Reactions O(3P, 1D) + HCCCN(X1Σ+) (Cyanoacetylene): Crossed-Beam and Theoretical Studies and Implications for the Chemistry of Extraterrestrial Environments.

Cyanoacetylene (HCCCN), the first member of the cyanopolyyne family (HCnN, where n = 3, 5, 7, ...), is of particular interest in astrochemistry being ubiquitous in space (molecular clouds, solar-type protostars, protoplanetary disks, circumstellar envelopes, and external galaxies) and also relatively abundant. It is also abundant in the upper atmosphere of Titan and comets. Since oxygen is the third most abundant element in space, after hydrogen and helium, the reaction O + HCCCN can be of relevance in the chemistry of extraterrestrial environments. Despite that, scarce information exists not only on the reactions of oxygen atoms with cyanoacetylene but with nitriles in general. Here, we report on a combined experimental and theoretical investigation of the reactions of cyanoacetylene with both ground 3P and excited 1D atomic oxygen and provide detailed information on the primary reaction products, their branching fractions (BFs), and the overall reaction mechanisms. More specifically, the reactions of O(3P, 1D) with HCCCN(X1Σ+) have been investigated under single-collision conditions by the crossed molecular beams scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy, Ec, of 31.1 kJ/mol. From product angular and time-of-flight distributions, we have identified the primary reaction products and determined their branching fractions (BFs). Theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) were performed to assist the interpretation of the experimental results and clarify the reaction mechanism. Adiabatic statistical calculations of product BFs for the decomposition of the main triplet and singlet intermediates have also been carried out. Merging together the experimental and theoretical results, we conclude that the O(3P) reaction is characterized by a minor adiabatic channel leading to OCCCN (cyanoketyl) + H (experimental BF = 0.10 ± 0.05), while the dominant channel (BF = 0.90 ± 0.05) occurs via intersystem crossing to the underlying singlet PES and leads to formation of 1HCCN (cyanomethylene) + CO. The O(1D) reaction is characterized by the same two channels, with the relative CO/H yield being slightly larger. Considering the recorded reactive signal and the calculated entrance barrier, we estimate that the rate coefficient for reaction O(3P) + HC3N at 300 K is in the 10-12 cm3 molec-1 s-1 range. Our results are expected to be useful to improve astrochemical and photochemical models. In addition, they are also relevant in combustion chemistry, because the thermal decomposition of pyrrolic and pyridinic structures present in fuel-bound nitrogen generates many nitrogen-bearing compounds, including cyanoacetylene.

Combined Experimental and Theoretical Studies of the O(3P) + 1-Butene Reaction Dynamics: Primary Products, Branching Fractions, and Role of Intersystem Crossing.

Information on the detailed mechanism and dynamics (primary products, branching fractions (BFs), and channel specific rate constants as a function of temperature) for many important combustion reactions of O(3P) with unsaturated hydrocarbons is still lacking. We report synergistic experimental/theoretical studies on the mechanism and dynamics of the O(3P) + 1-C4H8 (1-butene) reaction by combining crossed molecular beam (CMB) experiments with soft electron ionization mass-spectrometric detection and time-of-flight analysis at 10.5 kcal/mol collision energy (Ec) to high-level ab initio electronic structure calculations of the underlying triplet and singlet potential energy surfaces (PESs) and statistical Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) computations of BFs including intersystem crossing (ISC). The reactive interaction of O(3P) with 1-butene is found to mainly break apart the 4-carbon atom chain, leading to the radical product channels ethyl + vinoxy (BF = 0.34 ± 0.11), methyl + C3H5O (BF = 0.28 ± 0.09), formyl + propyl (BF = 0.17 ± 0.05), ethyl + acetyl (BF = 0.014 ± 0.007), and butanal radical (ethylvinoxy) + H (BF = 0.013 ± 0.004), and molecular product channels formaldehyde + propenylidene/propene (BF = 0.15 ± 0.05) and butenone (ethyl ketene) + H2 (BF = 0.037 ± 0.015). As some of these products can only be formed via ISC from triplet to singlet PESs, from BFs an extent of ISC of 50% is inferred. This value is significantly larger than that recently observed for O(3P) + propene (22%) at similar Ec, underlying the question of how important it is to consider nonadiabatic effects for these and similar combustion reactions. Comparison of the derived BFs with those of statistical (RRKM/ME) simulations on the ab initio coupled triplet/singlet PESs shows good agreement, warranting the use of the RRKM/ME approach to provide information on the variation of BFs with temperature and to derive channel specific rate constants as a function of temperature (T) and pressure (p). Notably, ISC is predicted to decrease strongly with increasing temperature (from about 70% at 300 K to 46% at Ec = 10.5 kcal/mol, and about 1% at 2000 K). The present results lead to a detailed understanding of the complex reaction mechanism of O(3P) + 1-butene and, by providing channel specific rate constants as a function of T and p, should facilitate the improvement of current fossil-fuel (1-butene) as well as biofuel (1-butanol) combustion models.

