The lowest-energy pathway in the α-peroxy network, shown in Figure involves a two-step HO 2 elimination of the peroxy radical. The first step, called αAdductFromRO2, forms a hydrogen bonded complex, called αadduct. This has an activation energy lower in energy than the separated products. The second step, αAdductSplit, involves breaking the hydrogen bond to form isobutanal and HO 2 . This barrierless reaction was included in the master equation by using an ILT of the collision-limited rate of the products: HO 2 and isobutanal. The TS for αAdductFromRO2 is a concerted reaction with the SOMO perpendicular to the COO plane; the analogous reaction of other α-hydroxy peroxy radicals has been discussed previously. The α-peroxy network also includes three intramolecular hydrogen abstraction reactions from the peroxy radical to form alkyl or alkoxy radicals. The lowest-energy direct hydrogen abstraction involves the formation of the alkoxy product, αQOOH [O]. Unlike the formation of αQOOHβ and αQOOHγ, this does not require the breaking of a hydrogen bond, allowing for a lower barrier despite a higher ring strain and larger bond dissociation energy. For the other hydrogen abstraction reactions, the reaction with a six-membered transition state forming αQOOHγ has a lower barrier than the one with a five-membered transition state forming αQOOHβ, due differences in ring strain.