The first question is the energy ordering of the relevant states and their structures. The triplet states can be well described by the CCSD method. Therefore, we used CCSD/cc-pVTZ for geometry optimizations. Fig. shows optimized structures of the lowest triplet state of C 2 H 4 O and C 3 H 5 O. The equilibrium structures are C 1 . For C 2 H 4 O, the structures optimized under C s constraint are very close to the C 1 one, both geometrically and energetically (these structures are shown in SI). In the propene-derived diradical, oxygen can attach to either terminal carbon or to the middle carbon atom, giving rise to nearly isoenergetic TC1 and TC2 isomers. For each structure, there are two conformers separated by relatively small barriers (about 1 kcal/mol). Fig. shows two lowest-energy structures of the C 2 H 2 O diradical ( 3 A ). Both structures are true minima. The barrier for rotation is about 7 kcal/mol.