Spin-orbit coupling effects on the zero field splitting of (π, π*) triplet states of conjugated enones

The optically detected paramagnetic resonance transitions of five cyclic conjugated enones have been observed in zero field. The $\mathrm{|E|/h\hspace{0.17em}c}$ parameters are small and nearly invariant for series, but the $\mathrm{|D|/\; h\hspace{0.17em}c}$ parameters range from 0.22 to 0.30 cm⁻¹. Simple theory suggests that both spin‐spin dipolar interactions and spin‐orbit interactions make significant contributions to the ZFS. The large variation in the ZFS of these enones is primarily due to the dependence of the spin‐orbit interaction on the energy gaps between the ³(π, π*) state and the ¹(n, π*) and ³(n, π*) states.