Whereas tropone (2) and a cyclopropanone 5 show rather normal ketone signals in their 17O‐NMR spectra, the two cyclopropenones 1a, and lb exhibit exceptionally high shielding, indicating a structure closer to a O− formula than to a ketone. For comparison, an enolate and several phenolates have been measured. In order to test the ketonic character, the nucleophilic addition of water was determined by the rate of isotopic O‐exchange between ketone and water: 2 exchanged ca. 20 times slower than acetophenone, whereas la reacted very much more slowly.
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Dahn et al. (1987) studied this question.