Radical-Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

Area of Science:

• Combustion Chemistry
• Chemical Kinetics
• Polycyclic Aromatic Hydrocarbons (PAHs)

Background:

• Hydrocarbon ring growth mechanisms in sooting environments are not fully understood.
• Radical-radical reactions are crucial for forming larger molecules in combustion.
• The phenyl (C6H5) + propargyl (H2CCCH) reaction serves as a model for ring-growth pathways.

Purpose of the Study:

• To experimentally investigate the phenyl + propargyl radical reaction mechanism.
• To determine isomer-resolved product branching fractions for C9H8 formation.
• To compare experimental findings with theoretical kinetics predictions.

Main Methods:

• Time-resolved multiplexed photoionization mass spectrometry (TRM-PIMS) from 300-1000 K and 4-10 Torr.
• Ab initio transition state theory-based master equation calculations.
• Variable reaction coordinate transition state theory (VRC-TST) for barrierless channels.

Main Results:

• Detected C9H8 and C9H7 + H product channels.
• Observed direct adducts at 300 K and increased indene formation at 1000 K.
• Experimental indene yields exceeded theoretical predictions, suggesting secondary reactions.

Conclusions:

• The phenyl + propargyl reaction directly or indirectly forms indene, a key PAH.
• H-atom reactions (recombination and isomerization) significantly impact product distribution at low pressures.
• Understanding these pathways is vital for modeling soot formation and PAH growth.