Measurements of Ionization Cross Sections by Molecular Beam Experiments: Information Content on the Imaginary Part of the Optical Potential.

In this work, we present and analyze in detail new and recent ionization cross section and mass spectrum determinations, collected in the case of He*, Ne*-H2O, -H2S, and -NH3 ionizing collisions. These sets of data, obtained under the same experimental conditions, are relevant to identify differences in the autoionization stereodynamics of the three hydrogenated molecules and on the selective role of the imaginary part of the optical potential. We demonstrate that in these autoionization processes hydrogen and halogen bonds are competing because they are controlling both real and imaginary components of the optical potential that drives the complete reaction dynamics. In particular, we found that both components critically depend on the angular and radial approach between the reagent partners in determining the collision dynamics.

Reaction Dynamics of O((3)P) + Propyne: I. Primary Products, Branching Ratios, and Role of Intersystem Crossing from Crossed Molecular Beam Experiments.

We performed synergic experimental/theoretical studies on the mechanism of the O((3)P) + propyne reaction by combining crossed molecular beams experiments with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy (Ec) with ab initio electronic structure calculations at a high level of theory of the relevant triplet and singlet potential energy surfaces (PESs) and statistical calculations of branching ratios (BRs) taking into account intersystem crossing (ISC). In this paper (I) we report the results of the experimental investigation, while the accompanying paper (II) shows results of the theoretical investigation with comparison to experimental results. By exploiting soft electron ionization detection to suppress/mitigate the effects of the dissociative ionization of reactants, products, and background gases, product angular and velocity distributions at different charge-to-mass ratios were measured. From the laboratory data angular and translational energy distributions in the center-of-mass system were obtained for the five competing most important product channels, and product BRs were derived. The reactive interaction of O((3)P) with propyne under single collision conditions is mainly leading to the rupture of the three-carbon atom chain, with production of the radical products methylketenyl + atomic hydrogen (BR = 0.04), methyl + ketenyl (BR = 0.10), and vinyl + formyl (BR = 0.11) and the molecular products ethylidene/ethylene + carbon monoxide (BR = 0.74) and propandienal + molecular hydrogen (BR = 0.01). Because some of the products can only be formed via ISC from the entrance triplet to the low-lying singlet PES, we infer from their BRs an amount of ISC larger than 80%. This value is dramatically large when compared to the negligible ISC reported for the O((3)P) reaction with the simplest alkyne, acetylene. At the same time, it is much larger than that (∼20%) recently observed in the related reaction of the three-carbon atom alkene, O((3)P) + propene at a comparable Ec. This poses the question of how important it is to consider nonadiabatic effects and their dependence on molecular structure for this kind of combustion reactions. The prevalence of the CH3 over the H displacement channels is not explained by invoking a preference for the addition on the methyl-substituted acetylenic carbon atom, but rather it is believed to be due to the different tendencies of the two addition triplet intermediates CH3CCHO (preferentially leading to H elimination) and CH3COCH (preferentially leading to CH3 elimination) to undergo ISC to the underlying singlet PES. It is concluded that the main coproduct of the CO forming channel is singlet ethylidene ((1)CH3CH) rather than ground-state ethylene. By comparing the derived BRs with those very recently derived from kinetics studies at room temperature and 4 Torr we obtained information on how BRs vary with collision energy. The extent of ISC is estimated to remain essentially constant (∼85%) with increasing Ec from ∼1 to ∼10 kcal/mol. The present experimental results shed light on the mechanism of the title reaction at energies comparable to those involved in combustion and, when compared with the results from the statistical calculations on the ab initio coupled PESs (see accompanying paper II), lead to an in-depth understanding of the rather complex reaction mechanism of O + propyne. The overall results are expected to contribute to the development of more refined models of hydrocarbon combustion.

Directed gas-phase formation of the ethynylsulfidoboron molecule.

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.

Gas-phase synthesis of phenyl oxoborane (C6H5BO) via the reaction of boron monoxide with benzene.

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.

