A chemical dynamics study of the reaction of the methylidyne radical (CH, X2Π) with dimethylacetylene (CH3CCCH3, X1A1g)

Competing Si2CH4-H2 and SiCH2-SiH4 Channels in the Bimolecular Reaction of Ground-State Atomic Carbon (C(3Pj)) with Disilane (Si2H6, X1A1g) under Single Collision Conditions

The bimolecular gas-phase reaction of ground-state atomic carbon (C(³P_j)) with disilane (Si₂H₆, X¹A_1g) was explored under single-collision conditions in a crossed molecular beam machine at a collision energy of 36.6 ± 4.5 kJ mol^-1. Two channels were observed: a molecular hydrogen elimination plus Si₂CH₄ (reaction 1) pathway and a silane loss channel along with the formation of SiCH₂ (reaction 2), with branching ratios of 20 ± 3 and 80 ± 4%, respectively. Both channels involved indirect scattering dynamics via long-lived Si₂CH₆ reaction intermediate(s); the latter eject molecular hydrogen and silane in "molecular" elimination channels within the rotational plane of the fragmenting intermediate nearly perpendicularly to the total angular momentum vector. These molecular elimination channels are associated with tight exit transition states as reflected in a significant electron rearrangement as visible from the chemical bonding in the light reaction products molecular hydrogen and silane. Once these hydrogenated silicon-carbide clusters are formed within the inner envelope of carbon stars such as of IRC + 10216, the stellar wind can drive both Si₂CH₄ and SiCH₂ to the outside sections of the envelope, where they can be photolyzed. This is of particular importance to unravel potential formation pathways to disilicon monocarbide (Si₂C) observed recently in the circumstellar shell of IRC + 10216.

Exotic Reaction Dynamics in the Gas-Phase Preparation of Anthracene (C14H10) via Spiroaromatic Radical Transients in the Indenyl-Cyclopentadienyl Radical-Radical Reaction

The gas-phase reaction between the 1-indenyl (C₉H₇^•) radical and the cyclopentadienyl (C₅H₅^•) radical has been investigated for the first time using synchrotron-based mass spectrometry coupled with a pyrolytic reactor. Soft photoionization with tunable vacuum ultraviolet photons afforded for the isomer-selective identification of the production of phenanthrene, anthracene, and benzofulvalene (C₁₄H₁₀). The classical theory prevalent in the literature proposing that radicals combine only at their specific radical centers is challenged by our discovery of an unusual reaction pathway that involves a barrierless combination of a resonantly stabilized hydrocarbon radical with an aromatic radical at the carbon atom adjacent to the traditional C1 radical center; this unconventional addition is followed by substantial isomerization into phenanthrene and anthracene via a category of exotic spiroaromatic intermediates. This result leads to a deeper understanding of the evolution of the cosmic carbon budget and provides new methodologies for the bottom-up synthesis of unique spiroaromatics that may be relevant for the synthesis of more complex aromatic carbon skeletons in deep space.

Gas-Phase Preparation of 1-Germavinylidene (H2CGe; X1A1), the Isovalent Counterpart of Vinylidene (H2CC; X1A1), via Non-adiabatic Dynamics through the Elementary Reaction of Ground State Atomic Carbon (C; 3P) with Germane (GeH4; X1A1)

1-Germavinylidene (H₂CGe; X¹A₁), the germanium analogue of vinylidene (H₂CC; X¹A₁), was prepared via a directed gas-phase synthesis through the bimolecular reaction of ground state atomic carbon (C; ³P) with germane (GeH₄; X¹A₁) under single-collision conditions. The reaction commences with the barrierless insertion of carbon into the Ge-H bond followed by intersystem crossing from the triplet to singlet surface and migration of atomic hydrogen to germylene (H₂GeCH₂), which predominantly decomposes via molecular hydrogen loss to 1-germavinylidene (H₂CGe; X¹A₁). Therefore, the replacement of a single carbon atom in the acetylene-vinylidene system by germanium critically impacts the chemical bonding, molecular structure, and thermodynamic stability of the carbene-type structures favoring 1-germavinylidene (H₂CGe) over germyne (HGeCH) by 160 kJ mol^-1. Hence, the carbon-germane system represents a benchmark in the exploration of the chemistries of main group 14 elements with germanium-bearing systems showing few similarities with the isovalent carbon system.

Ion-molecule reactions in the HBr+ + HCl (DCl) system: a combined experimental and theoretical study

Reactions in the system HBr⁺ + HCl (DCl) were investigated inside a guided ion-beam apparatus under single-collision conditions. In the HBr⁺ + HCl system, the proton transfer (PT_HCl) and charge transfer (CT) are observable. In the HBr⁺ + DCl system, proton transfer (PT_DCl) and deuterium abstraction (DA) are accessible. The cross sections for all reaction channels were measured as a function of the collision energy E_cm and of the rotational energy E_rot of the ion. The rotationally state-selective formation of the ionic species was realized by resonance-enhanced multiphoton ionization (REMPI). As expected, the PT-channels are exothermic, and the cross section decreases with increasing collision energy for both PT_HCl and PT_DCl. The cross section for DA also decreases with an increasing E_c.m.. In the case of a considerably endothermic CT-channel, the reaction efficiency increases with increasing collision energy but has an overall much smaller cross sections compared to PT and DA reactions. Both PT-reactions are hindered by ion rotation, whereas DA is independent of E_rot. The CT-channel shows a rotational enhancement near the thermochemical threshold. The experiment is complemented by theory, using ab initio molecular dynamics (AIMD, also known as direct dynamics) simulations and taking the rotational enhancement of HBr⁺ into account. The simulations show good agreement with the experimental results. The cross section of PT_HCl decreases with an increase of the rotational energy. Furthermore, the absolute cross sections are in the same order of magnitude. The CT channel shows no reactions in the simulation.

