Rate constants for the removal of O2(b1Σg+) by collisions with O2, N2, CO2, and H2O have been determined over the temperature range from 297 to 800 K. O2(b1Σg+) was excited by pulses from a tunable dye laser, and the deactivation kinetics were followed by observing the temporal behavior of the b1Σg+–X3Σg– fluorescence. The removal rate constants for CO2, N2, and H2O were not strongly dependent on temperature and could be represented by the expressions kCO2 = (1.18 ± 0.05) × 10–17 × T1.5 × exp, kN2 = (8 ± 0.3) × 10–20 × T1.5 × exp, and kH2O = (1.27 ± 0.08) × 10–16 × T1.5 × exp cm3 molecule–1 s–1. Rate constants for O2(b1Σg+) removal by O2(X), being orders of magnitude lower, demonstrated a sharp increase with temperature, represented by the fitted expression kO2 = (7.4 ± 0.8) × 10–17 × T0.5 × exp cm3 molecule–1 s–1. All of the rate constants measured at room temperature were found to be in good agreement with previously reported values.

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.

Exploiting a high temperature chemical reactor, we explored the pyrolysis of helium-seeded n-decane as a surrogate of the n-alkane fraction of Jet Propellant-8 (JP-8) over a temperature range of 1100–1600 K at a pressure of 600 Torr. The nascent products were identified in situ in a supersonic molecular beam via single photon vacuum ultraviolet (VUV) photoionization coupled with a mass spectroscopic analysis of the ions in a reflectron time-of-flight mass spectrometer (ReTOF). Our studies probe, for the first time, the initial reaction products formed in the decomposition of n-decane—including radicals and thermally labile closed-shell species effectively excluding mass growth processes. The present study identified 18 products: molecular hydrogen (H2), C2 to C7 1-alkenes [ethylene (C2H4) to 1-heptene (C7H14)], C1–C3 radicals [methyl (CH3), vinyl (C2H3), ethyl (C2H5), propargyl (C3H3), allyl (C3H5)], small C1–C3 hydrocarbons [methane (CH4), acetylene (C2H2), allene (C3H4), methylacetylene (C3H4)], along with higher-order reaction products [1,3-butadiene (C4H6), 2-butene (C4H8)]. On the basis of electronic structure calculations, n-decane decomposes initially by C–C bond cleavage (excluding the terminal C–C bonds) producing a mixture of alkyl radicals from ethyl to octyl. These alkyl radicals are unstable under the experimental conditions and rapidly dissociate by C–C bond β-scission to split ethylene (C2H4) plus a 1-alkyl radical with the number of carbon atoms reduced by two and 1,4-, 1,5-, 1,6-, or 1,7-H shifts followed by C–C β-scission producing alkenes from propene to 1-octene in combination with smaller 1-alkyl radicals. The higher alkenes become increasingly unstable with rising temperature. When the C–C β-scission continues all the way to the propyl radical (C3H7), it dissociates producing methyl (CH3) plus ethylene (C2H4). Also, at higher temperatures, hydrogen atoms can abstract hydrogen from C10H22 to yield n-decyl radicals, while methyl (CH3) can also abstract hydrogen or recombine with hydrogen to form methane. These n-decyl radicals can decompose via C–C-bond β-scission to C3 to C9 alkenes.

Formation of fulvene and benzene through the reaction of cyclopentadienyl (C5H5) with methyl radical (CH3) and consequent dissociation of its primary C6H7 products has been studied using ab initio and theoretical kinetics calculations. The potential energies and geometries of all involved species have been computed at the CCSD(T)-F12/cc-pVTZ-f12//B2PLYPD3/aug-cc-pVDZ level theory. Multichannel/multiwell RRKM-Master Equation calculations have been utilized to produce phenomenological pressure- and temperature-dependent absolute and individual-channel rate constants for various reactions at the C6H8 and C6H7 potential energy surfaces. The kinetic scheme combining the primary and secondary reactions has been used to generate the overall rate constants for the production of fulvene and benzene and their branching ratios. Analyses of the kinetic data revealed that at low pressures (0.01 atm) benzene formation prevails, with branching ratios exceeding 60%, whereas at the highest pressure (100 atm) fulvene formation is prevalent, with the branching ratio of benzene being lower than 40%. At intermediate pressures (1 and 10 atm) the two product channels compete and fulvene formation is preferable at temperatures above 1600 K. The results demonstrate that a five-member ring can be efficiently transformed into an aromatic six-member ring by methylation and corroborate the potentially important role of the methyl radical in the mechanism of PAH growth where CH3 additions alternate with H abstractions and acetylene additions.

We investigated temperature-dependent products in the pyrolysis of helium-seeded n-dodecane, which represents a surrogate of the n-alkane fraction of Jet Propellant-8 (JP-8) aviation fuel. The experiments were performed in a high temperature chemical reactor over a temperature range of 1200 K to 1600 K at a pressure of 600 Torr, with in situ identification of the nascent products in a supersonic molecular beam using single photon vacuum ultraviolet (VUV) photoionization coupled with the analysis of the ions in a reflectron time-of-flight mass spectrometer (ReTOF). For the first time, the initial decomposition products of n-dodecane—including radicals and thermally labile closed-shell species—were probed in experiments, which effectively exclude mass growth processes. A total of 15 different products were identified, such as molecular hydrogen (H2), C2 to C7 1-alkenes [ethylene (C2H4) to 1-heptene (C7H14)], C1–C3 radicals [methyl (CH3), ethyl (C2H5), allyl (C3H5)], small C1–C3 hydrocarbons [acetylene (C2H2), allene (C3H4), methylacetylene (C3H4)], as well as the reaction products [1,3-butadiene (C4H6), 2-butene (C4H8)] attributed to higher-order processes. Electronic structure calculations carried out at the G3(CCSD,MP2)//B3LYP/6-311G(d,p) level of theory combined with RRKM/master equation of rate constants for relevant reaction steps showed that n-dodecane decomposes initially by a nonterminal C–C bond cleavage and producing a mixture of alkyl radicals from ethyl to decyl with approximately equal branching ratios. The alkyl radicals appear to be unstable under the experimental conditions and to rapidly dissociate either directly by C–C bond β-scission to produce ethylene (C2H4) plus a smaller 1-alkyl radical with the number of carbon atoms diminished by two or via 1,5-, 1,6-, or 1,7- 1,4-, 1,9-, or 1,8-H shifts followed by C–C β-scission producing alkenes from propene to 1-nonene together with smaller 1-alkyl radicals. The stability and hence the branching ratios of higher alkenes decrease as temperature increases. The C–C β-scission continues all the way to the propyl radical (C3H7), which dissociates to methyl (CH3) plus ethylene (C2H4). In addition, at higher temperatures, another mechanism can contribute, in which hydrogen atoms abstract hydrogen from C12H26 producing various n-dodecyl radicals and these radicals then decompose by C–C bond β-scission to C3 to C11 alkenes.

Two ground-state CH radical reactions with the C3H4 isomers allene and methylacetylene occurring along the C4H5 potential energy surface (PES) were studied to probe the reaction mechanisms and final product distributions. The calculations were performed using a CCSD(T)-F12//B2PLYPD3 PES in combination with the 1-D chemical master equation. The reaction between the CH radical and allene was found to lead to exclusive “funneling” of the energized C4H5 intermediates into linear C4H5 configurations before reaching the exit channels, regardless of the specific nature of the initial bimolecular reactive encounter. In the case of the CH radical reaction with methylacetylene, energized C4H5 three-membered ring structures underwent H loss in significant amounts resulting in the production of a cyclic C4H4 methylenecyclopropene product, in accordance with experiments. The theoretical product distribution at room temperature for methylacetylene + CH was ∼35% methylenecyclopropene, ∼36% vinylacetylene, and ∼28% 1,2,3-butatriene, which is in agreement with the available experimental data. The distribution for allene + CH was ∼93% vinylacetylene, ∼4% 1,2,3-butatriene and ∼3% acetylene + vinyl, which overestimates the experimental yield of vinylacetylene and underestimates that of 1,2,3-butatriene by ∼10%. The possible reasons for this slight quantitative deviation of the theoretical results obtained within statistical treatment from the experiment are discussed.

