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A number of ketyls and semidiones containing 13C have been examined by ESR spectroscopy. Grossly different values of aCOC 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 aCOC, aαC, and aβC for keyls and semidiones are reporduced by the Karplus–Fraenkel equation if the term QCC′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 (aγC) have been detected in certain bicyclic semidiones possessing a nearly coplanar, zigzag arrangement of three carbon–carbon bonds with a pz orbital.

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