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Cyclic 1,4-semidiones (radical anions of A2-1,4-diones) have been prepared in the cyclopentane, cyclohexane, and cycloheptane systems and detected by esr spectroscopy. Conformational preference and mobility have been detected in the 1,4-semidiones derived from A8x9-1,5-diketohydrindan and 1,6-diketodecalin systems. Examples of valence isomerization are reported in the preparation of 1,4-semidiones from eucarvone, A3-2-acetoxycaren-5-one, 2.3-benzocyclohept-4-enone, and pentacyclo[6.2.2.027.04.'0.0j.9]dodecane-3,6-dione.

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.

Bicyclo[2.1.1]hexane-and bicyclo[3.1.1 ]heptane-2,3-semidiones have been examined by electron spin resonance spectroscopy in dimethyl sulfoxide solution. The long-range hyperfine splitting by the anti hydrogen atoms of the methano bridges decreases from 10 G in the hexane to 3.5 G in the heptane. Bicyclo[4.1.1 ]octane-3,4semidione was prepared and studied between -80 and +60°. Ring inversion was fast at all temperatures. A ring-expansion reaction of bicyclo[2.1.1 ]hexane-2,3-diones to bicyclo[3.1.1]heptane-2,3-diones by reaction with the methylsulfinylcarbanion has been detected.