Two sets of experiments were performed to unravel the high-temperature pyrolysis of tricyclo[5.2.1.02,6] decane (JP-10) exploiting high-temperature reactors over a temperature range of 1100 K to 1600 K, Advanced Light Source (ALS), and 927 K to 1083 K, National Synchrotron Radiation Laboratory (NSRL), with residence times of a few tens of microseconds (ALS) to typically 144 ms (NSRL). The products were identified in situ in supersonic molecular beams via single photon vacuum ultraviolet (VUV) photoionization coupled with mass spectroscopic detection in a reflectron time-of-flight mass spectrometer (ReTOF). These studies were designed to probe the initial (ALS) and also higher order reaction products (NSRL) formed in the decomposition of JP-10 – including radicals and thermally labile closed-shell species. Altogether 43 products were detected and quantified including C1–C4 alkenes, dienes, C3–C4 cumulenes, alkynes, eneynes, diynes, cycloalkenes, cyclo-dienes, aromatic molecules, and most importantly, radicals such as ethyl, allyl, and methyl produced at shorter residence times. At longer residence times, the predominant fragments were molecular hydrogen (H2), ethylene (C2H4), propene (C3H6), cyclopentadiene (C5H6), cyclopentene (C5H8), fulvene (C6H6), and benzene (C6H6). Accompanied by electronic structure calculations, the initial JP-10 decomposition via C–H bond cleavages resulting in the formation of the initial six C10H15 radicals was found to explain the formation of all products detected in both sets of experiments. These radicals are not stable under the experimental conditions and further decompose via C–C bond β-scission processes. These pathways result in ring opening in the initial tricyclic carbon skeletons of JP-10. Intermediates accessed after the first β-scission can further isomerize or dissociate. Complex PAH products in the NRLS experiment (naphthalene, acenaphthylene, biphenyl) are likely formed via molecular growth reactions at elevated residence times.

Density functional B3LYP/6-31G(d) and ab initio G3(MP2,CC) calculations have been carried out to determine thermal rate constants of direct H abstraction reactions from four- and five-ring polycyclic aromatic hydrocarbons (PAH) chrysene and benzo[a]pyrene by various radicals abundant in combustion flames, such as H, CH3, C3H3, and OH, using transition state theory. The results show that the H abstraction reactions with OH have the lowest barriers of ∼4 kcal mol−1, followed by those with H and CH3 with barriers of 16–17 kcal mol−1, and then with propargyl radicals with barriers of 24–26 kcal mol−1. Thus, the OH radical is predicted to be the fastest H abstractor from PAH. Even at 2500 K, the rate constant for H abstraction by H is still 34% lower than the rate constant for H abstraction by OH. The reaction with H is calculated to have rate constants 35–19 times higher than those for the reaction with CH3 due to a more favorable entropic factor. The reactions of H abstraction by C3H3 are predicted to be orders of magnitude slower than the other reactions considered and their equilibrium is strongly shifted toward the reactants, making propargyl an inefficient H abstractor from the aromatics. The calculations showed strong similarity of the reaction energetics in different H abstraction positions of benzo[a]pyrene and chrysene within armchair and zigzag edges in these molecules, but clear distinction between the armchair and zigzag sites. The zigzag sites appear to be more reactive, with H abstraction rate constants by H, CH3, and OH being respectively 37–42%, a factor of 2.1, and factors of 8–9 higher than the corresponding rate constants for the H abstraction reactions from armchair sites. Although the barrier heights for the two types of edges are similar, the entropic factor makes zigzag sites more favorable for H abstraction. Rate expressions have been generated for all studied reactions with the goal to rectify current combustion kinetics mechanisms.

Ab initio calculations of potential energy surfaces in conjunction with the RRKM-Master Equation theoretical approach have been employed to evaluate temperature- and pressure-dependent total and product specific rate constants and product branching ratios for unimolecular thermal decomposition of 2,4-cyclopentadienone C5H4O and for the C5H4O + H and C5H5 + O reactions. The formation of the cyclobutadiene + CO products via a ring contraction/CO elimination mechanism is shown to be the prevailing channel for the unimolecular decomposition of C5H4O. The unimolecular reaction is found to be relatively slow, but decomposition of cyclopentadienone can be greatly facilitated through bimolecular encounters with H atoms. The C5H4O + H reaction is predicted to be fast, with rate constants ranging from 4.6 × 10−12 to 1.8 × 10−10 cm3 molecule−1 s−1 at T = 500–2500 K and finite pressures. Cyclic C5H5O intermediates formed after the initial H addition undergo ring openings by β-scissions and then decompose to either butadienyl C4H5 + CO or 1-oxoprop-2-enyl H2CCHCO + C2H2, which are respectively predicted as the major and the minor reaction products. The calculations predict that thermal decomposition of the ortho and meta C5H5O radicals as well as pyranyl nearly exclusively forms the C4H5 + CO products, whereas decomposition of hydroxycyclopentadienyl C5H4OH predominantly produces cyclopentadienone + H. The C5H5 + O reaction is shown to proceed by barrierless oxygen addition to the ring followed by fast H migration, ring opening, and dissociation to C4H5 + CO. The C5H5 + O rate constant is calculated to be close to 1 × 10−10 cm3 molecule−1 s−1 and to be pressure-independent and nearly independent of temperature. Modified Arrhenius expressions for rate constants for all considered reactions at the high-pressure limit and at finite pressures are generated for kinetic modeling.

The mechanism of CH(X2Π) reaction with propene has been studied with ab initio CCSD(T)-F12/CBS//B3LYP/6-311G(d,p) calculations of the C4H7 potential energy surface and RRKM/master equation calculations of unimolecular rate constants for the various isomerization and dissociation steps available to the C4H7 radicals. Product branching ratios were calculated and were found to strongly depend on the initial chemically activated C4H7 complex formed in a barrierless entrance channel. If the reaction is initiated via either CH addition to the double bond in propene or CH insertions into the terminal sp2 C–H or single C–C bonds, then 1,3-butadiene + H are predicted to be the dominant products, ethene + C2H3 radical are minor but non-negligible products, and a small amount of 1,2-butadiene + H is also produced. The reaction then proceeds through a key CH3CHCH•CH2 intermediate, which loses an H atom to form either 1,3- or 1,2-butadiene or isomerizes to •CH2CH2CHCH2 and then dissociates to ethene + C2H3 radical. If CH inserts into a C–H bond in the CH3 group the •CH2CH2CHCH2 complex is formed directly and then the major reaction products are predicted to be ethene + C2H3 radical and 1,3-butadiene + H. Finally, if CH inserts into the middle sp2 C–H bond, a branched CH3C(•CH2)CH2 complex is produced, which predominantly decomposes to allene + CH3 radical. A comparison of the calculated reaction mechanism with available experimental data indicates that the CH addition entrance channel is favorable, in which case the computed branching ratios are in agreement with the experimental result of Loison and Bergeat, who measured the H elimination branching ratio of 78 ± 10%. However, the computed branching ratios quantitatively disagree with the experimental data by Trevitt et al., who observed a nearly 100% yield of the C4H6 + H products and also larger yields of 1,2-butadiene and 1-butyne than the calculations predict. The deviation of the theoretical results from experiment can be rationalized in terms of dynamical factors, which should favor direct dissociation of the CH3CHCH•CH2 precursor by H loss, especially to 1,2-butadiene, over its isomerization to •CH2CH2CHCH2 followed by the production of ethene + C2H3 radical, while 1-butyne might be formed through secondary H assisted isomerization of 1,2-butadiene. Overall, the calculations corroborate that the CH + C3H6 reaction could be a major source of 1,3-butadiene at low temperature and low pressure conditions in the interstellar medium and planetary atmospheres.

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.

The reactions of the p-tolyl radical with allene-d4 and methylacetylene-d4 as well as of the p-tolyl-d7 radical with methylacetylene-d1 and methylacetylene-d3 were carried out under single collision conditions at collision energies of 44–48 kJ mol−1 and combined with electronic structure and statistical (RRKM) calculations. Our experimental results indicated that the reactions of p-tolyl with allene-d4 and methylacetylene-d4 proceeded via indirect reaction dynamics with laboratory angular distributions spanning about 20° in the scattering plane. As a result, the center-of-mass translational energy distribution determined a reaction exoergicity of 149 ± 28 kJ mol−1 and exhibited a pronounced maximum at around 20 to 30 kJ mol−1. In addition, the center-of-mass angular flux distribution T(θ) depicted a forward–backward symmetry and indicated geometric constraints upon the decomposing complex(es). Combining with calculations, these results propose that the bicyclic polycyclic aromatic hydrocarbons, 6-methyl-1H-indene (p1) and 5-methyl-1H-indene (p2), are formed under single collision conditions at fractions of at least 85% in both reaction systems. For the p-tolyl–methylacetylene system, experiments with partially deuterated reactants also reveal the formation of a third isomer p5 (1-methyl-4-(1-propynyl)benzene) at levels of 5–10%, highlighting the importance in conducting reactions with partially deuterated reactants to elucidate the underlying reaction pathways comprehensively.

