Hydrogen ejection from hydrocarbons: Characterization and relevance in soot formation and interstellar chemistry

Area of Science:

• Chemistry
• Astrochemistry
• Combustion Science

Background:

• Polycyclic aromatic hydrocarbons (PAHs) are crucial in combustion, pyrolysis, and interstellar chemistry.
• Understanding hydrogen loss mechanisms in radical PAHs is vital for reaction pathways and particle formation.
• Direct C─H bond fission and bimolecular radical abstraction are the primary proposed pathways for hydrogen loss.

Purpose of the Study:

• To computationally investigate the role of hydrogen ejection in radical-centric hydrocarbon growth and particle formation.
• To characterize the preferred pathways for hydrogen loss in polycyclic aromatic hydrocarbon radicals.
• To assess the significance of microcanonical rates in astrochemical processes.

Main Methods:

• Electronic structure calculations were employed to determine C─H bond strengths in PAH radicals.
• Thermal rates for hydrogen ejection and H-abstraction were calculated and compared at temperatures above 1200 K.
• Microcanonical rates were computed, considering internal energy release from bond formation.

Main Results:

• C─H bond strengths in some C9 and C13 PAH radicals are found to be weaker than 30 kcal/mol.
• At temperatures > 1200 K, hydrogen ejection from weak C─H bonds on PAH radicals exhibits significantly higher rates than H-abstraction.
• Microcanonical rates, accounting for energy release from bond formation, are substantially higher than fully thermalized rates.

Conclusions:

• Direct hydrogen ejection is a dominant pathway for hydrogen loss in PAH radicals at high temperatures, influencing soot inception in combustion.
• Microcanonical rate considerations are essential for accurately modeling hydrocarbon growth in interstellar environments.
• The findings provide critical insights into PAH reactivity in both terrestrial and extraterrestrial chemical processes.