Facile pentagon formation in the dissociation of polyaromatics

Abstract

Energetic processing of gaseous polycyclic aromatic hydrocarbons (PAHs) plays a pivotal role in the chemistries of inter- and circumstellar environments, certain planetary atmospheres, and also in the chemistry of combustion and soot formation. Although the precursor PAH species have been extensively characterized, the products from these gaseous breakdown reactions have received far less attention. It has been particularly challenging to accurately determine their molecular structure in gas-phase experiments, where comparisons against theoretical modeling are best made. Here we report on a combined experimental and theoretical study of the dissociative ionization of two nitrogen containing polycyclic aromatic hydrocarbons of CHN composition, acridine and phenanthridine. The structures of HCN-loss fragments are resolved by infrared multiple-photon dissociation (IRMPD) spectroscopy of the mass-isolated products in an ion trap mass spectrometer. Quantum-chemical computations as well as reference IRMPD spectra are employed to unambiguously identify the molecular structures. Furthermore, computations at the density functional level of theory provide insight into chemical pathways leading to the observed products. Acenaphthylene˙ and benzopentalene˙ - two aromatic species containing pentagons - are identified as the main products, suggesting that such species are easily formed and may be abundant in regions where thermal or photoprocessing of polyaromatics occurs.

Spectroscopic Characterization of the Product Ions Formed by Electron Ionization of Adamantane

“…[36][37][38] Briefly, a radio frequency (RF, 1 MHz) Paul-type QIT is mounted inside a vacuum chamber at a pressure of < 10 À7 mbar. [36][37][38] Briefly, a radio frequency (RF, 1 MHz) Paul-type QIT is mounted inside a vacuum chamber at a pressure of < 10 À7 mbar.…”

“…Experiments were carried out on a quadrupole ion trap time-offlight (QIT-TOF) mass spectrometer coupled to the beamline of the IR free electron laser FELIX as described in detail elsewhere. [36][37][38] Briefly, a radio frequency (RF, 1 MHz) Paul-type QIT is mounted inside a vacuum chamber at a pressure of < 10 À7 mbar. Helium is supplied to the ion trap raising the pressure to 5 10 À5 mbar to thermalize and focus the ions to the center of the trap, improving the mass resolution of the mass spectrometer.…”

“…[36][37] Here, in absence of an intermediate electronic state in adamantane, we employ electron ionization to generate the ion and its fragments, which are subsequently mass isolated and interrogated by IR action spectroscopy. This methodology was successfully applied to structurally identify the product ions in ion chemistry studies, including those from resonance-enhanced two-photon (193 nm) dissociative ionization of (nitrogen-containing) aromatic hydrocarbons.…”

“…This methodology was successfully applied to structurally identify the product ions in ion chemistry studies, including those from resonance-enhanced two-photon (193 nm) dissociative ionization of (nitrogen-containing) aromatic hydrocarbons. [36][37] Here, in absence of an intermediate electronic state in adamantane, we employ electron ionization to generate the ion and its fragments, which are subsequently mass isolated and interrogated by IR action spectroscopy.…”

Fragmentation and isomerization of polycyclic aromatic hydrocarbons in the interstellar medium: Coronene as a case study

“…These processes are similar to the formation of pentagons in Bq + . Recent work has also shown that the loss of a HCN molecule in nitrogen-containing PAHs offers a facile pathway towards pentagon formation (de Haas et al 2017…”