Controlling soot and particulates production is one of the most stubborn problems in current combustion science. The molecular weight growth process that culminates in forming soot from aliphatic fuels hinges critically on the details of reactions of small unsaturated hydrocarbon radicals – especially resonance-stabilized radicals. First and second aromatic ring formation are particularly important steps in determining soot production. The iconic example is the reaction of two propargyl radicals, C3H3, to form isomers of C6H6. This reaction is one of the most important means for introducing the first aromatic ring, for the most stable C6H6 isomer is benzene. However, other C6H6 isomers are also formed in the reaction, and are more readily oxidized and more benign for particulate formation. Understanding the isomeric product formation in this and similar reactions as a function of pressure and temperature is crucially important for modeling the initial steps of molecular weight growth.