 1-phenyl-1-ethyl (C₆H₅CHCH₃) + ethyl (C₂H₅) and a minor amount of 1-phenyl-prop-1-yl (C₉H₁₁) + methyl (CH₃), and then 1-phenyl-1-ethyl (C₆H₅CHCH₃) and 1-phenyl-prop-1-yl (C₉H₁₁) rapidly dissociate to styrene (C₈H₈) + hydrogen (H) and styrene (C₈H₈) + methyl (CH₃), respectively. t-Butylbenzene decomposes nearly exclusively to 2-phenyl-prop-2-yl (C₉H₁₁) + methyl (CH₃), and further, 2-phenyl-prop-2-yl (C₉H₁₁) rapidly eliminates a hydrogen atom to form 2-phenylpropene (C₉H₁₀). If hydrogen atoms or other reactive radicals are available to make a direct hydrogen-atom abstraction from butylbenzenes possible, the C₁₀H₁₃ radicals (1-phenyl-but-1-yl, 2-phenyl-but-2-yl, and t-phenyl-isobutyl) can be formed as the primary products from n-, s-, and t-butylbenzene, respectively. The secondary decomposition of 1-phenyl-but-1-yl leads to styrene (C₈H₈) + ethyl (C₂H₅), whereas 2-phenyl-but-2-yl and t-phenyl-isobutyl dissociate to 2-phenylpropene (C₉H₁₀) + methyl (CH₃). Thus, the three butylbenzene isomers produce distinct but overlapping nascent pyrolysis fragments, which likely affect the successive oxidation mechanism and combustion kinetics of these JP-8 fuel components. Temperature- and pressure-dependent rate constants generated for the initial stages of pyrolysis of butylbenzenes are recommended for kinetic modeling.

Atmospheric Reaction of Cl with 4-Hydroxy-2-pentanone (4H2P): A Theoretical Study

The kinetics and the mechanism of the reaction of 4-hydroxy-2-pentanone (4H2P) with Cl atom were investigated using quantum theoretical calculations. Density functional theory, CBS-QB3, and G3B3 methods are used to explore the reaction pathways. Rice-Ramsperger-Kassel-Marcus theory is employed to obtain rate constants of the reaction at atmospheric pressure and the temperature range 278-400 K. This study provides the first theoretical and kinetic determination of Cl rate constant for reactions with 4H2P over a large temperature range. The obtained rate constant 1.47 × 10^-10 cm³ molecule^-1 s^-1 at 298 K is in reasonable agreement with those obtained for C4-C5 hydroxyketones both theoretically and experimentally. The results regarding the structure-reactivity relationship and the atmospheric implications are discussed.

Rearrangements leading to fragmentations of hydrocinnamate and analogous nitrogen-containing anions upon collision-induced dissociation

Tandem mass spectrometry (MS/MS) confirmed decarboxylation as the major collision-induced dissociation (CID) pathway of deprotonated hydrocinnamic acid (C6H5CH2CH2CO2H), N-phenylglycine (C6H5NHCH2CO2H) and 3-pyridin-2-ylpropanoic acid (C5H4NCH2CH2CO2H). The structure and stability of isomeric precursor and product anions were examined using density functional theory and ab initio methods. Geometry optimizations and frequency calculations were performed using the B3LYP/6-31++G(2d,p) level of theory and basis set with additional single point energies calculated at the MP2/6-311++G(2d,p) level. The formation of a delocalized product anion by carboxyl group-mediated migration of a benzylic proton to the ortho position of the ring and subsequent Cα-CO2(-) bond cleavage was energetically more favorable than direct decarboxylation and rearrangements of anions within ion-neutral complexes with carbon dioxide. The energy barrier for rearrangement of the delocalized product anion to the more stable benzylic anion was lowest in the fragmentation pathway of 3-pyridin-2-ylpropanoate. More energetically demanding fragmentation processes were indicated by the formation of other product anions at higher collision energy. Computations supported the feasibility of the formation of hydroxycarbonyl, styrene, and phenide ions from the benzylic anion of hydrocinnamate and the corresponding product anions from the nitrogen-containing analogues. The loss of dihydrogen from decarboxylated 3-pyridin-2-ylpropanoate was characterized computationally as hydride abstraction of an aryl proton. Overall, the results highlight the importance of exploring rearrangements in the fragmentation pathways of ions formed by electrospray ionization (ESI).