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.

To gain qualitative and quantitative understanding of oxidation processes of large polycyclic aromatics, soot particles, and graphene edges, a theoretical study is reported for the pyrenyl–O2 reaction system. First, possible reaction pathways and their energetics were investigated using high-level ab initio calculations. The results were utilized in RRKM–master equation calculations of rate coefficients and relative product yields at temperatures and pressures relevant to combustion. Finally, the deduced oxidation mechanisms of six- and five-member rings and the computed rate coefficients were employed in kinetic Monte Carlo simulations of oxidation of a graphene “molecule” evolving in flame-like environments. Among the major findings from the latter simulations are the following: The oxidation system exhibits two basic pathways, thermal decomposition and regeneration of oxyradicals. Their competition is temperature-dependent, with the former dominating at higher and the latter at lower temperatures. The overall oxidation of the graphene substrate is computed to be time-dependent, with the initial rates consistent with the known experimental data.

The classification of chemical reactions based on shared characteristics is at the heart of the chemical sciences, and is well exemplified by Langmuir's concept of isovalency, in which ‘two molecular entities with the same number of valence electrons have similar chemistries’. Within this account we further investigate the ramifications of the isovalency of four radicals with the same X2Σ+ electronic structure – cyano (CN), boron monoxide (BO), silicon nitride (SiN), and ethynyl (C2H), and their reactions with simple prototype hydrocarbons acetylene (C2H2) and ethylene (C2H4). The fact that these four reactants own the same X2Σ+ electronic ground state should dictate the outcome of their reactions with prototypical hydrocarbons holding a carbon–carbon triple and double bond. However, we find that other factors come into play, namely, atomic radii, bonding orbital overlaps, and preferential location of the radical site. These doublet radical reactions with simple hydrocarbons play significant roles in extreme environments such as the interstellar medium and planetary atmospheres (CN, SiN and C2H), and combustion flames (C2H, BO).

As a member of the organo sulfidoboron (RBS) family, the hitherto elusive ethynylsulfidoboron molecule (HCCBS) has been formed via the bimolecular reaction of the boron monosulfide radical (BS) with acetylene (C2H2) under single collision conditions in the gas phase, exploiting the crossed molecular beams technique. The reaction mechanism follows indirect dynamics via a barrierless addition of the boron monosulfide radical with its boron atom to the carbon atom of the acetylene molecule, leading to the trans-HCCHBS intermediate. As predicted by ab initio electronic structure calculations, the initial collision complex either isomerizes to its cis-form or undergoes a hydrogen atom migration to form H2CCBS. The cis-HCCHBS intermediate either isomerizes via hydrogen atom shift from the carbon to the boron atom, leading to the HCCBHS isomer, or decomposes to ethynylsulfidoboron (HCCBS). Both H2CCBS and HCCBHS intermediates were predicted to fragment to ethynylsulfidoboron via atomic hydrogen losses. Statistical (RRKM) calculations report yields to form the ethynylsulfidoboron molecule from cis-HCCHBS, H2CCBS, and HCCBHS to be 21%, 7%, and 72%, respectively, under current experimental conditions. Our findings open up an unconventional path to access the previously obscure class of organo sulfidoboron molecules, which are difficult to access through “classical” formation.

The reaction dynamics of the dicarbon radical C2(a3Πu/X1Σg+) in the singlet and triplet state with C4H6 isomers 2-butyne, 1-butyne and 1,2-butadiene were investigated at collision energies of about 26 kJ mol−1 using the crossed molecular beam technique and supported by ab initio and RRKM calculations. The reactions are all indirect, forming C6H6 complexes through barrierless additions by dicarbon on the triplet and singlet surfaces. Isomerization of the C6H6 reaction intermediate leads to product formation by hydrogen loss in a dicarbon–hydrogen atom exchange mechanism forming acyclic C6H5 reaction products through loose exit transition states in overall exoergic reactions.

We probed the reaction of the 4-methylphenyl radical with isoprene under single collision conditions at a collision energy of 58 kJ mol−1 by exploiting the crossed molecular beam technique. Supported by the electronic structure calculations, the reaction was found to initially lead to a van-der-Waals complex without any barrier which can then isomerize by addition of the 4-methylphenyl radical to any one of the four carbon atoms of the 1,3-butadiene moiety of isoprene. The initial addition products isomerize with formal addition products preferentially to C1 and C4 carbon atoms of the isoprene. These structures further isomerize via hydrogen migration and cyclization; the reaction is terminated by a hydrogen atom elimination from the 4-methylphenyl moiety via tight exit transition states leading to two dimethyl-dihydronaphthalene isomers as the dominating products. This study presents one of the very first bimolecular reactions of the 4-methylphenyl radical with unsaturated hydrocarbons and opens a path for the investigation of this reaction class in future experiments.

•Results of the bimolecular reaction of dicarbon with 1,3-pentadiene are reported.•Reaction dynamics, energetics and mechanism are presented.•Reaction was indirect via complex formation and rearrangement.•At least three cyclic C7H7 radical products were determined.We report on the crossed molecular beam reaction of dicarbon, C2 (X1Σg+, a3Πu), with 1,3-pentadiene (C5H8; X1A′) conducted at a collision energy of 43 kJ mol−1 under single collision conditions and studied by ab initio and statistical calculations. The reactions involve indirect scattering dynamics initiated by the barrierless addition of dicarbon to the carbon–carbon double bond of 1,3-pentadiene followed by successive rearrangements leading eventually through hydrogen atom elimination to distinct C7H7 radical species. The experimental reaction exoergicity of 412 ± 52 kJ mol−1 is consistent with the formation of cycloheptatrienyl, m-tolyl, and/or benzyl radicals predicted as the major products by theory.

The C4H9 potential energy surface accessed by the reaction of methylidyne radical, CH (X2Π), with propane, C3H8, including possible intermediates, transition states and dissociation products, has been studied by ab initio and density functional calculations at the CCSD(T)/CBS//B3LYP/6-311G(d,p) level of theory. The computed relative energies and molecular parameters were utilized to calculate collision-energy-dependent unimolecular rate constants at the zero-pressure limit for isomerization and dissociation channels of the C4H9 adducts formed in the entrance reaction channels. The rate constants were used to evaluate the product branching ratios in the CH + C3H8 reaction under single-collision conditions. The results show that the reaction can produce mostly ethene (C2H4) + ethyl radical (C2H5) and propene (C3H6) + methyl radical (CH3), and up to 14% of various butene isomers (C4H8) + H. The product branching ratios are sensitive to the initial reaction adduct (a butyl radical, C4H9) formed in the entrance channels via barrierless insertion of the CH radical into the terminal and middle C–H bonds of propane or, possibly, into the single C–C bonds. A more definite answer on relative contributions of various available CH insertion channels can be obtained through ab initio quasiclassical trajectory calculations, which are proposed for the future. The results allowed us to conclude that the CH + C3H8 reaction does not result in major amounts in the direct growth of the carbon-skeleton to four-carbon C4H8 products via the CH-for-H exchange because C–C bond cleavages in C4H9 radicals are generally more preferable than C–H bond cleavages.

The reactions of the 4-tolyl radical (C6H4CH3) and of the D7-4-tolyl radical (C6D4CD3) with 1,2-butadiene (C4H6) have been probed in crossed molecular beams under single collision conditions at a collision energy of about 54 kJ mol–1 and studied theoretically using ab initio G3(MP2,CC)//B3LYP/6-311G** and statistical RRKM calculations. The results show that the reaction proceeds via indirect scattering dynamics through the formation of a van-der-Waals complex followed by the addition of the radical center of the 4-tolyl radical to the C1 or C3 carbon atoms of 1,2-butadiene. The collision complexes then isomerize by migration of the tolyl group from the C1 (C3) to the C2 carbon atom of the 1,2-butadiene moiety. The resulting intermediate undergoes unimolecular decomposition via elimination of a hydrogen atom from the methyl group of the 1,2-butadiene moiety through a rather loose exit transition state leading to 2-para-tolyl-1,3-butadiene (p4), which likely presents the major reaction product. Our observation combined with theoretical calculations suggest that one methyl group (at the phenyl group) acts as a spectator in the reaction, whereas the other one (at the allene moiety) is actively engaged in the underlying chemical dynamics. On the contrary to the reaction of the phenyl radical with allene, which leads to the formation of indene, the substitution of a hydrogen atom by a methyl group in allene essentially eliminates the formation of bicyclic PAHs such as substituted indenes in the 4-tolyl plus 1,2-butadiene reaction.

