Rate constants for H abstraction from benzo(a)pyrene and chrysene: a theoretical study

Area of Science:

• Combustion Chemistry
• Atmospheric Chemistry
• Computational Chemistry

Background:

• Polycyclic Aromatic Hydrocarbons (PAHs) are significant components in combustion processes and atmospheric aerosols.
• Understanding the reactivity of PAHs with radicals is crucial for modeling combustion kinetics and PAH degradation pathways.
• Previous studies have explored PAH reactions, but detailed comparisons of radical abstractor efficiency and site-specific reactivity are needed.

Purpose of the Study:

• To computationally determine the thermal rate constants for direct H abstraction reactions from chrysene and benzo[a]pyrene by key combustion radicals.
• To compare the reactivity of different radicals (H, CH3, C3H3, OH) and identify the most efficient abstractor.
• To investigate the influence of different H-abstraction sites (armchair vs. zigzag edges) on reaction rates and energetics.

Main Methods:

• Density functional B3LYP/6-31G(d) and ab initio G3(MP2,CC) calculations were employed.
• Transition State Theory was used to calculate thermal rate constants.
• Energetics and kinetic parameters for H abstraction reactions were analyzed.

Main Results:

• Hydroxyl (OH) radicals exhibit the lowest activation barriers and are the fastest H abstractors from PAHs.
• Zigzag sites on PAHs show significantly higher H abstraction rate constants compared to armchair sites, driven by favorable entropic factors.
• Propargyl (C3H3) radicals are inefficient H abstractors due to high barriers and unfavorable equilibrium.

Conclusions:

• OH radical is the dominant species for initiating H abstraction from PAHs in combustion environments.
• The reactivity of PAHs is site-specific, with zigzag positions being more susceptible to radical attack.
• Generated rate expressions can improve the accuracy of current combustion kinetic mechanisms.