Low temperature formation of naphthalene and its role in the synthesis of PAHs (polycyclic aromatic hydrocarbons) in the interstellar medium

Polycyclic aromatic hydrocarbons (PAHs) are regarded as key molecules in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest prototype-naphthalene (C(10)H(8))-has remained an open question. Here, we show in a combined crossed beam and theoretical study that naphthalene can be formed in the gas phase via a barrierless and exoergic reaction between the phenyl radical (C(6)H(5)) and vinylacetylene (CH(2) = CH-C ≡ CH) involving a van-der-Waals complex and submerged barrier in the entrance channel. Our finding challenges conventional wisdom that PAH-formation only occurs at high temperatures such as in combustion systems and implies that low temperature chemistry can initiate the synthesis of the very first PAH in the interstellar medium. In cold molecular clouds, barrierless phenyl-type radical reactions could propagate the vinylacetylene-mediated formation of PAHs leading to more complex structures like phenanthrene and anthracene at temperatures down to 10 K.

Reactions of 1-naphthyl radicals with acetylene. Single-pulse shock tube experiments and quantum chemical calculations. Differences and similarities in the reaction with ethylene

The reactions of 1-naphthyl radicals with acetylene were studied behind reflected shock waves in a single-pulse shock tube, covering the temperature range 950-1200 K at overall densities behind the reflected shocks of approximately 2.5 x 10(-5) mol/cm3. 1-Iodonaphthalene served as the source for 1-naphthyl radicals. The [acetylene]/[1-iodonaphthalene] ratio in all of the experiments was approximately 100 to channel the free radicals into reactions with acetylene rather than iodonaphthalene. Only two major products resulting from the reactions of 1-naphthyl radicals with acetylene and with hydrogen atoms were found in the post shock samples. They were acenaphthylene and naphthalene. Some low molecular weight aliphatic products at rather low concentrations, resulting from an attack of various free radicals on acetylene, were also found in the shocked samples. In view of the relatively low temperatures employed in the present experiments, the unimolecular decomposition rate of acetylene is negligible. One potential energy surface describes the production of acenaphthylene and 1-naphthyl acetylene, although the latter was not found experimentally due to the high barrier (calculated) required for its production. Using quantum chemical methods, the rate constants for three unimolecular elementary steps on the surface were calculated using transition state theory. A kinetics scheme containing 16 elementary steps was constructed, and computer modeling was performed. An excellent agreement between the experimental yields of the two major products and the calculated yields was obtained. Differences and similarities in the potential energy surfaces of 1-naphthyl radical + acetylene and those of ethylene are presented, and the kinetics mechanisms are discussed.

The Products of the Reactions of o-Benzyne with Ethene, Propene, and Acetylene: A Combined Mass Spectrometric and Quantum Chemical Study

The primary products of the bimolecular reactions of ortho-benzyne, o-C6H4 (1,2-dehydrobenzene), with ethene, propene, and acetylene have been detected by molecular beam mass spectrometry at a combustion relevant temperature of T = 1475 K. o-Benzyne was produced by flash pyrolysis of phthalic anhydride in the absence and presence of the respective reactant. Potential reaction pathways of the addition reactions were investigated by quantum chemical calculations. Channels with biradical intermediates were found to be energetically more favorable than alternative quasi-concerted [2+1] cycloaddition and concerted H-transfer pathways. Bicyclic benzocyclobutene and benzocyclobutadiene were identified as the main products of the reactions with C2H4 and C2H2, respectively. At combustion temperatures, however, these cyclic products are likely to undergo sequential ring opening. In the case of propene, the presence of an allylic H atom initiates a favorable ene-type reaction sequence yielding the open-chain product allylbenzene. Overall, hydrocarbon reactivity was found to increase in the order C2H2, C2H4 to 3H8. The range of the estimated bimolecular rate constants is comparable to the rate constants of the corresponding phenyl radical reactions and hence point out a potentially important role of o-C6H4 reactions in flame and soot formation chemistry.