Ab initio G3(MP2,CC)/B3LYP/6-311G** calculations of potential energy surfaces (PESs) for the reactions of cyano and ethynyl radicals with styrene and N-methylenebenzenamine have been performed to investigate a possible formation mechanism of the prototype nitrogen-containing polycyclic aromatic compounds: (substituted) 1- and 2-azanaphthalenes. The computed PESs and molecular parameters have been used for RRKM and RRKM-Master Equation calculations of reaction rate constants and product branching ratios under single-collision conditions and at pressures from 3 to 10–6 mbar and temperatures of 90–200 K relevant to the organic aerosol formation regions in the stratosphere of a Saturn’s moon Titan. The results show that ethynyl-substituted 1- and 2-azanaphthalenes can be produced by consecutive CN and C2H additions to styrene or by two C2H additions to N-methylenebenzenamine. All CN and C2H radical addition complexes are formed in the entrance channels without barriers, and the reactions are computed to be exothermic, with all intermediates and transition states along the favorable pathways residing lower in energy than the respective initial reactants. The reactions are completed by dissociation of chemically activated radical intermediates via H losses, so that collisional stabilization of the intermediates is not required to form the final products. These features make the proposed mechanism viable even at very low temperatures and under single-collision conditions and especially significant for astrochemical environments. In Titan’s stratosphere, collisional stabilization of the initial CN + styrene reaction adducts may be significant, but substantial amounts of 2-vinylbenzonitrile and 2-ethynyl-N-methylenebenzenamine can still be produced and then react with C2H to form substituted azanaphthalenes.

•Methyl-cyclopentanone ionization/dissociation is studied in strong fs laser fields.•Mass spectra are analyzed to understand the effects of methyl substitution.•Generalized-KFR theory calculations are performed for the photoionization mechanism.•Ab initio/RRKM calculations are used to unravel the photodissociation patterns.Ionization and dissociation of 2- and 3-methyl cyclopentanones have been investigated in molecular beams by their irradiation with intense 394 and 788 nm laser fields with pulse duration of 90 fs and intensity of 3 × 1013–4 × 1014 W/cm2. The analysis of the resulting mass spectra allowed us to discern the effects of methyl substitution and its position on the outcome of ionization/dissociation processes induced by the intense femtosecond laser field. Generalized Keldysh-Faisal-Reiss (g-KFR) and ab initio G3(MP2,CCSD)//B3LYP/6–31G∗/RRKM theoretical calculations helped to uncover the formation mechanism of major ionic fragments observed in the mass spectra including C5H10+, C4H6O+, C3H3O+, C3H4O+, C3Hx+, and C2Hx+.

•Rate constants for H and D atom tunneling in formic acid in noble gas matrices are computed.•H/D kinetic isotope effect is evaluated in the temperature range of 5–25 K.•Formalism is based on theory of radiationless transitions and accounts for media reorganization.Intramolecular tunneling of hydrogen and deuterium atoms in noble gas matrices (Ar, Kr and Xe) has been studied theoretically for cis–trans isomerization of formic acid at low temperatures. The temperature dependence of the tunneling rate constant and quantum kinetic isotope effect taking into account the media reorganization upon the H/D atomic transfer is described in terms of the modified theory of radiationless transitions. The theoretical results are found to be in a reasonable agreement with the literature experimental data on the rate constants and the anomalous quantum kinetic H/D isotope effect.

Organyl oxoboranes (RBO) are valuable reagents in organic synthesis due to their role in Suzuki coupling reactions. However, organyl oxoboranes (RBO) are only found in trimeric forms (RBO3) commonly known as boronic acids or boroxins; obtaining their monomers has proved a complex endeavor. Here, we demonstrate an oligomerization-free formation of organyl oxoborane (RBO) monomers in the gas phase by a radical substitution reaction under single-collision conditions in the gas phase. Using the cross molecular beams technique, phenyl oxoborane (C6H5BO) is formed through the reaction of boronyl radicals (BO) with benzene (C6H6). The reaction is indirect, initially forming a van der Waals complex that isomerizes below the energy of the reactants and eventually forming phenyl oxoborane by hydrogen emission in an overall exoergic radical–hydrogen atom exchange mechanism.

Ab initio CCSD(T)/cc-pVTZ(CBS)//B3LYP/6-311g(d,p) calculations of the C5H6N potential energy surface have been performed to investigate the reaction mechanism of cyano radical (CN) with C4H6 isomers 1- and 2-butyne and 1,2-butadiene. They were followed by RRKM calculations of the reaction rate constants and product branching ratios under single-collision conditions in the 0–5 kcal/mol collision energy range. With the assumption of equal probabilities of the barrierless terminal and central addition of the cyano radical to 1-butyne, 2-cyano-1,3-butadiene + H, and cyanoallene + CH3 are predicted to be the major reaction products with a branching ratio of ∼2:1. The terminal CN addition to C1 favors the formation of cyanoallene + CH3, whereas the central CN addition to C2 enhances the formation of 2-cyano-1,3-butadiene + H. For the CN + 2-butyne reaction, the dominant product is calculated to be 1-cyano-prop-1-yne + CH3, and the CH3 loss occurs directly from the initial adduct formed by the barrierless CN addition to either of the two acetylenic carbon atoms. A small amount of the H loss product, 3-cyano-1,2-butadiene (1-cyano-1-methylallene), can be also formed as was observed in earlier crossed molecular beam experiments. Three different products are predicted for the CN + 1,2-butadiene reaction, which also occurs without entrance barriers. If various initial complexes formed by the CN addition to C1, C2, C3, or to the C═C double bonds in 1,2-butadiene are produced in the entrance channel with equal probabilities, the dominating product (70–60%) is 2-cyano-1,3-butadiene + H, and the other significant products include 1-cyano-prop-3-yne + CH3 (19–25%) favored by the initial CN addition to C1 and cyanoallene + CH3 (11–15%) preferred for the CN addition to C3. The H abstraction HCN + C4H5 products may also be formed either from the initial CN addition adducts through a CN roaming mechanism or via certain trajectories directly from the initial reactants, but their yield is not expected to be significant, at least at low temperatures. The energetics, mechanisms, and product branching ratios of the cyano radical reactions with various C4H6 isomers and their analogous isoelectronic C2H + C4H6 reactions have been summarized and compared.

The hitherto elusive silaisocyanoacetylene molecule (HCCNSi)—a member of the silaisocyanide family—has been synthesized for the first time through the reaction of the silicon nitride radical (SiN) with acetylene (C2H2) in the gas phase under single collision conditions. Compared to the isoelectronic reaction of the cyano radical (CN) with acetylene, the replacement of the carbon atom in the cyano group by an isovalent silicon atom has a pronounced effect on the reactivity. Whereas the silicon nitride radical was found to pass an entrance barrier and adds with the nitrogen atom to the acetylene molecule, the cyano radical adds barrierlessly with the carbon atom forming the HCCH(NSi) and HCCH(CN) intermediates, respectively. These structures undergo hydrogen loss to form the linear silaisocyanoacetylene (HCCNSi) and cyanoacetylene molecules (HCCCN), respectively. Therefore, the isovalency of the silicon atom was found to bear little resemblance with the carbon atom having a dramatic effect not only on the reactivity, but also on the reaction mechanism, thermochemistry, and chemical bonding of the isoelectronic silaisocyanoacetylene and cyanoacetylene products, effectively reversing the thermodynamical stability of the nitrile versus isonitrile and silanitrile versus isosilanitrile isomer pairs.

The gas-phase reaction between the silicon nitride radical (SiN) and the prototypical olefin—ethylene—is investigated experimentally and theoretically for the first time. Silicon nitride (SiN) and the cyano radical (CN) are isoelectronic; however, their chemical reactivities and structures are drastically different from each other. Through the use of the cross molecular beam technique, we were able to study the notoriously refractory silicon nitride radical in reaction with ethylene under single-collision conditions. We investigated the similarities and also the distinct differences with the cyano radical–ethylene system. We find that the silicon nitride radical bonds by the nitrogen atom to the double bond of ethylene; in comparison, the cyano radical adds via its carbon atom. The silicon nitride addition is barrierless, forming a long-lived SiNCH2CH2 collision complex, which is also able to isomerize via a hydrogen shift to the SiNCHCH3 intermediate. Both isomers can emit a hydrogen atom via tight transition states to form the silaisocyanoethylene (SiNC2H3) molecule in an overall exoergic reaction. This presents the very first experiment in which the silaisocyanoethylene molecule—a member of the silaisocyanide family—has been formed via a directed synthesis under gas-phase single-collision conditions. In comparison with the isoelectronic cyano–ethylene system, the cyanoethylene (C2H3CN) isomer is formed. Therefore, the replacement of a single carbon atom by an isovalent silicon atom, i.e. shifting from the cyano (CN) to the silicon nitride (SiN) radical, has a dramatic influence not only on the reactivity with ethylene (carbon atom versus nitrogen atom addition) but also on the final reaction products. In the reactions of ethylene with silicon nitride and the cyano radical, the silaisonitrile over the silanitrile and the nitrile over the isonitrile reaction products are favored, respectively. This reaction provides rare experimental data for investigating the chemistry of bimolecular reactions of silicon nitride diatomics in chemical vapor deposition techniques and interstellar environments.