A crossed beam and ab initio investigation on the formation of boronyldiacetylene (HCCCC11BO; X1Σ+) via the reaction of the boron monoxide radical (11BO; X2Σ+) with diacetylene (C4H2; X1Σg(+)).

The reaction dynamics of the boron monoxide radical ((11)BO; X(2)Σ(+)) with diacetylene (C4H2; X(1)Σg(+)) were investigated at a nominal collision energy of 17.5 kJ mol(-1) employing the crossed molecular beam technique and supported by ab initio and statistical (RRKM) calculations. The reaction is governed by indirect (complex forming) scattering dynamics with the boron monoxide radical adding with its boron atom to the carbon-carbon triple bond of the diacetylene molecule at one of the terminal carbon atoms without entrance barrier. This leads to a doublet radical intermediate (C4H2(11)BO), which undergoes unimolecular decomposition through hydrogen atom emission from the C1 carbon atom via a tight exit transition state located about 18 kJ mol(-1) above the separated products. This process forms the hitherto elusive boronyldiacetylene molecule (HCCCC(11)BO; X(1)Σ(+)) in a bimolecular gas phase reaction under single collision conditions. The overall reaction was determined to be exoergic by 62 kJ mol(-1). The reaction dynamics are compared to the isoelectronic diacetylene (C4H2; X(1)Σg(+))-cyano radical (CN; X(2)Σ(+)) system studied previously in our group. The characteristics of boronyl-diacetylene and the boronyldiacetylene molecule (HCCCC(11)BO; X(1)Σ(+)) as well as numerous intermediates are reported for the first time.

The dynamics of allyl radical dissociation.

Dissociation of the allyl radical, CH(2)CHCH(2), and its deuterated isotopolog, CH(2)CDCH(2), have been investigated using trajectory calculations on an ab initio ground-state potential energy surface calculated for 97,418 geometries at the coupled cluster single and double and perturbative treatment of triple excitations, with the augmented correlation consistent triple-ζ basis set level (CCSD(T)/AVTZ). At an excitation energy of 115 kcal/mol, corresponding to optical excitation at 248 nm, the primary channel is hydrogen loss with a quantum yield of 0.94 to give either allene or propyne in a ratio of 6.4:1. The total dissociation rate for CH(2)CHCH(2) is 6.3 × 10(10) s(-1), corresponding to a 1/e time of 16 ps. Methyl and C(2)H(2) are produced with a quantum yield of 0.06 by three different mechanisms: a 1,3 hydrogen shift followed by C-C cleavage to give methyl and acetylene, a double 1,2 shift followed by C-C cleavage to give methyl and acetylene, or a single 1,2 hydrogen shift followed by C-C cleavage to give methyl and vinylidene. In this last channel, the vinylidene eventually isomerizes to give internally excited acetylene, and the kinetic energy distribution is peaked at much lower energy (6.4 kcal/mol) than that for the other two channels (18 kcal/mol). The trajectory results also predict the v-J correlation, the anisotropy of dissociation, and distributions for the angular momentum of the fragments. The v-J correlation for the CH(3) + HCCH channel is strongest for high rotational levels of acetylene, where v is perpendicular to J. Methyl elimination is anisotropic, with ß = 0.66, whereas hydrogen elimination is nearly isotropic. In the hydrogen elimination channel, allene is rotationally excited with a total angular momentum distribution peaked near J = 17. In the methyl elimination channel, the peak of the methyl rotational distribution is at J ≈ 12, whereas the peak of the acetylene rotational distribution is at J ≈ 28.

An Experimental and Theoretical Investigation of 1-Butanol Pyrolysis.

Bioalcohols are a promising family of biofuels. Among them, 1-butanol has a strong potential as a substitute for petrol. In this manuscript, we report on a theoretical and experimental characterization of 1-butanol thermal decomposition, a very important process in the 1-butanol combustion at high temperatures. Advantage has been taken of a flash pyrolysis experimental set-up with mass spectrometric detection, in which the brief residence time of the pyrolyzing mixture inside a short, resistively heated SiC tube allows the identification of the primary products of the decomposing species, limiting secondary processes. Dedicated electronic structure calculations of the relevant potential energy surface have also been performed and RRKM estimates of the rate coefficients and product branching ratios up to 2,000 K are provided. Both electronic structure and RRKM calculations are in line with previous determinations. According to the present study, the H2O elimination channel leading to 1-butene is more important than previously believed. In addition to that, we provide experimental evidence that butanal formation by H2 elimination is not a primary decomposition route. Finally, we have experimental evidence of a small yield of the CH3 elimination channel.

