A Theoretical Investigation of the Triplet Carbon Atom C(3P) + Vinyl Radical C2H3(2A‘) Reaction and Thermochemistry of C3Hn (n = 1−4) Species

Gas-Phase Formation of 1-Methylcyclopropene and 3-Methylcyclopropene via the Reaction of the Methylidyne Radical (CH; X2Π) with Propylene (CH3CHCH2; X1A')

The crossed molecular beam reactions of the methylidyne radical (CH; X²Π) with propylene (CH₃CHCH₂; X¹A') along with (partially) substituted reactants were conducted at collision energies of 19.3 kJ mol^-1. Combining our experimental data with ab initio electronic structure and statistical calculations, the methylidyne radical is revealed to add barrierlessly to the carbon-carbon double bond of propylene reactant resulting in a cyclic doublet C₄H₇ intermediate with a lifetime longer than its rotation period. These adducts undergo a nonstatistical unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming the cyclic products 1-methylcyclopropene and 3-methylcyclopropene with overall reaction exoergicities of 168 ± 25 kJ mol^-1. These C₄H₆ isomers are predicted to exist even in low-temperature environments such as cold molecular clouds like TMC-1, since the reaction is barrierless and exoergic, all transition states are below the energy of the separated reactants, and both the methylidyne radical (CH; X²Π) and propylene reactant were detected in cold molecular clouds such as TMC-1.

A combined experimental and computational study on the reaction dynamics of the 1-propynyl radical (CH3CC; X2A1) with ethylene (H2CCH2; X1A1g) and the formation of 1-penten-3-yne (CH2CHCCCH3; X1A')

The crossed molecular beam reactions of the 1-propynyl radical (CH₃CC; X²A₁) with ethylene (H₂CCH₂; X¹A_1g) and ethylene-d₄ (D₂CCD₂; X¹A_1g) were performed at collision energies of 31 kJ mol^-1 under single collision conditions. Combining our laboratory data with ab initio electronic structure and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations, we reveal that the reaction is initiated by the barrierless addition of the 1-propynyl radical to the π-electron density of the unsaturated hydrocarbon of ethylene leading to a doublet C₅H₇ intermediate(s) with a life time(s) longer than the rotation period(s). The reaction eventually produces 1-penten-3-yne (p1) plus a hydrogen atom with an overall reaction exoergicity of 111 ± 16 kJ mol^-1. About 35% of p1 originates from the initial collision complex followed by C-H bond rupture via a tight exit transition state located 22 kJ mol^-1 above the separated products. The collision complex (i1) can also undergo a [1,2] hydrogen atom shift to the CH₃CHCCCH₃ intermediate (i2) prior to a hydrogen atom release; RRKM calculations suggest that this pathway contributes to about 65% of p1. In higher density environments such as in combustion flames and circumstellar envelopes of carbon stars close to the central star, 1-penten-3-yne (p1) may eventually form the cyclopentadiene (c-C₅H₆) isomer via hydrogen atom assisted isomerization followed by hydrogen abstraction to the cyclopentadienyl radical (c-C₅H₅) as an important pathway to key precursors to polycyclic aromatic hydrocarbons (PAHs) and to carbonaceous nanoparticles.

Are Nonadiabatic Reaction Dynamics the Key to Novel Organosilicon Molecules? The Silicon (Si(3P))-Dimethylacetylene (C4H6(X1A1g)) System as a Case Study

The bimolecular gas phase reaction of ground-state silicon (Si; ³P) with dimethylacetylene (C₄H₆; X¹A_1g) was investigated under single collision conditions in a crossed molecular beams machine. Merged with electronic structure calculations, the data propose nonadiabatic reaction dynamics leading to the formation of singlet SiC₄H₄ isomer(s) and molecular hydrogen (H₂) via indirect scattering dynamics along with intersystem crossing (ISC) from the triplet to the singlet surface. The reaction may lead to distinct energetically accessible singlet SiC₄H₄ isomers (¹p8-¹p24) in overall exoergic reaction(s) (-107_-20⁺¹² kJ mol^-1). All feasible reaction products are either cyclic, carry carbene analogous silylene moieties, or carry C-Si-H or C-Si-C bonds that would require extensive isomerization from the initial collision complex(es) to the fragmenting singlet intermediate(s). The present study demonstrates the first successful crossed beams study of an exoergic reaction channel arising from bimolecular collisions of silicon, Si(³P), with a hydrocarbon molecule.

