El-Sayed's rules 44 predict different magnitude of SOCs for the two triplet states in C 2 H 4 O: the coupling between 3 A and 1 A is expected to be much larger than that between 3 A and 1 A or between 3 A and 1 A because rotation of the p(O) orbital is needed for significant coupling. The calculations yield two orders of magnitude difference between SOCC( 3 A , 1 A ) and SOCC( 3 A , 1 A ), whereas SOCC( 3 A , 1 A ) has an intermediate value. To rationalize these couplings, we use El-Sayed's rule and NTOs. Figs. and show the NTOs, norms of oneparticle transition density matrix, and SOCCs for several states in ethylene-derived diradical and for the lowest triplet and singlet states in various isomers and conformers of the propylenederived diradicals.