Radical Stereochemistry: Accounting for Diastereomers in Kinetic Mechanism Development

Area of Science:

• Chemical kinetics
• Atmospheric chemistry
• Computational chemistry

Background:

• Accurate modeling of reacting flows in combustion and atmospheric chemistry requires distinguishing between diastereomers.
• Existing kinetic mechanisms often lack the capability to resolve stereoisomers, leading to potential inaccuracies.

Purpose of the Study:

• To present an open-source framework for incorporating stereoisomer resolution into kinetic mechanisms.
• To develop algorithms for labeling, enumerating, and describing stereoisomers and transition states.
• To introduce a novel chemical identifier (AMChI) for stereoisomers and transition states.

Main Methods:

• Developed definitions and algorithms for stereoisomer labeling and enumeration.
• Generalized stereoisomer description to transition states (TS), accounting for fleeting stereochemistry.
• Introduced the AutoMech Chemical Identifier (AMChI) for stereoisomers and reaction TSs.
• Derived an analytic formula to remove redundancy in stereoresolved mechanisms with racemic mixtures.

Main Results:

• Applied methodology to NUIGMech1.1 subsets, reducing extra species in large-fuel oxidation from 2231 to 694.
• Found that over a quarter of species in expanded pyrolysis mechanisms require AMChI for proper identification.
• Observed that approximately 25% of large-fuel oxidation reactions and 33% of pyrolysis reactions involve fleeting TS stereochemistry.

Conclusions:

• The developed framework effectively reduces mechanism size and improves species identification.
• The AMChI identifier is crucial for accurately describing complex chemical species and reactions, especially those with stereochemistry.
• Fleeting TS stereochemistry is prevalent and may significantly impact reaction kinetics, necessitating its inclusion in detailed chemical models.