13C-Hyperfine Splitting in Semidiones

A number of ketyls and semidiones containing ¹³C have been examined by ESR spectroscopy. Grossly different values of $a_{\mathrm{CO}}^C$ in the ketyls (∼ 50 G) and the semidiones (∼ 1 G) allow for an easy differentiation of the two species. This is important because strained ring ketyls can undergo a decomposition reaction leading to the semidione with one additional carbonyl group added to the ketyl. Values of $a_{\mathrm{CO}}^C{\mathrm{,\; a}}_{\alpha }^C,\mathrm{and}{a}_{\beta }^C$ for keyls and semidiones are reporduced by the Karplus–Fraenkel equation if the term $Q_{\mathrm{CC\prime }}^C$ is allowed to increase by 15% when C′ is altered from methyl to t‐butyl. No evidence for the nonplanarity of di‐t‐butyl ketyl could be deduced. Some unusual long‐range ¹³C hfsc ${\mathrm{(a}}_{\gamma }^C)$ have been detected in certain bicyclic semidiones possessing a nearly coplanar, zigzag arrangement of three carbon–carbon bonds with a $p_z$ orbital.