The potential energy diagram (Fig. ) also provides an explanation for the relatively low yields of the other three labelings, especially (2,3)-HAA and (4,5)-HAA, for which no previous studies have provided corresponding reaction mechanisms. Although these isotopic pathways have similar overall barriers as (1,2)-HAA, the absolute barriers are much higher. Comparing the formation of (2,3)-HAA and (4,5)-HAA, the formation pathway of (2,3)-HAA requires an additional transformation from INI5 to INI6 (glyceraldehyde), which leads to both a higher overall barrier and more reaction steps. This comparison also illustrates that the absolute barrier can sometimes be misleading, since one exothermic reaction can reduce the absolute barriers of all following steps. The occurrence of competing reactions also affects the yield of each pathway. Based on the entire reaction network (Fig. ), one, two, and two relatively low barrier (with 2-14 kcal/mol reductions in ∆G † max ) reactions compete with the formation of (3,4)-HAA, (2,3)-HAA, and (4,5)-HAA, respectively (Fig. ). On the one hand, as for (3,4) HAA, the formation of a 5-membered ring (node 4-5 in Fig. ) has lower barrier than the formation of the intermediate INI3. However, this competing intermediate is less stable than the two HAA molecules produced throughout the degradation process (i.e. [3,4]-HAA and [5,6]-HAA), and has only one kinetic accessible reaction pathway, which is converting back to INI2. On the other hand, two competing reactions correspond to the formation of (2,3)-HAA and (4,5)-HAA are both kinetically and thermodynamically more favorable. Specifically, the formation of DHA has similar barrier compared to the formation of (4,5)-HAA, but with 6 kcal/mol reduction in ∆G r . Similarly, the formation of INI13 (which can be further converted into a more stable isomer, species 5-2 in Fig. ) exhibits 13.6 and 12.2 kcal/mol reductions in ∆G † and ∆G r , respectively, compared to the formation of (2,3)-HAA. The presence of these competing reaction pathways explains the experimental observation that (2,3)-HAA and (4,5)-HAA are only observed at higher temperatures (e.g. 500 • C, Table . 1).