Potential energy surfaces (PESs) of the reactions of 1- and 2-naphthyl radicals with molecular oxygen have been investigated at the G3(MP2,CC)//B3LYP/6-311G** level of theory. Both reactions are shown to be initiated by barrierless addition of O2 to the respective radical sites of C10H7. The end-on O2 addition leading to 1- and 2-naphthylperoxy radicals exothermic by 45–46 kcal/mol is found to be more preferable thermodynamically than the side-on addition. At the subsequent reaction step, the chemically activated 1- and 2-C10H7OO adducts can eliminate an oxygen atom leading to the formation of 1- and 2-naphthoxy radical products, respectively, which in turn can undergo unimolecular decomposition producing indenyl radical + CO via the barriers of 57.8 and 48.3 kcal/mol and with total reaction endothermicities of 14.5 and 10.2 kcal/mol, respectively. Alternatively, the initial reaction adducts can feature an oxygen atom insertion into the attacked C6 ring leading to bicyclic intermediates a10 and a10′ (from 1-naphthyl + O2) or b10 and b10′ (from 2-naphthyl + O2) composed from two fused six-member C6 and seven-member C6O rings. Next, a10 and a10′ are predicted to decompose to C9H7 (indenyl) + CO2, 1,2-C10H6O2 (1,2-naphthoquinone) + H, and 1-C9H7O (1-benzopyranyl) + CO, whereas b10 and b10′ would dissociate to C9H7 (indenyl) + CO2, 2-C9H7O (2-benzopyranyl) + CO, and 1,2-C10H6O2 (1,2-naphthoquinone) + H. On the basis of this, the 1-naphthyl + O2 reaction is concluded to form the following products (with the overall reaction energies given in parentheses): 1-naphthoxy + O (−15.5 kcal/mol), indenyl + CO2 (−123.9 kcal/mol), 1-benzopyranyl + CO (−97.2 kcal/mol), and 1,2-naphthoquinone + H (−63.5 kcal/mol). The 2-naphthyl + O2 reaction is predicted to produce 2-naphthoxy + O (−10.9 kcal/mol), indenyl + CO2 (−123.7 kcal/mol), 2-benzopyranyl + CO (−90.7 kcal/mol), and 1,2-naphthoquinone + H (−63.2 kcal/mol). Simplified kinetic calculations using transition-state theory computed rate constants at the high-pressure limit indicate that the C10H7O + O product channels are favored at high temperatures, while the irreversible oxygen atom insertion first leading to the a10 and a10′ or b10 and b10′ intermediates and then to their various decomposition products is preferable at lower temperatures. Among the decomposition products, indenyl + CO2 are always most favorable at lower temperatures, but the others, 1,2-C10H6O2 (1,2-naphthoquinone) + H (from a10 and b10′), 1-C9H7O (1-benzopyranyl) + CO (from a10′), and 2-C10H7O (2-benzopyranyl) + O (from b10 and minor from b10′), may notably contribute or even become major products at higher temperatures.

Ab initio CCSD(T)/cc-pVTZ(CBS)//B3LYP/6-311G** calculations of the C6H7 potential energy surface are combined with RRKM calculations of reaction rate constants and product branching ratios to investigate the mechanism and product distribution in the C2H + 1-butyne/2-butyne reactions. 2-Ethynyl-1,3-butadiene (C6H6) + H and ethynylallene (C5H4) + CH3 are predicted to be the major products of the C2H + 1-butyne reaction. The reaction is initiated by barrierless ethynyl additions to the acetylenic C atoms in 1-butyne and the product branching ratios depend on collision energy and the direction of the initial C2H attack. The 2-ethynyl-1,3-butadiene + H products are favored by the central C2H addition to 1-butyne, whereas ethynylallene + CH3 are preferred for the terminal C2H addition. A relatively minor product favored at higher collision energies is diacetylene + C2H5. Three other acyclic C6H6 isomers, including 1,3-hexadiene-5-yne, 3,4-hexadiene-1-yne, and 1,3-hexadiyne, can be formed as less important products, but the production of the cyclic C6H6 species, fulvene, and dimethylenecyclobut-1-ene (DMCB), is predicted to be negligible. The qualitative disagreement with the recently measured experimental product distribution of C6H6 isomers is attributed to a possible role of the secondary 2-ethynyl-1,3-butadiene + H reaction, which may generate fulvene as a significant product. Also, the photoionization energy curve assigned to DMCB in experiment may originate from vibrationally excited 2-ethynyl-1,3-butadiene molecules. For the C2H + 2-butyne reaction, the calculations predict the C5H4 isomer methyldiacetylene + CH3 to be the dominant product, whereas very minor products include the C6H6 isomers 1,1-ethynylmethylallene and 2-ethynyl-1,3-butadiene.

Reactions of dicarbon molecules (C2) with C4H6 isomers such as 1,3-butadiene represent a potential, but hitherto unnoticed, route to synthesize the first aromatic C6 ring in hydrocarbon flames and in the interstellar medium where concentrations of dicarbon transient species are significant. Here, crossed molecular beams experiments of dicarbon molecules in their X1Σg+ electronic ground state and in the first electronically excited a3Πu state have been conducted with 1,3-butadiene and two partially deuterated counterparts (1,1,4,4-D4-1,3-butadiene and 2,3-D2-1,3-butadiene) at two collision energies of 12.7 and 33.7 kJ mol−1. Combining these scattering experiments with electronic structure and RRKM calculations on the singlet and triplet C6H6 surfaces, our investigation reveals that the aromatic phenyl radical is formed predominantly on the triplet surface via indirect scattering dynamics through a long-lived reaction intermediate. Initiated by a barrierless addition of triplet dicarbon to one of the terminal carbon atoms of 1,3-butadiene, the collision complex undergoes trans−cis isomerization followed by ring closure and hydrogen migration prior to hydrogen atom elimination, ultimately forming the phenyl radical. The latter step emits the hydrogen atom almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. On the singlet surface, smaller contributions of phenyl radical could not be excluded; experiments with partially deuterated 1,3-butadiene indicate the formation of the thermodynamically less stable acyclic H2CCHCCCCH2 isomer. This study presents the very first experimental evidence, contemplated by theoretical studies, that under single collision conditions an aromatic hydrocarbon molecule can be formed in a bimolecular gas-phase reaction via reaction of two acyclic molecules involving cyclization processes at collision energies highly relevant to combustion flames.

Ab initio CCSD(T)/cc-pVTZ//B3LYP/6-311G** calculations of the C5H5 potential energy surface have been performed to investigate the reaction mechanism of ethynyl radical (C2H) with C3H4 isomers, allene and methylacetylene. They were followed by RRKM calculations of reaction rate constants and product branching ratios under single-collision conditions. The results show that the C2H + CH2CCH2 reaction in a case of statistical behavior is expected to produce 1,4-pentadiyne (56–63%), ethynylallene (22–24%), and pentatetraene (10–15%), with the most favorable pathways including H losses from the initial HCCCH2CCH2 adduct leading to either 1,4-pentadiyne or ethynylallene, and a multistep route HCCC(CH2)2 → four-member ring → CH2CCCHCH2 → CH2CCCCH2 + H featuring a formal insertion of C2H into a double bond of allene followed by H elimination giving rise to pentatetraene. On the contrary, the C2H + CH3CCH reaction produces diacetylene + methyl (21–61%) by CH3 loss from the HCCC(CH)CH3 initial adduct as well as methyldiacetylene + H (27–56%) and ethynylallene + H (11–22%) by H eliminations from CHCCHCCH3. The calculated product branching ratios are in general agreement with the available experimental data, although some quantitative deviations from experiment and possible reasons for them are also discussed. The present calculations confirm that the C2H + C3H4 reactions proceed without entrance barriers and lead, via intermediates and transition states residing lower in energy than the initial reactants, to the C5H4 + H and C4H2 + CH3 products exothermic by 20–36 kcal mol−1, with strong dependence of the product distribution on the reacting C3H4 isomer, making these reactions fast under low-temperature conditions of Titan’s atmosphere where they can serve as a source of more complex unsaturated hydrocarbons.

