For the adducts derived from acetylene and propyne, the difference in isomer energetics is slightly larger, up to 3 kcal/mol. As discussed above, the electronic structure of these adducts is very different from ethylene-derived species and vertical singlet-triplet energy gaps are greater than 20 kcal/mol. There is a significant difference between the MECP heights in acetylenederived and propyne-derived intermediates, sometimes up to a factor of two. This result is also consistent with the experimental finding: the branching ratio indicates much smaller yield of the singlet products in C 2 H 2 O than in C 2 H 4 O. The difference between the branching ratios in the propyne+O and acetylene+O reactions can be explained by the MECP heights as well: smaller yields of singlet products in the acetylene+O reaction can be attributed to higher MECPs relative to propyne.