Low-temperature gas-phase formation of cyclopentadiene and its role in the formation of aromatics in the interstellar medium.

The cyclopentadiene (CH) molecule has emerged as a molecular building block of nonplanar polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanostructures such as corannulene (CH), nanobowls (CH), and fullerenes (C) in deep space. However, the underlying elementary gas-phase processes synthesizing cyclopentadiene from acyclic hydrocarbon precursors have remained elusive. Here, by merging crossed molecular beam experiments with rate coefficient calculations and comprehensive astrochemical modeling, we afford persuasive testimony on an unconventional low-temperature cyclization pathway to cyclopentadiene from acyclic precursors through the reaction of the simplest diatomic organic radical-methylidyne (CH)-with 1,3-butadiene (CH) representing main route to cyclopentadiene observed in TaurusMolecular Cloud.

Experimental and theoretical study of the Sn-O bond formation between atomic tin and molecular oxygen.

The merging of the electronic structure calculations and crossed beam experiments expose the reaction dynamics in the tin (Sn, P) - molecular oxygen (O, XΣ-g) system yielding tin monoxide (SnO, XΣ) along with ground state atomic oxygen O(P). The reaction can be initiated on the triplet and singlet surfaces addition of tin to the oxygen atom leading to linear, bent, and/or triangular reaction intermediates. On both the triplet and singlet surfaces, formation of the tin dioxide structure is required prior to unimolecular decomposition to SnO(XΣ) and O(P).

Elucidating the chemical dynamics of the elementary reactions of the 1-propynyl radical (CHCC; XA) with 2-methylpropene ((CH)CCH; XA).

Exploiting the crossed molecular beam technique, we studied the reaction of the 1-propynyl radical (CHCC; XA) with 2-methylpropene (isobutylene; (CH)CCH; XA) at a collision energy of 38 ± 3 kJ mol. The experimental results along with and statistical calculations revealed that the reaction has no entrance barrier and proceeds indirect scattering dynamics involving CH intermediates with lifetimes longer than their rotation period(s). The reaction is initiated by the addition of the 1-propynyl radical with its radical center to the π-electron density at the C1 and/or C2 position in 2-methylpropene.

One Collision-Two Substituents: Gas-Phase Preparation of Xylenes under Single-Collision Conditions.

The fundamental reaction pathways to the simplest dialkylsubstituted aromatics-xylenes (C H (CH ) )-in high-temperature combustion flames and in low-temperature extraterrestrial environments are still unknown, but critical to understand the chemistry and molecular mass growth processes in these extreme environments. Exploiting crossed molecular beam experiments augmented by state-of-the-art electronic structure and statistical calculations, this study uncovers a previously elusive, facile gas-phase synthesis of xylenes through an isomer-selective reaction of 1-propynyl (methylethynyl, CH CC) with 2-methyl-1,3-butadiene (isoprene, C H ). The reaction dynamics are driven by a barrierless addition of the radical to the diene moiety of 2-methyl-1,3-butadiene followed by extensive isomerization (hydrogen shifts, cyclization) prior to unimolecular decomposition accompanied by aromatization via atomic hydrogen loss.

Low-Temperature Gas-Phase Formation of Methanimine (CHNH; XA')─the Simplest Imine─under Single-Collision Conditions.

The D1-methanimine molecule (CHDNH; XA')─the simplest (deuterated) imine─has been prepared through the elementary reaction of the D1-methylidyne (CD; XΠ) with ammonia (NH; XA) under single collision conditions. As a highly reactive species with a carbon-nitrogen double bond and a key building block of biomolecules such as amino acids and nucleobases, methanimine is of particular significance in coupling the nitrogen and carbon chemistries in the interstellar medium and in hydrocarbon-rich atmospheres of planets and their moons. However, the underlying formation mechanisms of methanimine in these extreme environments are still elusive.

The Role of Methylaryl Radicals in the Growth of Polycyclic Aromatic Hydrocarbons: The Formation of Five-Membered Rings.

The regions of the CH potential energy surface (PES) related to the unimolecular isomerization and decomposition of the 1-methylbiphenylyl radical and accessed by the 1-/2-methylnaphthyl + CH reactions have been explored by ab initio G3(MP2,CC)//B3LYP/6-311G(d,p) calculations. The kinetics of these reactions relevant to the growth of polycyclic aromatic hydrocarbons (PAH) under high-temperature conditions in circumstellar envelopes and in combustion flames has been studied employing the RRKM-Master Equation approach. The unimolecular reaction of 1-methylbiphenylyl proceeding via a five-membered ring closure followed by H elimination is predicted to be very fast, on a submicrosecond scale above 1000 K and to result in the formation of an embedded five-membered ring in the 9-fluorene product.

The Reaction of o-Benzyne with Vinylacetylene: An Unexplored Way to Produce Naphthalene.

The mechanism and kinetics of the reaction of ortho-benzyne with vinylacetylene have been studied by ab initio and density functional CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the pertinent potential energy surface combined with Rice-Ramsperger-Kassel-Marcus - Master Equation calculations of reaction rate constants at various temperatures and pressures. Under prevailing combustion conditions, the reaction has been shown to predominantly proceed by the biradical acetylenic mechanism initiated by the addition of C H to one of the C atoms of the triple bond in ortho-benzyne by the acetylenic end, with a significant contribution of the concerted addition mechanism. Following the initial reaction steps, an extra six-membered ring is produced and the rearrangement of H atoms in this new ring leads to the formation of naphthalene, which can further dissociate to 1- or 2-naphthyl radicals.

Theoretical Study of the Phenoxy Radical Recombination with the O(P) Atom, Phenyl plus Molecular Oxygen Revisited.

Quantum chemical calculations of the CHO potential energy surface (PES) were carried out to study the mechanism of the phenoxy + O(P) and phenyl + O reactions. CASPT2(15e,13o)/CBS//CASSCF(15e,13o)/DZP multireference calculations were utilized to map out the minimum energy path for the entrance channels of the phenoxy + O(P) reaction. Stationary points on the CHO PES were explored at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311++G** level for the species with a single-reference character of the wave function and at the CASPT2(15e,13o)/CBS//B3LYP/6-311++G** level of theory for the species with a multireference character of the wave function.