Kinetics of Propargyl Radical Dissociation

Due to the prominent role of the propargyl radical for hydrocarbon growth within combustion environments, it is important to understand the kinetics of its formation and loss. The ab initio transition state theory-based master equation method is used to obtain theoretical kinetic predictions for the temperature and pressure dependence of the thermal decomposition of propargyl, which may be its primary loss channel under some conditions. The potential energy surface for the decomposition of propargyl is first mapped at a high level of theory with a combination of coupled cluster and multireference perturbation calculations. Variational transition state theory is then used to predict the microcanonical rate coefficients, which are subsequently implemented within the multiple-well multiple-channel master equation. A variety of energy transfer parameters are considered, and the sensitivity of the thermal rate predictions to these parameters is explored. The predictions for the thermal decomposition rate coefficient are found to be in good agreement with the limited experimental data. Modified Arrhenius representations of the rate constants are reported for utility in combustion modeling.

Dynamics of the C/H and C/F exchanges in the reaction of 3P carbon atoms with vinyl fluoride

Two product channels C3H2F + H and C3H3 + F were identified in the reaction of C((3)P) atoms with vinyl fluoride (C2H3F) at collision energy 3.7 kcal mol(-1) in a crossed molecular-beam apparatus using selective photoionization. Time-of-flight (TOF) spectra of products C3H2F and C3H3 were measured at 12-16 laboratory angles as well as a TOF spectrum of atomic F, a counter part of C3H3, was recorded at single laboratory angle. From the best simulation of product TOF spectra, translational-energy distributions at seven scattering angles and a nearly isotropic (forward and backward peaked) angular distribution were derivable for exit channel C3H2F + H (C3H3 + F) that has average kinetic-energy release of 14.5 (4.9) kcal mol(-1). Products C3H2F + H and C3H3 + F were estimated to have a branching ratio of ~53:47. Furthermore, TOF spectra and photoionization spectra of products C3H2F and C3H3 were measured at laboratory angle 62° with photoionization energy ranging from 7 eV to 11.6 eV. The appearance of TOF spectra is insensitive to photon energy, implying that only single species overwhelmingly contributes to products C3H2F and C3H3. HCCCHF (H2CCCH) was identified as the dominant species based on the measured ionization threshold of 8.3 ± 0.2 (8.6 ± 0.2) eV and the maximal translational-energy release. The C/H and C/F exchange mechanisms are stated.

Absolute photoionization cross-section of the propargyl radical

Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C(3)H(3)) radical, σ(propargyl) (ion)(E), relative to the known absolute cross-section of the methyl (CH(3)) radical. We generated a stoichiometric 1:1 ratio of C(3)H(3):CH(3) from 193 nm photolysis of two different C(4)H(6) isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of σ(propargyl)(ion)(10.213 eV)=(26.1±4.2) Mb and σ(propargyl)(ion)(10.413 eV)=(23.4±3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of σ(propargyl)(ion)(10.213 eV)=(23.6±3.6) Mb and σ(propargyl)(ion)(10.413 eV)=(25.1±3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.

Crossed beam study of the atom-radical reaction of ground state carbon atoms (C(3P)) with the vinyl radical (C2H3(X2A'))

The atom-radical reaction of ground state carbon atoms (C((3)P)) with the vinyl radical (C(2)H(3)(X(2)A')) was conducted under single collision conditions at a collision energy of 32.3 ± 2.9 kJ mol(-1). The reaction dynamics were found to involve a complex forming reaction mechanism, which is initiated by the barrier-less addition of atomic carbon to the carbon-carbon-double bond of the vinyl radical forming a cyclic C(3)H(3) radical intermediate. The latter has a lifetime of at least 1.5 times its rotational period and decomposes via a tight exit transition state located about 45 kJ mol(-1) above the separated products through atomic hydrogen loss to the cyclopropenylidene isomer (c-C(3)H(2)) as detected toward cold molecular clouds and in star forming regions.

Slow electron velocity-map imaging of negative ions: applications to spectroscopy and dynamics

Anion photoelectron spectroscopy (PES) has become one of the most versatile techniques in chemical physics. This article briefly reviews the history of anion PES and some of its applications. It describes efforts to improve the resolution of this technique, including anion zero electron kinetic energy (ZEKE) and the recently developed method of slow electron velocity-map imaging (SEVI). Applications of SEVI to studies of vibronic coupling in open-shell systems and the spectroscopy of prereactive van der Waals complexes are then discussed.

