The triplet PES of the acetylene-derived diradical (Fig. , bottom) is very different: the barrier is an order of magnitude higher than in C 2 H 4 O. This difference can be explained by the orbital pattern: the π-subsystem of the triplet state resembles that of the allyl radical, giving rise to an increased CC bond order (which is also manifested by a shorter CC distance). The gap between the triplet and the lowest singlet is also larger than in C 2 H 4 O (see SI for the state orderings at selected geometries). The character of the singlet state depends on geometry: near the triplet 3 A minimum this state has an open-shell 1 A -like character, but in the structures with rotated CH, it acquires a closed-shell character. The scans (Fig. ) suggest a higher-lying singlet-triplet MECP, which is confirmed by the calculations.