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The attention of investigators has been attracted in recent years to nitrogen derivatives of carbonyl compounds-which, one would think, had already been studied thoroughly-and also to 1,3-dicarbonyl compounds (oximes, substituted hydrazones, semicarbazones, thiosemicarbazones), for which a tendency has been found not only toward keto-enol (A-C) or imine-enamine (A-B) tautomerism, but also to ring-chain tautomerism (A-D) [1-4] (Scheme 1). However, these studies were performed in solutions, where, it had been shown, the predominance of one tautomeric form or the other is highly dependent on the polarity of the solvent. In the gas phase, with no solvent present, mass-spectrometric studies have identified both cyclic and linear tautomeric forms [5][6][7][8]; in those studies, the reliability of interpretation of ion characteristics was not always adequate.

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