Chemistry of energetically activated cumulenes - from allene (H2CCCH2) to hexapentaene (H2CCCCCCH2)

During the last decade, experimental and theoretical studies on the unimolecular decomposition of cumulenes (H(2)C(n)H(2)) from propadiene (H(2)CCCH(2)) to hexapentaene (H(2)CCCCCCH(2)) have received considerable attention due to the importance of these carbon-bearing molecules in combustion flames, chemical vapor deposition processes, atmospheric chemistry, and the chemistry of the interstellar medium. Cumulenes and their substituted counterparts also have significant technical potential as elements for molecular machines (nanomechanics), molecular wires (nano-electronics), nonlinear optics, and molecular sensors. In this review, we present a systematic overview of the stability, formation, and unimolecular decomposition of chemically, photo-chemically, and thermally activated small to medium-sized cumulenes in extreme environments. By concentrating on reactions under gas phase thermal conditions (pyrolysis) and on molecular beam experiments conducted under single-collision conditions (crossed beam and photodissociation studies), a comprehensive picture on the unimolecular decomposition dynamics of cumulenes transpires.

Slow electron velocity-map imaging spectroscopy of the 1-propynyl radical

High resolution photoelectron spectra of the 1-propynyl and 1-propynyl-d(3) anions acquired with slow electron velocity-map imaging are presented. The electron affinity is determined to be 2.7355+/-0.0010 eV for the 1-propynyl radical and 2.7300+/-0.0010 eV for 1-propynyl-d(3). Several vibronic transitions are observed and assigned using the isotopic shifts and results from ab initio calculations. Good agreement between experimental spectra and calculations suggests a C(3v) geometry for the 1-propynyl radical. No evidence is found for strong vibronic coupling between the ground electronic state and the low-lying first excited state.

Thermochemistry of Key Soot Formation Intermediates: C3H3 Isomers

Accurate standard enthalpies of formation for allene, propyne, and four C3H3 isomers involved in soot formation mechanisms have been determined through systematic focal point extrapolations of ab initio energies. Auxiliary corrections have been applied for anharmonic zero-point vibrational energy, core electron correlation, the diagonal Born-Oppenheimer correction (DBOC), and scalar relativistic effects. Electron correlation has been accounted for via second-order Z-averaged perturbation theory (ZAPT2) and primarily through coupled-cluster theory, including single, double, and triple excitations, as well as a perturbative treatment of connected quadruple excitations [ROCCSD, ROCCSD(T), ROCCSDT, and UCCSDT(Q)]. The correlation-consistent hierarchy of basis sets, cc-pVXZ (X = D, T, Q, 5, 6), was employed. The CCSDT(Q) corrections do not exceed 0.12 kcal mol(-)1 for the relative energies of the systems considered here, indicating a high degree of electron correlation convergence in the present results. Our recommended values for the enthalpies of formation are as follows: Delta(f)H(o)(0)(propargyl) = 84.76, Delta(f)H(o)(0) (1-propynyl) = 126.60, Delta(f)H(o)(0) (cycloprop-1-enyl) = 126.28, Delta(f)H(o)(0)(cycloprop-2-enyl) = 117.36, Delta(f)H(o)(0)(allene) = 47.41, and Delta(f)H(o)(0)(propyne) = 46.33 kcal mol(-1), with estimated errors no larger than 0.3 kcal mol(-1). The corresponding C3H3 isomerization energies are about 1 kcal mol(-1) larger than previous coupled-cluster results and several kcal mol(-1) below those previously obtained using density functional theory.

Unimolecular dissociation of the propargyl radical intermediate of the CH+C2H2 and C+C2H3 reactions

This paper examines the unimolecular dissociation of propargyl (HCCCH2) radicals over a range of internal energies to probe the CH+HCCH and C+C2H3 bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by 157 nm photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and H2 elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 (+5-10) kcal/mol as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical's dissociation and also lower than calculated overall reaction enthalpies. Products from both H2+HCCC and H+C3H2 channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C-H fission channel were also observed in the photolysis of propargyl chloride.

Photodissociation of propargyl bromide and photoionization of the propargyl radical

Velocity map imaging was used to study the 193 nm photodissociation of propargyl bromide C(3)H(3)Br as well as the photoionization dynamics of the resulting propargyl radical C(3)H(3). Images were recorded by using single-photon vacuum ultraviolet ionization of the propargyl radical and by using two-photon resonant, three-photon ionization of the ground state Br((2)P(32)) and spin-orbit excited Br(*)((2)P(12)) atoms. Analysis of these data allowed the determination of the branching ratio Br:Br(*) as well as the photofragment angular distributions. Images of C(3)H(3) produced by the photodissociation of both C(3)H(3)Br and C(3)H(3)Cl were recorded at several energies between 8.97 and 9.12 eV, as well as at 9.86 eV, and showed no obvious internal energy dependence of the relative photoionization cross sections.