Reaction dynamics of phenyl radicals in extreme environments: a crossed molecular beam study

Polycyclic aromatic hydrocarbons (PAHs)organic compounds that consist of fused benzene ringsand their hydrogen-deficient precursors have attracted extensive interest from combustion scientists, organic chemists, astronomers, and planetary scientists. On Earth, PAHs are toxic combustion products and a source of air pollution. In the interstellar medium, research suggests that PAHs play a role in unidentified infrared emission bands, diffuse interstellar bands, and the synthesis of precursor molecules to life. To build clean combustion devices and to understand the astrochemical evolution of the interstellar medium, it will be critical to understand the elementary reaction mechanisms under single collision conditions by which these molecules form in the gas phase. Until recently, this work had been hampered by the difficulty in preparing a large concentration of phenyl radicals, but the phenyl radical represents one of the most important radical species to trigger PAH formation in high-temperature environments. However, we have developed a method for producing these radical species and have undertaken a systematic experimental investigation. In this Account, we report on the chemical dynamics of the phenyl radical (C(6)H(5)) reactions with the unsaturated hydrocarbons acetylene (C(2)H(2)), ethylene (C(2)H(4)), methylacetylene (CH(3)CCH), allene (H(2)CCCH(2)), propylene (CH(3)CHCH(2)), and benzene (C(6)H(6)) utilizing the crossed molecular beams approach. For nonsymmetric reactants such as methylacetylene and propylene, steric effects and the larger cones of acceptance drive the addition of the phenyl radical to the nonsubstituted carbon atom of the hydrocarbon reactant. Reaction intermediates decomposed via atomic hydrogen loss pathways. In the phenyl-propylene system, the longer lifetime of the reaction intermediate yielded a more efficient energy randomization compared with the phenyl-methylacetylene system. Therefore, two reaction channels were open: hydrogen losses from the vinyl and from the methyl groups. All fragmentation pathways involved tight exit transition states. In the range of collision energies investigated, the reactions are dictated by phenyl radical addition-hydrogen atom elimination pathways. We did not observe ring closure processes with the benzene ring. Our investigations present an important step toward a systematic investigation of phenyl radical reactions under single collision conditions similar to those found in combustion flames and in high-temperature interstellar environments. Future experiments at lower collision energies may enhance the lifetimes of the reaction intermediates, which could open up competing ring closure channels to form bicyclic reaction products.

Reaction Dynamics on the Formation of Styrene: A Crossed Molecular Beam Study of the Reaction of Phenyl Radicals with Ethylene

The reaction dynamics of phenyl radicals (C6H5) with ethylene (C2H4) and D4-ethylene (C2D4) were investigated at two collision energies of 83.6 and 105.3 kJ mol-1 utilizing a crossed molecular beam setup. The experiments suggested that the reaction followed indirect scattering dynamics via complex formation and was initiated by an addition of the phenyl radical to the carbon-carbon double bond of the ethylene molecule forming a C6H5CH2CH2 radical intermediate. Under single collision conditions, this short-lived transient species was found to undergo unimolecular decomposition via atomic hydrogen loss through a tight exit transitions state to synthesize the styrene molecule (C6H5C2H3). Experiments with D4-ethylene verified that in the corresponding reaction with ethylene the hydrogen atom was truly emitted from the ethylene unit but not from the phenyl moiety. The overall reaction to form styrene plus atomic hydrogen from the reactants was found to be exoergic by 25 +/- 12 kJ mol(-1). This study provides solid evidence that in combustion flames the styrene molecule, a crucial precursor to form polycyclic aromatic hydrocarbons (PAHs), can be formed within a single neutral-neutral collision, a long-standing theoretical prediction which has remained to be confirmed by laboratory experiments under well-defined single collision conditions for the last 50 years.

Experimental and Computational Studies of the Phenyl Radical Reaction with Allene

The kinetics for the gas-phase reaction of phenyl radicals with allene has been measured by cavity ring-down spectrometry (CRDS), and the mechanism and initial product branching have been elucidated with the help of quantum-chemical calculations. The absolute rate constant measured by the CRDS technique can be expressed by the following Arrhenius equation: kallene (T=301-421 K)=(4.07+/-0.38)x10(11) exp[-(1865+/-85)/T] cm3 mol(-1) s(-1). Theoretical calculations, employing high level G2M energetic and IRCMax(RCCSD(T)//B3LYP-DFT) molecular parameters, indicate that under our experimental conditions the most preferable reaction channel is the addition of phenyl radicals to the terminal carbon atoms in allene. Predicted total rate constants agree with the experimental values within 40%. Calculated total and branching rate constants are provided for high-T kinetic modeling.

