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Abstract: Polycyclic aromatic hydrocarbons (PAHs) represent the link between resonance‐stabilized free radicals and carbonaceous nanoparticles generated in incomplete combustion processes and in circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars. Although these PAHs resemble building blocks of complex carbonaceous nanostructures, their fundamental formation mechanisms have remained elusive. By exploring these reaction mechanisms of the phenyl radical with biphenyl/naphthalene theoretically and experimentally, we provide compelling evidence on a novel phenyl‐addition/dehydrocyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene. PAC operates efficiently at high temperatures leading through rapid molecular mass growth processes to complex aromatic structures, which are difficult to synthesize by traditional pathways such as hydrogen‐abstraction/acetylene‐addition. The elucidation of the fundamental reactions leading to PAHs is necessary to facilitate an understanding of the origin and evolution of the molecular universe and of carbon in our galaxy.

The fundamental reaction pathways to the simplest dialkylsubstituted aromatics—xylenes (C6H4(CH3)2)—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, CH3CC) with 2‐methyl‐1,3‐butadiene (isoprene, C5H8). 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. This overall exoergic reaction affords a preparation of xylenes not only in high‐temperature environments such as in combustion flames and around circumstellar envelopes of carbon‐rich Asymptotic Giant Branch (AGB) stars, but also in low‐temperature cold molecular clouds (10 K) and in hydrocarbon‐rich atmospheres of planets and their moons such as Triton and Titan. Our study established a hitherto unknown gas‐phase route to xylenes and potentially more complex, disubstituted benzenes via a single collision event highlighting the significance of an alkyl‐substituted ethynyl‐mediated preparation of aromatic molecules in our Universe. [ABSTRACT FROM AUTHOR]

The simplest polycyclic aromatic hydrocarbon (PAH) carrying a five‐membered ring—9H‐fluorene (C13H10)—is produced isomer‐specifically in the gas phase by reacting benzyl (C7H7⋅) with phenyl (C6H5⋅) radicals in a pyrolytic reactor coupled with single photon ionization mass spectrometry. The unconventional mechanism of reaction is supported by theoretical calculations, which first produces diphenylmethane and unexpected 1‐(6‐methylenecyclohexa‐2,4‐dienyl)benzene intermediates (C13H12) accessed via addition of the phenyl radical to the ortho position of the benzyl radical. These findings offer convincing evidence for molecular mass growth processes defying conventional wisdom that radical‐radical reactions are initiated through recombination at their radical centers. The structure of 9H‐fluorene acts as a molecular building block for complex curved nanostructures like fullerenes and nanobowls providing fundamental insights into the hydrocarbon evolution in high temperature settings. [ABSTRACT FROM AUTHOR]

The simplest polycyclic aromatic hydrocarbon (PAH) carrying a five‐membered ring—9H‐fluorene (C13H10)—is produced isomer‐specifically in the gas phase by reacting benzyl (C7H7⋅) with phenyl (C6H5⋅) radicals in a pyrolytic reactor coupled with single photon ionization mass spectrometry. The unconventional mechanism of reaction is supported by theoretical calculations, which first produces diphenylmethane and unexpected 1‐(6‐methylenecyclohexa‐2,4‐dienyl)benzene intermediates (C13H12) accessed via addition of the phenyl radical to the ortho position of the benzyl radical. These findings offer convincing evidence for molecular mass growth processes defying conventional wisdom that radical‐radical reactions are initiated through recombination at their radical centers. The structure of 9H‐fluorene acts as a molecular building block for complex curved nanostructures like fullerenes and nanobowls providing fundamental insights into the hydrocarbon evolution in high temperature settings. [ABSTRACT FROM AUTHOR]

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The hitherto elusive disilavinylidene (H2SiSi) molecule, which is in equilibrium with the mono-bridged (Si(H)SiH) and di-bridged (Si(H2)Si) isomers, was initially formed in the gas-phase reaction of ground-state atomic silicon (Si) with silane (SiH4) under single-collision conditions in crossed molecular beam experiments. Combined with state-of-the-art electronic structure and statistical calculations, the reaction was found to involve an initial formation of a van der Waals complex in the entrance channel, a submerged barrier to insertion, intersystem crossing (ISC) from the triplet to the singlet manifold, and hydrogen migrations. These studies provide a rare glimpse of silicon chemistry on the molecular level and shed light on the remarkable non-adiabatic reaction dynamics of silicon, which are quite distinct from those of isovalent carbon systems, providing important insight that reveals an exotic silicon chemistry to form disilavinylidene. [ABSTRACT FROM AUTHOR]

Contemporary mechanisms for the spontaneous formation of glycerol have not been able to explain its existence on early Earth. The exogenous origin and delivery of organic molecules to early Earth presents an alternative route to their terrestrial in situ formation since biorelevant molecules like amino acids, carboxylic acids, and alkylphosphonic acids have been recovered from carbonaceous chondrites. Reported herein is the first in situ identification of glycerol, the key building block of all cellular membranes, formed by exposure of methanol-based - interstellar model ices to ionizing radiation in the form of energetic electrons. These results provide compelling evidence that the radiation-induced formation of glycerol in low-temperature interstellar model ices is facile. Synthesized on interstellar grains and eventually incorporated into the 'building material' of solar systems, biorelevant molecules such as glycerol could have been dispensed to habitable planets such as early Earth by comets and meteorites. [ABSTRACT FROM AUTHOR]

Dicarbon (C2), the simplest bare carbon molecule, is ubiquitous in the interstellar medium and in combustion flames. A gas-phase synthesis is presented of the benzyl radical (C6H5CH2) by the crossed molecular beam reaction of dicarbon, C2(X1Σg+, a3Πu), with 2-methyl-1,3-butadiene (isoprene; C5H8; X1A′) accessing the triplet and singlet C7H8 potential energy surfaces (PESs) under single collision conditions. The experimental data combined with ab initio and statistical calculations reveal the underlying reaction mechanism and chemical dynamics. On the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the barrierless addition of dicarbon to the carbon-carbon double bond of the 2-methyl-1,3-butadiene molecule. These initial addition complexes rearrange via multiple isomerization steps, leading eventually to the formation of C7H7 radical species through atomic hydrogen elimination. The benzyl radical (C6H5CH2), the thermodynamically most stable C7H7 isomer, is determined as the major product. [ABSTRACT FROM AUTHOR]

The cover page of the periodical "Angewandte Chemie" is presented.

Dicarbon (C2), the simplest bare carbon molecule, is ubiquitous in the interstellar medium and in combustion flames. A gas-phase synthesis is presented of the benzyl radical (C6H5CH2) by the crossed molecular beam reaction of dicarbon, C2(X1Σg+, a3Πu), with 2-methyl-1,3-butadiene (isoprene; C5H8; X1A′) accessing the triplet and singlet C7H8 potential energy surfaces (PESs) under single collision conditions. The experimental data combined with ab initio and statistical calculations reveal the underlying reaction mechanism and chemical dynamics. On the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the barrierless addition of dicarbon to the carbon-carbon double bond of the 2-methyl-1,3-butadiene molecule. These initial addition complexes rearrange via multiple isomerization steps, leading eventually to the formation of C7H7 radical species through atomic hydrogen elimination. The benzyl radical (C6H5CH2), the thermodynamically most stable C7H7 isomer, is determined as the major product. [ABSTRACT FROM AUTHOR]