Ab initio G3(MP2,CC)//B3LYP/6-311G** calculations have been performed to investigate the potential energy surface and mechanism of the reaction of phenyl radical with 1,2-butadiene followed by kinetic RRKM-ME calculations of the reaction rate constants and product branching ratios at various temperatures and pressures. The results show that the reaction can proceed by direct hydrogen abstraction to produce benzene and C4H5 radicals or by addition of phenyl to different carbon atoms in CH2CCHCH3 followed by isomerizations of C10H11 adducts and their dissociation by H or CH3 losses. The H abstraction channels are found to be kinetically preferable and to contribute 70−90% to the total product yield in the 300−2500 K temperature range, with the products including C6H6 + CH2CHCCH2 (∼40%), C6H6 + CH3CHCCH (5−31%), and C6H6 + CH2CCCH3 (24−20%). The phenyl addition channels are calculated to be responsible for 10−30% of the total product yield, with their contribution decreasing as the temperature increases. The products of the addition channels include collisionally stabilized C10H11 adducts, 1-phenyl-2-buten-2-yl, 3-phenyl-2-buten-2-yl, and 2-phenyl-2-buten-1-yl/2-phenyl-1-buten-3-yl, which are favored under low temperatures, as well as their dissociation products, 1-phenyl-propyne + CH3, phenylallene + CH3, and 2-phenyl-1,3-butadiene + H, preferred at higher temperatures. Indene is predicted to be a very minor reaction product at the temperatures relevant to combustion, with the maximal calculated yield of only 2% at 700 K and 7.6 Torr. Our calculations showed that at typical combustion temperatures product branching ratios are practically independent of pressure, and collisional stabilization of reaction intermediates does not play a significant role. Three-parameter modified Arrhenius expressions have been generated for the total reaction rate constants and rate constants for the most important product channels, which can be utilized in future kinetic modeling of reaction networks related to the growth of hydrocarbons in combustion processes.

Intramolecular tunneling of a hydrogen atom in formic acid at low temperatures has been studied theoretically on the basis of quantum-chemical modeling of HCOOH@Nb12 clusters. Three noble matrixes (Ar, Kr, and Xe) are considered. Energetic and geometric parameters as well as vibrational frequencies for the formic acid in cis and trans configurations surrounded by 12 Nb atoms are calculated within the frame of the MP2 approach with extended basis sets. The rate constant of HCOOH cis−trans conversion is analyzed by taking into account matrix reorganization and the change of HCOOH position in the cluster. The matrix reorganization is considered within the Debye model of lattice vibrations, whereas the external motion of HCOOH in the cluster is treated using the Einstein model of solids. It has been shown that the literature experimental data on the cis to trans tunneling reaction in the formic acid can be accounted for within the proposed mechanism, which describes the matrix reorganization and the change of the HCOOH position in the noble gas matrix, with fitting parameters of the suggested theoretical model attaining reasonable values.

Ab initio and density functional calculations using a variety of theoretical methods (CASSCF, B3LYP, CASPT2, CCSD(T), and G3(MP2,CC)) have been carried out to unravel the mechanism of unimolecular isomerization and dissociation of 9,10-dihydrofulvalene C10H10 (S0) formed by barrierless recombination of two cyclopentadienyl radicals. Different reaction pathways on the C10H10 potential energy surface (PES) are found to lead to the production of 9-H-fulvalenyl radical + H, 9-H-naphthyl radical (a naphthalene precursor) + H, and naphthalene + H2. RRKM calculations of thermal rate constants and product branching ratios at the high pressure limit show that at temperatures relevant to combustion the 9-H-fulvalenyl radical formed by a direct H loss from S0 with endothermicity of 76.3 kcal/mol is expected to be the dominant reaction product. The naphthalene precursor 9,10-dihydronaphthalene (D3) can be produced from the initial S0 adduct by a multistep diradical mechanism involving the formation of a metastable tricyclic diradical intermediate, followed by its three-step opening to a 10-member ring structure, which then undergoes ring contraction producing the naphthalene core structure in D3, with the highest barrier on this pathway being 70.3 kcal/mol. D3 can lose molecular hydrogen producing naphthalene via a barrier of 77.7 kcal/mol relative to the initial adduct. Another possibility is a hydrogen atom elimination in D3 giving rise to the 9-H-naphthyl radical without exit barrier and with overall endothermicity of 59.2 kcal/mol. The pathway to 9-H-naphthyl appears to be preferable as compared to the direct route to 9-H-fulvalenyl at temperatures below 600 K, but the rate constants at these temperatures are too slow for the reaction to be significant. The naphthalene + H2 channel is not viable at any temperature. The following reaction sequence is suggested for kinetic models to account for the recombination of two cyclopentadienyl radicals: We conclude that naphthalene can be produced from the recombination of two cyclopentadienyl radicals and is expected to be a favorable product of this reaction sequence at T < 1000 K, but this molecule would be formed through isomerizations and H atom loss on the C10H9 PES (after the initial H elimination from C10H10 S0) and not in conjunction with molecular hydrogen. The alternative product, fulvalene, can potentially contribute to the growth of cyclopentafused polycyclic aromatic hydrocarbons.

Quantum chemical calculations of the geometric and electronic structures and vertical transition energies for several low-lying excited states of the neutral and negatively charged vacancy-related point defects in diamond containing two and three nitrogen atoms (N2V0, N2V−, and N3V0) have been performed employing various theoretical methods (time-dependent density functional theory, equation-of-motion coupled cluster, and multireference perturbation theory) and different basis sets and using C21H28, C35H36, and C51H52 finite model clusters. In the ground states, the vacancy-related atoms are found to be shifted away from the vacancy center by ∼0.1 Å, whereas the positions of atoms from the second layer around the vacancy remain nearly unchanged, indicating a local character of geometry relaxation due to defects. The lowest excited states are formed with participation of the stretched (N2V) or broken (N3V) C−C bond and nonbonding combinations of nitrogen lone pairs as donors, with the C−C antibonding molecular orbital (MO) in N2V0, broken C−C bond in N3V0, and diffuse vacancy-related MOs serving as acceptors. Normally, the first excited states have a valence character, but the diffuse states are rather close in energy, especially for N3V0 (22A1 and 12E excited states). The first optically active excitation in the N2V0 defect with the calculated energy of ∼2.6 eV (in close agreement with the experiment) is formed by the electronic transition from the stretched C−C bond to the antibonding C−C MO, with an additional contribution from the combination of nitrogen lone pairs. For the negatively charged N2V− system, the lowest excitation to the 12A1 state is predicted to occur from the singly occupied antibonding b1 MO to the empty diffuse a1 orbital, but the CASPT2 calculated excitation energy, ∼0.9 eV, underestimates the experimental zero phonon line observed at 1.26 eV. The lowest excited states of N3V0, 22A1, and 12E correspond to transitions from the singly occupied MO (SOMO) to the diffuse lowest vacant orbital and from the nonbonding combination of nitrogen lone pairs to SOMO, respectively, and have similar energies of about 3.1−3.3 eV, in agreement with the experimental photoabsorption band maximum at ∼3 eV.

The photodissociation of cyanoacetylene, one of the key minor constituents in Titan’s atmosphere, was studied in a molecular beam under collisionless conditions using direct current slice ion imaging at 121.6, 193.3, and 243.2 nm. The experimental results were augmented by high-level theoretical calculations of stationary points on the ground-state and second excited singlet potential surfaces, and by statistical calculations of the dissociation rates and product branching on the ground-state surface. Results at 121.6 and 243.2 nm are nearly identical, suggesting that the 243.2 nm photodissociation is the result of a two-photon process. The translational energy distributions show only a modest fraction of the available energy in translation and are consistent with barrierless dissociation from the ground state. The results at 193.3 nm are quite distinct, showing up to half of the available energy in translation, implying dissociation with an exit barrier. The 193 nm result is ascribed to dissociation on the S1 potential energy surface. The theoretical calculations show significant rates for H loss on the ground state at 193 nm and significant branching to CN + CCH at 157 nm and higher.