Experimental and theoretical investigation of rate coefficients of the reaction S(P3)+OCS in the temperature range of 298–985K

The reaction S(3P)+OCS in Ar was investigated over the pressure range of 50-710 Torr and the temperature range of 298-985 K with the laser photolysis technique. S atoms were generated by photolysis of OCS with light at 248 nm from a KrF excimer laser; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO2. At pressures less than 250 Torr, our measurements give k(298 K)=(2.7+/-0.5)x10(-15) cm3 molecule-1 s-1, in satisfactory agreement with a previous report by Klemm and Davis [J. Phys. Chem. 78, 1137 (1974)]. New data determined for 407-985 K connect rate coefficients reported previously for T>or=860 and T<or=478 K and show a non-Arrhenius behavior. Combining our results with data reported at high temperatures, we derived an expression k(T)=(6.1+/-0.3)x10(-18) T1.97+/-0.24 exp[-(1560+/-170)/T] cm3 molecule-1 s-1 for 298<or=TK<or=1680. At 298 K and P>or=500 Torr, the reaction rate was enhanced. Theoretical calculations at the G2M(CC2) level, using geometries optimized with the B3LYP6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) calculations agree satisfactorily with experimental observations. According to our calculations, the singlet channel involving formation of SSCO followed by direct dissociation into S2(a 1Deltag)+CO dominates below 2000 K; SSCO is formed via intersystem crossing from the triplet surface. At low temperature and under high pressure the stabilization of OCS2, formed via isomerization of SSCO, becomes important; its formation and further reaction with S atoms partially account for the observed increase in the rate coefficient under such conditions.

Experimental and Theoretical Studies of the Phenyl Radical Reaction with Propene

The kinetics for the reaction of C6H5 with propene has been measured by cavity ring-down spectrometry (CRDS) at temperatures 296-496 K under an Ar pressure of 40 Torr. The total rate constant can be given by the following Arrhenius expression (in units of cm3 mol(-1) s(-1)): k(C6H5 + C3H6) = 10(11.93+/-0.06) exp[-(1512 +/- 51)/T]. Density functional and higher level of theory calculations (up to the G2M level) have been carried out to provide additional insights about the mechanism of this reaction, and we also performed transition state theory (TST) calculation for the rate constant prediction. Our theoretical kinetic calculations predict that the C6H5 addition to the terminal =CH2 site in propene is dominant at the temperature range of our CRDS measurements. However, the H-abstraction channel forming benzene and the allyl radical becomes increasingly important at higher temperatures. The total high-pressure limiting rate constant calculated on the basis of the G2M reaction barriers is in reasonable agreement with the experimental values.

Experimental and Computational Studies of the Phenyl Radical Reaction with Propyne

The kinetics for the gas-phase reaction of phenyl radical with propyne has been measured by cavity ring-down spectrometry (CRDS), and the mechanism and initial product branching have been elucidated with the help of quantum chemical calculations. Absolute rate constants measured by the CRDS technique can be expressed by the following Arrhenius equation: (k/cm(3) mol(-1) s(-1)): k(propyne)(T=301-428 K)=(3.68+/-0.92) x 10(11)exp[-(1685+/-80)/T]. The experiment is unable to distinguish between the possible reactive channels, but theory indicates that phenyl radicals preferably add to the unsaturated terminal carbon atom in propyne under our experimental conditions. Theoretical kinetic calculations, employing high-level G2M(RCC, RMP2) and G3 energetic and IRCMax(RCCSD(T)//B3LYP-DFT) molecular parameters, reproduce the total experimental rate constants within a factor of three. Calculated total and branching rate constants are provided for high-T kinetic modeling. Addition reactions of phenyl to C3H4 are estimated to be less important molecular-growth pathways in high-T conditions (T>1000 K) in comparison to the C6H5 + C2H2 reaction.