Pinenes. The two pinene isomers are identical except for the position of one C-C double bond, but they have very different YSIs (207.1 for a-PN vs. 136.9 for b-PN). The measured YSI of a-PN is much larger than its predicted YSI (207.1 vs. 149.6), indicating a soot-enhancing NNNI, while the YSI of b-PN is well-predicted (136.9 vs. 141.8), indicating no NNNI. Theoretical calculations were performed to understand these differences. All the bond dissociation free energies (BDFE) of both molecules were obtained, which showed that opening of the four-membered ring requires less energy than the other dissociating events (SI I ). Potential energy surfaces of pathways initiated with the unimolecular ring opening were calculated with DFT. The pathways with the lowest activation barriers are shown in Fig. . Relative Gibbs free energy using different levels of theory and analysis of spin contamination are in SI J. Kinetically, low activation barriers (up to 32.9 kcal•mol -1 ) make both pathways favorable. Thermodynamically, the a-PN pathways are more favorable due to a more negative ΔG from the formation of more stabilized allyl-type radicals. a-PN forms α-Ar (toluene) as an aromatic species with a measured YSI of 170.9 while b-PN forms β-Int-V (5-methylene-1,3-cyclohexadiene) as a non-aromatic species that is likely less sooty than toluene. This difference qualitatively agrees with the YSI difference between a-PN and b-PN. Since more steps are needed for β-Int-V, the YSI difference between a-PN and b-PN is mainly affected by the identity of the products. SI-K show alternative pathways that compete with those in Fig. but are expected to be less important. A similar conclusion can be drawn that a-PN forms an aromatic species while b-PN forms a non-aromatic species. The YSI of a-PN (207.1) is between that of the product from this pathway (4-Isopropenyltoluene; 389.6), and the product from the main pathway (toluene; 170.9), which further validates these pathways.