Competing channels in the thermal decomposition of azidoacetone studied by pyrolysis in combination with molecular beam mass spectrometric techniques.

The thermal decomposition of azidoacetone (CH3COCH2N3) was studied using a combined experimental and computational approach. Flash pyrolysis at a range of temperatures (296-1250 K) was used to induce thermal decomposition, and the resulting products were expanded into a molecular beam and subsequently analyzed using electron bombardment ionization coupled to a quadrupole mass spectrometer. The advantages of this technique are that the parent molecules spend a very short time in the pyrolysis zone (20-30 mus) and that the subsequent expansion permits the stabilization of thermal products that are not observable using conventional pyrolysis methods. A detailed analysis of the mass spectra as a function of pyrolysis temperature revealed the participation of five thermal decomposition channels. Ab initio calculations on the stable structures and transition states of the azidoacetone system in combination with an analysis of the dissociative ionization pattern of each channel allowed the identity and mechanism of each channel to be elucidated. At low temperatures (296-800 K) the azide decomposes principally by the loss of N2 to yield the imine (CH3COCHNH), which can further decompose to CH3CO and CHNH. At low and intermediate temperatures a process involving the loss of N2 to yield CH3CHO and HCN is also open. Finally, at high temperatures (800-1250 K) a channel in which the azide decomposes to a stable cyclic amine (CO(CH2)2NH) (after loss of N2) is active. The last channel involves subsequent thermal decomposition of this cyclic amine to ketene (H2CCO) and methanimine (H2CNH).

Combined Experimental-Theoretical Study of the OH + CO → H + CO2 Reaction Dynamics.

A combined experimental-theoretical study is performed to advance our understanding of the dynamics of the prototypical tetra-atom, complex-forming reaction OH + CO → H + CO2, which is also of great practical relevance in combustion, Earth's atmosphere, and, potentially, Mars's atmosphere and interstellar chemistry. New crossed molecular beam experiments with mass spectrometric detection are analyzed together with the results from previous experiments and compared with quasi-classical trajectory (QCT) calculations on a new, full-dimensional potential energy surface (PES). Comparisons between experiment and theory are carried out both in the center-of-mass and laboratory frames. Good agreement is found between experiment and theory, both for product angular and translational energy distributions, leading to the conclusion that the new PES is the most accurate at present in elucidating the dynamics of this fundamental reaction. Yet, small deviations between experiment and theory remain and are presumably attributable to the QCT treatment of the scattering dynamics.

Isomer-Specific Chemistry in the Propyne and Allene Reactions with Oxygen Atoms: CH3CH + CO versus CH2CH2 + CO Products.

We report direct experimental and theoretical evidence that, under single-collision conditions, the dominant product channels of the O((3)P) + propyne and O((3)P) + allene isomeric reactions lead in both cases to CO formation, but the coproducts are singlet ethylidene ((1)CH3CH) and singlet ethylene (CH2CH2), respectively. These data, which settle a long-standing issue on whether ethylidene is actually formed in the O((3)P) + propyne reaction, suggest that formation of CO + alkylidene biradicals may be a common mechanism in O((3)P) + alkyne reactions, in contrast to formation of CO + alkene molecular products in the corresponding isomeric O((3)P) + diene reactions, either in combustion or other gaseous environments. These findings are of fundamental relevance and may have implications for improved combustion models. Moreover, we predict that the so far neglected (1)CH3CH + CO channel is among the main reaction routes also when the C3H4O singlet potential energy surface is accessed from the OH + C3H3 (propargyl) entrance channel, which are radical species playing a key role in many combustion systems.

Crossed beam studies of the reactions of atomic oxygen in the ground 3P and first electronically excited 1D states with hydrogen sulfide.

The reactions of both ground, (3)P, and electronically excited, (1)D, oxygen atoms with hydrogen sulfide, H(2)S, have been investigated by means of the crossed molecular beams method with mass spectrometric detection at different collision energies. Amongst the possible reaction channels those leading to HSO+H for the O((3)P) reaction and to HSO/HOS+H and SO+H(2) for the O((1)D) reaction have been identified and investigated. The dynamics of the channels leading to HSO/HOS+H are elucidated for the reactions of both states and the trend with increasing the collision energy analyzed. Noteworthily, the formation of SO+H(2) products appears to be an open channel for the O((1)D) reaction, at least for the highest collision energy investigated (11.8 kcal/mol). Finally, the recent experimental and theoretical estimates of the enthalpy of formation of the HSO radical have been critically analyzed to evaluate their conformity with the present experimental data.