So why does isoprene give the highest yield among the acyclic dienes? The yields decrease both when a second methyl group is attached to a central C atom of 2 leading to 3 and when the methyl group of 2 is removed leading to 1. First we consider the relative E(T1) of 1 -3 (0.6, 0.0, and 0.3 kcal mol -1 , respectively) which do not reveal a gradual stabilization of the T1 state with increasing methyl substitution. In the most stable conformer of 3 2, the methyl group is positioned at one of the ends of the allyl radical (at the inner end). In 3 3, both methyl groups are bonded to the internal C atoms, and one of these groups will be attached to the C2 atom of the allyl moiety of 3 3. At this position the singly occupied molecular orbital (SOMO) has no coefficient, and this methyl group provides no additional stabilization when compared to 2. One should also compare the two isomers 2 and 4, and although E(T1) of 4 and 2 are the same, the methyl group in the most stable conformer of 3 4 sits at the alkyl radical moiety in contrast to 3 2 where it must sit at the allyl radical. Finally, when going to the dimethyl substituted 5 and 6, one can note lowered E(T1) for some of the isomers. However, for Z,Z-5