The Accuracy of Semi-Empirical Quantum Chemistry Methods on Soot Formation Simulation

Soot molecules are hazardous compounds threatening human health. Computational chemistry provides efficient tools for studying them. However, accurate quantum chemistry calculation is costly for the simulation of large-size soot molecules and high-throughput calculations. Semi-empirical (SE) quantum chemistry methods are optional choices for balancing computational costs. In this work, we validated the performances of several widely used SE methods in the description of soot formation. Our benchmark study focuses on, but is not limited to, the validation of the performances of SE methods on reactive and non-reactive MD trajectory calculations. We also examined the accuracy of SE methods of predicting soot precursor structures and energy profiles along intrinsic reaction coordinate(s) (IRC). Finally, we discussed the spin density predicted by SE methods. The SE methods validated include AM1, PM6, PM7, GFN2-xTB, DFTB2, with or without spin-polarization, and DFTB3. We found that the shape of MD trajectory profiles, the relative energy, and molecular structures predicted by SE methods are qualitatively correct. We suggest that SE methods can be used in massive reaction soot formation event sampling and primary reaction mechanism generation. Yet, they cannot be used to provide quantitatively accurate data, such as thermodynamic and reaction kinetics ones.

A crossed molecular beams and computational study on the unusual reactivity of banana bonds of cyclopropane (c-C3H6; ) through insertion by ground state carbon atoms (C(3Pj))

The mechanism and chemical dynamics of the reaction of ground electronic state atomic carbon C(³P_j) with cyclopropane c-C₃H₆ have been explored by combining crossed molecular beams experiments with electronic structure calculations of the pertinent triplet C₄H₆ potential energy surface and statistical computations of product branching ratios under single-collision conditions. The experimental findings suggest that the reaction proceeds via indirect scattering dynamics through triplet C₄H₆ reaction intermediate(s) leading to C₄H₅ product(s) plus atomic hydrogen via a tight exit transition state, with the overall reaction exoergicity evaluated as 231 ± 52 kJ mol^-1. The calculations indicate that C(³P_j) can easily insert into one of the three equivalent C-C 'banana' bonds of cyclopropane overcoming a low barrier of only 2 kJ mol^-1 following the formation of a van der Waals reactant complex stabilized by 15 kJ mol^-1. The carbon atom insertion into one of the six C-H bonds is also feasible via a slightly higher barrier of 5 kJ mol^-1. These results highlight an unusual reactivity of cyclopropane's banana C-C bonds, which behave more like unsaturated C-C bonds with a π-character than saturated σ C-C bonds, which are known to be generally unreactive toward the ground electronic state atomic carbon such as in ethane (C₂H₆). The statistical theory predicts the overall product branching ratios at the experimental collision energy as 50% for 1-butyn-4-yl, 33% for 1,3-butadien-2-yl, i-C₄H₅, and 11% for 1,3-butadien-1-yl, n-C₄H₅, with i-C₄H₅ (230 kJ mol^-1 below the reactants) favored by the C-C insertion providing the best match with the experimentally observed reaction exoergicity. The C(³P_j) + c-C₃H₆ reaction is predicted to be a source of C₄H₅ radicals under the conditions where its low entrance barriers can be overcome, such as in planetary atmospheres or in circumstellar envelopes but not in cold molecular clouds. Both i- and n-C₄H₅ can further react with acetylene eventually producing the first aromatic ring and hence, the reaction of the atomic carbon with c-C₃H₆ can be considered as an initial step toward the formation of benzene.

The Potential Energy Profile of the HBr+ + HCl Bimolecular Collision

Recently, the HBr⁺ + HCl bimolecular reaction has been exploited by guided ion beam studies to probe the effect of rotational excitations and collision energies on the dynamics of the ion-molecule reactions. The current manuscript employs high-level ab initio calculations and reports the potential energy of pathways leading to various products, including HBr + HCl⁺, H₂Cl⁺ + Br, H₂Br⁺ + Cl, and H₂ + BrCl⁺. The study shows that the intermediates involved in this reaction are connected by low-lying transition states, thus frequent isomerizations and diverse products are expected. Further, this manuscript screens the performance of 192 different combinations of computationally efficient methods and basis sets in order to identify the optimal quantum chemical method for further dynamics simulations.

Directed gas phase preparation of ethynylallene (H2CCCHCCH; X1A′) via the crossed molecular beam reaction of the methylidyne radical (CH; X2Π) with vinylacetylene (H2CCHCCH; X1A′)

The elementary reaction of the methylidyne radical with vinylacetylene leading to the predominant formation of ethynylallene and atomic hydrogen via indirect scattering dynamics.