It is unclear if there are stable minima on the singlet PES of the diradicals derived from ethylene and propylene. Our attempts to optimize the singlet structure by CCSD was unsuccessful. EOM-SF-CCSD and EOM-DIP-CCSD optimizations yield structures that might correspond to a very shallow minimum; tiny displacements lead to hydrogen shifts toward aldehyde or keton products. Fig. shows potential energy profiles of the lowest electronic states of C 2 H 4 O and C 2 H 2 O along torsional motion of the CH 2 group. In C 2 H 4 O the triplet surface exhibits a rather small barrier (about 1 kcal/mol) for internal rotation of the CH 2 group. The rotation in the triplet C 2 H 4 O is nearly free because there is no additional CC bonding (CC bond length is close to the single bond in ethane). Due to the Pauli principle, the two unpaired electrons in the triplet state cannot occupy the same bonding MO, leading to non-bonding interactions between the 2 radical centers. The rotation leads to the crossing with the lowest singlet surface, giving an upper bound of the MECP energy of 0.05 eV. The singlet PESs show much larger variations along the torsional coordinate. Importantly, at large dihedral angles the two singlets are scrambled and one cannot correlate the individual states with the 1 A or 1 A states. That is, almost all configurations from Fig. are present in the respective wavefunctions. At the symmetric geometry, closed-shell configuration (1) from Fig. , which contributes to bonding between the radical centers, has a weight comparable to the open-shell determinants. As the ring at symmetric geometry closes, the energy drops and the weight of determinant (1) becomes dominant. We also note that the gap between the two triplets remains nearly constant (∼0.2 eV). The vertical state orderings at the selected geometries is given in SI.