The ESO Diffuse Interstellar Bands Large Exploration Survey: EDIBLES I. Project description, survey sample and quality assessment

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

The regions of the C₁₃H₁₁ potential energy surface (PES) related to the unimolecular isomerization and decomposition of the 1-methylbiphenylyl radical and accessed by the 1-/2-methylnaphthyl + C₂H₂ 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 9H-fluorene product. The 1-/2-methylnaphthyl + C₂H₂ reaction mechanism involves acetylene addition to the radical on the methylene group followed by a six- or five-membered ring closure and aromatization via an H atom loss. Despite of the complexity of the C₁₃H₁₁ PES, these straightforward pathways are dominant in the high-temperature regime (above ∼1000 K), with the prevailing products being phenalene, with a significant contribution of 1H-cyclopenta(a)naphthalene, for 1-methylnaphthyl + C₂H₂, and 1H-cyclopenta(b)naphthalene and 3H-cyclopenta(a)naphthalene, for 2-methylnaphthyl + C₂H₂. The methylnaphthyl reactions with acetylene represent a clean source of the three-ring PAHs, but they are relatively slow owing to the high entrance barriers of ∼10 kcal/mol, with the rate constants of about an order of magnitude lower as compared to those for naphthyl + allene and σ-aryl + C₂H₂. The 1-methylnaphthyl + C₂H₂ and 2-methylnaphthyl + C₂H₂ reactions represent prototypes for PAH growth by an extra six- and five-membered ring on a zigzag edge or a corner of PAH and the generated modified Arrhenius expressions are recommended for kinetic modeling of PAH expansion by the mechanism of acetylene addition to methylaryl radicals.

Low-temperature gas-phase formation of indene in the interstellar medium

Polycyclic aromatic hydrocarbons (PAHs) are fundamental molecular building blocks of fullerenes and carbonaceous nanostructures in the interstellar medium and in combustion systems. However, an understanding of the formation of aromatic molecules carrying five-membered rings-the essential building block of nonplanar PAHs-is still in its infancy. Exploiting crossed molecular beam experiments augmented by electronic structure calculations and astrochemical modeling, we reveal an unusual pathway leading to the formation of indene (C₉H₈)-the prototype aromatic molecule with a five-membered ring-via a barrierless bimolecular reaction involving the simplest organic radical-methylidyne (CH)-and styrene (C₆H₅C₂H₃) through the hitherto elusive methylidyne addition-cyclization-aromatization (MACA) mechanism. Through extensive structural reorganization of the carbon backbone, the incorporation of a five-membered ring may eventually lead to three-dimensional PAHs such as corannulene (C₂₀H₁₀) along with fullerenes (C₆₀, C₇₀), thus offering a new concept on the low-temperature chemistry of carbon in our galaxy.

Electronic spectra of ions of astrochemical interest: from fast overview spectra to high resolution

The combination of cryogenic ion traps with suitable light sources and standard tools of mass spectrometry has led to many innovative applications in previous years. This paper presents the combination of our versatile instrument with a supercontinuum laser for the rapid identification of ions that might be of special interest, e.g. as candidates for diffuse interstellar bands carriers. Using a linear wire quadrupole ion trap at 3 K, routine He-tagging, long irradiation times, and the brilliance and wide spectral range of a crystal fiber laser, mass selected ions have been exposed to spectral fluencies larger than 10 mJ (nm cm2)-1. These conditions result in an unsurpassed sensitivity, allowing us to find out within a few minutes and with nm accuracy, where photo absorption occurs with cross sections above 10-18 cm2. In this contribution, we present a variety of ions, probed between 420 and 720 nm. They have been generated by electron- or electrospray ionization of (polycyclic) aromatic hydrocarbons. For selected candidates, we recorded spectra with higher resolution and in the IR range. The anthracene dication has been selected to present a detailed analysis of our new results.

Near- and Mid-IR Gas-Phase Absorption Spectra of H2@C60+-He

Near- and mid-IR absorption spectra of endohedral H₂@C₆₀⁺ have been measured using He-tagging. The samples have been prepared using a "molecular surgery" synthetic approach and were ionized and spectroscopically characterized in the gas phase. In contrast to neutral C₆₀ and H₂@C₆₀, the corresponding He-tagged cationic species show distinct spectral differences. Shifts and line splittings in the near- and mid-IR regions indicate the influence of the caged hydrogen molecule on both the electronic ground and excited states. Possible relevance to astronomy is discussed.