Hydrogen Abstraction Acetylene Addition and Diels-Alder Mechanisms of PAH Formation:  A Detailed Study Using First...

Area of Science:

• Combustion Chemistry
• Theoretical Chemistry
• Chemical Kinetics

Background:

• Polycyclic Aromatic Hydrocarbons (PAHs) are crucial in combustion processes and atmospheric chemistry.
• Understanding PAH formation mechanisms is vital for controlling soot and emissions.
• The hydrogen abstraction acetylene addition (HACA) and Diels-Alder pathways are proposed routes for PAH growth.

Purpose of the Study:

• To investigate and compare the HACA and Diels-Alder mechanisms for naphthalene, acenaphthalene, phenanthrene, and pyrene formation.
• To calculate key energetic parameters and reaction rate constants for various reaction pathways.
• To determine the dominant mechanism for PAH growth under combustion conditions.

Main Methods:

• Extensive ab initio Gaussian-3-type calculations for potential energy surfaces (PES).
• Statistical theory calculations for reaction rate constants.
• Generation of barrier heights, reaction energies, and molecular parameters for reactants, products, intermediates, and transition states.

Main Results:

• HACA mechanism pathways exhibit low energy barriers and high rate constants under combustion conditions.
• Diels-Alder pathways have high energy barriers and low rate constants, rendering them non-competitive.
• Calculated parameters were compared with existing experimental and theoretical data.

Conclusions:

• The HACA mechanism is the predominant pathway for PAH formation in combustion environments.
• Diels-Alder reactions are insignificant contributors to PAH growth at high temperatures.
• The study provides crucial data for future comparative analyses of HACA mechanisms in PAH growth.