Photodissociation of Propargyl Chloride at 193 nm

The photodissociation of propargyl chloride (C3H3Cl) has been studied at 193 nm. Ion imaging experiments with state-selective detection of the Cl atoms and single-photon ionization of the C3H3 radicals were performed, along with measurements of the Cl + C3H3 and HCl + C3H2 recoil kinetic energy distributions, using a scattering apparatus with electron bombardment ionization detection to resolve the competing Cl and HCl elimination channels. The experiments allow the determination of the Cl (2P3/2) and Cl (2P1/2) (hereafter Cl) branching fractions associated with the C-Cl bond fission, which are determined to be 0.5 +/- 0.1 for both channels. Although prior translational spectroscopy studies by others had concluded that the low velocity signal at the Cl+ mass was due to daughter fragments of the HCl elimination products, the present work shows that Cl atoms are produced with a bimodal recoil kinetic energy distribution. The major C-Cl bond fission channel, with a narrow recoil kinetic energy distribution peaking near 40 kcal/mol, produces both Cl and Cl, whereas the minor (5%) channel, partitioning much less energy to relative kinetic energy, produces only ground spin-orbit state Cl atoms. The maximum internal energy of the radicals produced in the low-recoil-kinetic-energy channel is consistent with this channel producing electronically excited propargyl radicals. Finally, in contrast to previous studies, the present work determines the HCl recoil kinetic energy distribution and identifies the possible contribution to this spectrum from propargyl radicals cracking to C3+ ions in the mass spectrometer.

Potential Energy Surface and Product Branching Ratios for the Reaction of Dicarbon, C2(XΣg+), with Methylacetylene, CH3CCH(X1A1): An Ab Initio/RRKM Study

Ab initio calculations of the potential energy surface for the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction have been carried at the G2M level of theory. The calculations show that the dicarbon molecule in the ground singlet electronic state can add to methylacetylene without a barrier producing a three-member or a four-member ring intermediate, which can rapidly rearrange to the most stable H(3)CCCCCH isomer on the C(5)H(4) singlet surface. This isomer can then lose a hydrogen atom (H) or molecular hydrogen (H(2)) from the CH(3) group with the formation of H(2)CCCCCH and HCCCCCH, respectively. Alternatively, H atom migrations and three-member-ring closure/opening rearrangements followed by H and H(2) losses can lead to other isomers of the C(5)H(3) and C(5)H(2) species. According to the calculated energetics, the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH reaction is likely to be a major source of the C(5)H(3) radicals (in particular, the most stable H(2)CCCCCH and HCCCHCCH isomers, which are relevant to the formation of benzene through the reactions with CH(3)). Among heavy-fragment product channels, only C(3)H(3) + C(2)H and c-C(3)H(2) + C(2)H(2) might compete with C(5)H(3) + H and C(5)H(2) + H(2). RRKM calculations of reaction rate constants and product branching ratios depending on the reactive collision energy showed that the major reaction products are expected to be H(2)CCCCCH + H (64-66%) and HCCCHCCH + H (34-30%), with minor contributions from HCCCCCH + H(2) (1-2%), HCCCHCC + H(2) (up to 1%), C(3)H(3) + C(2)H (up to 1%), and c-C(3)H(2) + C(2)H(2) (up to 0.1%) if the energy randomization is complete. The calculations also indicate that the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction can proceed by direct H-abstraction of a methyl hydrogen to form C(3)H(3) + C(2)H almost without a barrier.

An experimental and computational evaluation of the energetics of the isomeric methoxyphenylcarbenes generated in carbon atom reactions

Carbon atom reactions with anisole and methoxybenzaldehyde demonstrate the reversible ring expansion of methoxyphenylcarbene (CH(3)O-C(6)H(4)-C-H). Trapping with HBF(4) yields the methoxytropylium ion, analogous to the well-known reactions of phenylcarbene. For instance, in the reaction of carbon atom with p-methoxybenzaldehyde, which proceeds by deoxygenation of the carbonyl group and formation of the corresponding arylcarbene, the products formed are methoxytropylium fluoroborate, p-methoxytoluene and m-methoxytoluene in yields of 69.4, 7.6, and 22.9%, respectively. Gas-phase density functional theory calculations were also carried out. The observed product yields from the uniquely generated p- and m-methoxyphenylcarbenes are in good agreement with the calculations. In the case of o-methoxyphenylcarbene, however, the calculations indicate that formation of dihydrobenzofuran is the most facile rearrangement on the ground-state singlet surface. In contrast, ring expansion is observed to be the major reaction path experimentally. The exothermicity of the deoxygenation step for carbon atom reaction with methoxybenzaldehyde ( approximately 100 kcal/mol) can allow for an excited singlet state to be formed initially. This excited singlet state of the methoxyphenylcarbene will resemble open-shell phenylnitrene, which is known to undergo ring expansion much more readily than phenylcarbene. On the basis of this analogy, we reconcile the difference in reactivity of the arylcarbene predicted by density functional theory calculations with the reactivity observed experimentally.