Propenols have been found to be common intermediates in the hydrocarbon combustion and they are present in substantial concentrations in a wide range of flames. However, the kinetics properties of these species in combustion flames have not received much attention. In this work, the mechanism and kinetics of the OH hydrogen abstraction from propenols are investigated. Three stable conformations of propenols, (E)-1-propenol, (Z)-1-propenol, and syn-propen-2-ol, are taken into consideration. The potential energy profiles for the three reaction systems have been first investigated by the CCSD(T) method. The geometric parameters and relative energies of the reactants, reactant complexes, transition states, product complexes, and products have been investigated theoretically. The rate constants are calculated in the temperature range of 200−3000 K by the Variflex code based on the weak collision master equation/microcanonical variational RRKM theory. For all considered reactions, our results support a stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel. In the reaction of OH with (E)-1-propenol, the hydrogen abstractions from the −CH3 and −OH sites are dominant and competitive with each other in the temperature range from 500 to 2000 K. Above 2000 K, the hydrogen abstraction from the −CH group bonded to O atom becomes dominant with a relative yield of 51.1% at 3000 K. In the reaction of OH with (Z)-1-propenol, the hydrogen abstractions from −CH3, −CH bonded to O atom, and −OH are preferable in the temperature range from 500 to 1800 K, with the first two channels being competitive with each other. Above 1800 K, the hydrogen abstraction reaction from the CH group bonded to the CH3 group becomes dominant with the branching ratio of 90.3% at 3000 K. In the reaction of OH with syn-propen-2-ol, the abstractions from the −CH3 and −OH sites are competitive with each other when the temperature is higher than 500 K, and they become dominant above 800 K with the relative yields of 70.5% and 29.5% at 3000 K, respectively. The predicted total rate constants at the pressure of 1 atm fitted by modified three-parameter Arrhenius expressions in two different temperature ranges are also provided.

Ab initio G3(MP2,CC)//B3LYP/6-311G∗∗ calculations have been performed to investigate the C2H + C6H6 and C4H3 + C4H4 reactions on the C8H7 potential energy surface. The results demonstrate that C2H reacts with benzene without a barrier and then the C6H6(C2H) adduct produced loses atomic hydrogen to form phenylacetylene with overall reaction exothermicity of 28.4 kcal/mol. The reaction can be a major source of phenylacetylene under low-temperature conditions of Titan’s atmosphere. The reactions of vinylacetylene and butatriene with i-C4H3, producing phenylacetylene, pentalene, or benzocyclobutene, are predicted to be unlikely at low temperatures because of significant barriers but may be important in combustion flames.Barrierless ethynyl radical addition to benzene serves as a major source for the formation of phenylacetylene in Titan’s atmosphere.

Quantum chemical calculations of geometry relaxation in lowest excited states of neutral (NV0) and negatively charged (NV−) nitrogen-vacancy point defects in diamond have been performed employing the CASSCF method with a finite NC19H28 model cluster. Vibrations activated by the electronic transitions are determined by comparing calculated atomic displacements in the excited states with normal mode vectors, and the activated frequencies are evaluated as ∼600 cm−1. The barrier for N migration through the vacancy in NV− is estimated at the TD-B3LYP level and no significant decrease of this barrier (∼5 eV in the ground state) is found due to electronic excitations.Ab initio calculations of diamond nitrogen-vacancy defects describe geometry relaxation upon electronic excitation and vibrational spacing in absorption/emission spectra.

High-level ab initio calculations of the potential energy surface for the F(2P)+CH3(X2A2″) reaction show that the CH3F intermediate can be formed without a barrier and then dissociate via four product channels, including F + CH3, HF + CH2(1A1), H2 + CHF, and H + CH2F. RRKM and transition state theories have been applied to compute rate constants and branching ratios of the F + CH3/CH2 + HF/CHF + H2/CH2F + H products at various collision energies and temperatures. H + CH2F are predicted to be the major reaction products (except at low temperatures) followed by H2 + CHF. The H abstraction mechanism leading to HF + CH2(3B1) over a low, 1.4 kcal/mol, barrier is also important at high collision energies and temperatures.

Density functional B3LYP calculations have been performed to investigate proton transport in orthoperiodic and orthotellurium acids, their salts MIO6H4 (M = Li, Rb, Cs) and CsH5TeO6, dimers of the salt*acid type MIO6H4*H5IO6 (M = Rb, Cs), CsIO6H4*H6TeO6, CsHSO4*H6TeO6, Cs2SO4*H6TeO6, and also in double-substituted and binary salts Rb2H3IO6 and Rb4H2I2O10. It has been shown that the energy of salt dimerization is 33–35 kcal mol−1 and the activation barrier for proton migration between the neighboring octahedrons of the salt*acid → acid*salt type is calculated to be 3–13 kcal mol−1. The activation energy of the proton migration along the octahedron, 20–30 kcal mol−1, is comparable with the barrier for water molecule separation. Quantum-chemical calculations correlate with the results of X-ray and electrochemical studies.

Density functional B3LYP and ab initio G3(MP2,CCSD)//B3LYP calculations have been performed to study isomerization and dissociation pathways of C6H123+ in relation to the Coulomb explosion mechanism of cyclohexane. Cyclohexane trication is found to be metastable kinetically as the highest barrier on its decomposition pathway corresponding to the 2,1-H shift in the initial open-chain intermediate formed spontaneously after multiphoton ionization of cyclohexane is only 3.0 kcal/mol. The most favorable dissociation channels lead to the C4H82++C2H4+,C5H92++CH3+,andC3H62++C3H6+ products and all of them share the same reaction step with the highest barrier.

RRKM-Master Equation calculations have been performed to evaluate temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the C6H5, C6H4C2H, C6H5C2H2, and C6H4C2H3 radicals. These calculations indicate a strong pressure dependence for the role of various Hydrogen-Abstraction-C2H2-Addition (HACA) sequences for the formation of naphthalene from benzene. At atmospheric and lower pressures the C8H7 radicals, C6H4C2H3 and C6H5C2H2, cannot be stabilized above 1650 K. As a result, both the Bittner–Howard HACA route, in which a second acetylene molecule adds to C6H5C2H2, and the modified Frenklach route, where a second C2H2 adds to the aromatic ring of C6H4C2H3 obtained by internal hydrogen abstraction, are unrealistic under low pressure flame conditions. At the higher pressures of some practical combustion devices (e.g., 100 atm) these routes may be operative. Naphthalene is predicted to be the main product of the C6H5C2H2 + C2H2 and C6H4C2H3 + C2H2 reactions in the entire 500–2500 K temperature range independent of pressure (ignoring the issues related to the instability of C8H7 species). Frenklach's original HACA route, where the second C2H2 molecule adds to the aromatic ring activated by intermolecular H abstraction from C8H6, involves the C6H4C2H + C2H2 reaction, which is shown to predominantly form dehydrogenated species with a naphthalene core (naphthyl radicals or naphthynes) at T < 2000 K and diethynylbenzene at higher temperatures. The temperature and pressure dependence of rate coefficients for the various reaction channels has been analyzed and the results clearly demonstrate the importance of pressure for the reaction outcome. Thus, one must use caution when using low-pressure flame studies to validate PAH mechanisms for use in broader ranges of pressure.

Theoretical VRC-TST/RRKM-ME calculations were performed to evaluate total rate coefficients and product branching ratios for the oxidation of phenyl and 1- and 2-naphthyl radicals with O2 at temperatures relevant to combustion (1500, 2000, and 2500 K) and pressures of 0.01, 0.1, 1.0, and 10 atm. The results give the rate coefficients in the range of 3.0–5.5 × 10−11 cm3 molecule−1 s−1 with slightly positive temperature dependence, activation energies varying within 2.3–3.3 kcal/mol, and pre-exponential factors of 7–10 × 10−11 cm3 molecule−1 s−1. The dominant reaction channel in all three cases is elimination of the oxygen atom from peroxy complexes formed at the initial O2 addition step and leading to the phenoxy and naphthoxy radical products. The contribution of this channel increases with temperature. Chemically-activated phenoxy and naphthoxy radicals either decompose to the cyclopentadienyl + CO and indenyl + CO products, respectively, or undergo thermal equilibration. The relative yields of the decomposition/equilibration products strongly depend on temperature and pressure in the way that a temperature growth favors decomposition, whereas an increase in pressure favors equilibration. At the lowest temperature considered, 1500 K, the reactions also yield significant amounts of pyranyl + CO (phenyl + O2) or 1-benzopyranyl + CO (1-naphthyl + O2). A comparison of the phenyl + O2 and naphthyl + O2 reactions reveals that although the general trends in the oxidation kinetics of phenyl and naphthyl radicals are similar, the size and especially the position of the radical site in the aromatic moiety may affect the details of the mechanism and relative product yields.

