and on the activation and reaction energies of the step that leads to the dimer radical pair intermediate. In their T1 states, the acyclic conjugated dienes can be described as triplet radical pairs composed of two non-interacting radicals oriented perpendicularly to each other; one alkyl radical and one allyl radical (Fig. and Fig. ). The product of the first reaction step, on the other hand, can be described as a bis(allyl) radical pair dimer with triplet multiplicity (Fig. ), a short-lived intermediate that undergoes intersystem crossing to the singlet radical pair and then closes to the observed dimers. The methyl substituents have two effects; they hyperconjugatively stabilize the alkyl and allyl radicals that develop in T1, however the more methyl substituents the more extensive the steric congestion at the transition state that leads to the dimer radical-pair intermediate. For the triplet state dienes and the dimer radical pair intermediates at their least congested (most stable) conformers, the methyl substituents only provide hyperconjugative stabilization. The steric effects, on the other hand, play a role at the transition states. The first three acyclic dienes (1 -3) adopt lowest energy conformers described as one primary alkyl radical and one allyl radical, whereas 4 -6 adopt triplet state conformers described as one secondary alkyl radical and one allyl radical.