Carvacrol. CVC is a benzenoid aromatic with a hydroxy group attached to the ring. Its measured YSI (193.1) is much lower than its GCM-predicted YSI (253.3), which suggests it undergoes a NNNI that suppresses soot formation. We have observed similar soot suppression for other alkylphenols, e.g., [47]. The likely mechanism is that C-H fission in the hydroxy group produces resonantly-stabilized phenoxy-like radicals that interfere with growth to multi-ring aromatics. β-Ocimene. b-OM is a non-cyclic species but it has one of the largest YSIs measured in this study. Its measured YSI (213.6) is much larger than its GCM-predicted YSI (89.6), which suggests it undergoes a NNNI that enhances soot formation. Structurally, it is a substituted 1,4-pentadiene which has two C-C single bonds sandwiched between C-C double bonds (see the right side of Fig. ). The C-H bonds on the central carbon are extremely weak (BDE = 69.2 kcal/mol ) since their fission produces a "triple-allyl" radical with four resonance structures. This highly-conjugated C10 radical can likely cyclize directly to substituted aromatic hydrocarbons, which provides the soot-enhancing NNNI. Consistent with this hypothesis, we observed larger aromatics production from 1,4-hexadiene than other hexadienes in an earlier mass spectrometry (MS) study of doped flames. Furthermore, we observed very little formation of C3 and C4 products from 1,4-hexadiene.