Two ground-state CH radical reactions with the C3H4 isomers allene and methylacetylene occurring along the C4H5 potential energy surface (PES) were studied to probe the reaction mechanisms and final product distributions. The calculations were performed using a CCSD(T)-F12//B2PLYPD3 PES in combination with the 1-D chemical master equation. The reaction between the CH radical and allene was found to lead to exclusive “funneling” of the energized C4H5 intermediates into linear C4H5 configurations before reaching the exit channels, regardless of the specific nature of the initial bimolecular reactive encounter. In the case of the CH radical reaction with methylacetylene, energized C4H5 three-membered ring structures underwent H loss in significant amounts resulting in the production of a cyclic C4H4 methylenecyclopropene product, in accordance with experiments. The theoretical product distribution at room temperature for methylacetylene + CH was ∼35% methylenecyclopropene, ∼36% vinylacetylene, and ∼28% 1,2,3-butatriene, which is in agreement with the available experimental data. The distribution for allene + CH was ∼93% vinylacetylene, ∼4% 1,2,3-butatriene and ∼3% acetylene + vinyl, which overestimates the experimental yield of vinylacetylene and underestimates that of 1,2,3-butatriene by ∼10%. The possible reasons for this slight quantitative deviation of the theoretical results obtained within statistical treatment from the experiment are discussed.

Two sets of experiments were performed to unravel the high-temperature pyrolysis of tricyclo[5.2.1.02,6] decane (JP-10) exploiting high-temperature reactors over a temperature range of 1100 K to 1600 K, Advanced Light Source (ALS), and 927 K to 1083 K, National Synchrotron Radiation Laboratory (NSRL), with residence times of a few tens of microseconds (ALS) to typically 144 ms (NSRL). The products were identified in situ in supersonic molecular beams via single photon vacuum ultraviolet (VUV) photoionization coupled with mass spectroscopic detection in a reflectron time-of-flight mass spectrometer (ReTOF). These studies were designed to probe the initial (ALS) and also higher order reaction products (NSRL) formed in the decomposition of JP-10 – including radicals and thermally labile closed-shell species. Altogether 43 products were detected and quantified including C1–C4 alkenes, dienes, C3–C4 cumulenes, alkynes, eneynes, diynes, cycloalkenes, cyclo-dienes, aromatic molecules, and most importantly, radicals such as ethyl, allyl, and methyl produced at shorter residence times. At longer residence times, the predominant fragments were molecular hydrogen (H2), ethylene (C2H4), propene (C3H6), cyclopentadiene (C5H6), cyclopentene (C5H8), fulvene (C6H6), and benzene (C6H6). Accompanied by electronic structure calculations, the initial JP-10 decomposition via C–H bond cleavages resulting in the formation of the initial six C10H15 radicals was found to explain the formation of all products detected in both sets of experiments. These radicals are not stable under the experimental conditions and further decompose via C–C bond β-scission processes. These pathways result in ring opening in the initial tricyclic carbon skeletons of JP-10. Intermediates accessed after the first β-scission can further isomerize or dissociate. Complex PAH products in the NRLS experiment (naphthalene, acenaphthylene, biphenyl) are likely formed via molecular growth reactions at elevated residence times.

The classification of chemical reactions based on shared characteristics is at the heart of the chemical sciences, and is well exemplified by Langmuir's concept of isovalency, in which ‘two molecular entities with the same number of valence electrons have similar chemistries’. Within this account we further investigate the ramifications of the isovalency of four radicals with the same X2Σ+ electronic structure – cyano (CN), boron monoxide (BO), silicon nitride (SiN), and ethynyl (C2H), and their reactions with simple prototype hydrocarbons acetylene (C2H2) and ethylene (C2H4). The fact that these four reactants own the same X2Σ+ electronic ground state should dictate the outcome of their reactions with prototypical hydrocarbons holding a carbon–carbon triple and double bond. However, we find that other factors come into play, namely, atomic radii, bonding orbital overlaps, and preferential location of the radical site. These doublet radical reactions with simple hydrocarbons play significant roles in extreme environments such as the interstellar medium and planetary atmospheres (CN, SiN and C2H), and combustion flames (C2H, BO).

The reactions of the p-tolyl radical with allene-d4 and methylacetylene-d4 as well as of the p-tolyl-d7 radical with methylacetylene-d1 and methylacetylene-d3 were carried out under single collision conditions at collision energies of 44–48 kJ mol−1 and combined with electronic structure and statistical (RRKM) calculations. Our experimental results indicated that the reactions of p-tolyl with allene-d4 and methylacetylene-d4 proceeded via indirect reaction dynamics with laboratory angular distributions spanning about 20° in the scattering plane. As a result, the center-of-mass translational energy distribution determined a reaction exoergicity of 149 ± 28 kJ mol−1 and exhibited a pronounced maximum at around 20 to 30 kJ mol−1. In addition, the center-of-mass angular flux distribution T(θ) depicted a forward–backward symmetry and indicated geometric constraints upon the decomposing complex(es). Combining with calculations, these results propose that the bicyclic polycyclic aromatic hydrocarbons, 6-methyl-1H-indene (p1) and 5-methyl-1H-indene (p2), are formed under single collision conditions at fractions of at least 85% in both reaction systems. For the p-tolyl–methylacetylene system, experiments with partially deuterated reactants also reveal the formation of a third isomer p5 (1-methyl-4-(1-propynyl)benzene) at levels of 5–10%, highlighting the importance in conducting reactions with partially deuterated reactants to elucidate the underlying reaction pathways comprehensively.

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.

The reaction dynamics of the dicarbon radical C2(a3Πu/X1Σg+) in the singlet and triplet state with C4H6 isomers 2-butyne, 1-butyne and 1,2-butadiene were investigated at collision energies of about 26 kJ mol−1 using the crossed molecular beam technique and supported by ab initio and RRKM calculations. The reactions are all indirect, forming C6H6 complexes through barrierless additions by dicarbon on the triplet and singlet surfaces. Isomerization of the C6H6 reaction intermediate leads to product formation by hydrogen loss in a dicarbon–hydrogen atom exchange mechanism forming acyclic C6H5 reaction products through loose exit transition states in overall exoergic reactions.

We probed the reaction of the 4-methylphenyl radical with isoprene under single collision conditions at a collision energy of 58 kJ mol−1 by exploiting the crossed molecular beam technique. Supported by the electronic structure calculations, the reaction was found to initially lead to a van-der-Waals complex without any barrier which can then isomerize by addition of the 4-methylphenyl radical to any one of the four carbon atoms of the 1,3-butadiene moiety of isoprene. The initial addition products isomerize with formal addition products preferentially to C1 and C4 carbon atoms of the isoprene. These structures further isomerize via hydrogen migration and cyclization; the reaction is terminated by a hydrogen atom elimination from the 4-methylphenyl moiety via tight exit transition states leading to two dimethyl-dihydronaphthalene isomers as the dominating products. This study presents one of the very first bimolecular reactions of the 4-methylphenyl radical with unsaturated hydrocarbons and opens a path for the investigation of this reaction class in future experiments.

Ab initio CCSD(T)/cc-pVTZ//B3LYP/6-311G** calculations of the C5H5 potential energy surface have been performed to investigate the reaction mechanism of ethynyl radical (C2H) with C3H4 isomers, allene and methylacetylene. They were followed by RRKM calculations of reaction rate constants and product branching ratios under single-collision conditions. The results show that the C2H + CH2CCH2 reaction in a case of statistical behavior is expected to produce 1,4-pentadiyne (56–63%), ethynylallene (22–24%), and pentatetraene (10–15%), with the most favorable pathways including H losses from the initial HCCCH2CCH2 adduct leading to either 1,4-pentadiyne or ethynylallene, and a multistep route HCCC(CH2)2 → four-member ring → CH2CCCHCH2 → CH2CCCCH2 + H featuring a formal insertion of C2H into a double bond of allene followed by H elimination giving rise to pentatetraene. On the contrary, the C2H + CH3CCH reaction produces diacetylene + methyl (21–61%) by CH3 loss from the HCCC(CH)CH3 initial adduct as well as methyldiacetylene + H (27–56%) and ethynylallene + H (11–22%) by H eliminations from CHCCHCCH3. The calculated product branching ratios are in general agreement with the available experimental data, although some quantitative deviations from experiment and possible reasons for them are also discussed. The present calculations confirm that the C2H + C3H4 reactions proceed without entrance barriers and lead, via intermediates and transition states residing lower in energy than the initial reactants, to the C5H4 + H and C4H2 + CH3 products exothermic by 20–36 kcal mol−1, with strong dependence of the product distribution on the reacting C3H4 isomer, making these reactions fast under low-temperature conditions of Titan’s atmosphere where they can serve as a source of more complex unsaturated hydrocarbons.