Myrcene. MC is identical to b-OM except for the position of one C-C double bond, but it has a much lower YSI (103.9 vs. 213.6). Its measured YSI is reasonably predicted by the GCM (103.9 vs. 84.2), which suggests that it does not experience any NNNIs. Structurally it is a substituted 1,5-hexadiene with three C-C single bonds sandwiched between two C-C double bonds (see the left side of Fig. ). The weakest bond is the central C-C bond (BDE = 62.8 kcal/mol ) since its fission produces two allylic radicals. These radicals only contain five carbon atoms, so they must undergo addition reactions with other species to form benzenoid aromatics, which causes soot production to be slower than in b-OM and other terpenes. This hypothesis is also consistent with our hexadiene study , where 1,5-hexadiene produced relatively low concentrations of aromatics but high concentrations of C3 species. It is also consistent with the flame-flame molecular-beam MS (MBMS) study of Bierkandt et al. , where MC produced relatively large amounts of C5 species.