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Radical-cations of iminopropadienones (RN=C=C=C=O+') have been generated by dissociative ionization of isoxazole precursors and structurally characterized by collisional activation mass spectrometry; the corresponding neutral cumulenes have also been produced in a tandem mass spectrometer by neutralization and/or flashvacuum pyrolysis experiments.Dissociative ionization of heterocyclic compounds constitutes a virtually inexhaustible source of new ionic systems.' This is again exemplified in this report which describes the production from isoxazole derivatives of hitherto unreported heterocumulenes; ionized as well as neutral iminopropadienones, RN=C=C=C=O. Structural analysis of these new molecules has been performed using tandem mass spectrometry techniques,* uiz collisional activation (CA)3 and neutralization-reionization (NR) mass ~pectrometry.~ RESULTSThe electron ionization mass spectra of 3-phenylisoxazolo[5,4-d]pyrimidine-4(5H)ones la-d invariably feature a base peak at rnlz 143 (Table 1). This common behaviour together with high resolution mass measurement suggest the composition C,H,NO for this ion. This can be further specified as [C6H5, C3, N, 0 1 because the base peaks of the CA spectra of the massselected rnlz 143 ions correspond to the phenyl cation (mlz 77, see Table 2 and inset in Fig. l(a)). A competitive loss of CO from mlz 143 (giving mlz 115) indicates the interconnection of the atoms to be that of the iminopropadienone structure 3a". The other possible candidate, ionized phenylcyanoketene, 4+', can be excluded on the basis of the CA data. This ketene was produced directly in the ion source of the spectrometer by flash-vacuum pyrolysis (FVP) of ethyl phenylcyanoacetate C6HSCH(CN)CO2C2HS (5). Although 4 was not the major FVP product (losses of C2H4 and C 0 2 are favoured over the C2H50H loss at 800 "C '), the signal at rnlz 143 was sufficiently intense that its CA spectrum, which is very different from that of 3a+', could be recorded. In particular, a much weaker peak at rnlz 77 and a much stronger one at mlz 115 are observed for 4+' (Table 2). All the data thus indicate that the phenyliminopropadienone radical cation is a stable species in the gas phase in the mass spectrometer.Neutralization of a fast beam of these mlz 143 ions (3a+') with ammonia followed by reionization with oxygen in a separate collision cell afforded the NR mass spectrum shown in Fig. l(a). The observation of a strong recovery signal at rnlz 143 ('survivor ions') and the close similarity of the NR and C A mass spectra prove that neutral phenyliminopropadienone is also * Author to whom correspondence should be addressed. stable in the rarefied gas phase on the microsecond time scale of the mass spectrometer. One significant difference between the C A and NR spectra is observed at mlz 66, which corresponds to reionization of the openshell NCCCO radical formed by unimolecular dissociation in the neutralization step. This peculiar behaviour has also been noted and used in the characterization of analogous compounds, the carbo...

Using a combination of mass spectrometric techniques, it is shown that 5-amino-4-cyanoisoxazole molecular ions (I+'), lose isocyanic acid HNC=O, not fulminic acid, HCGN + 0. Metastable ion fragmentations (unimolecular and collision induced) and deuterium-labelling experiments are in agreement with the formation of a cumulenic structure, HN=C=C=C=NH (3a"). The hitherto unreported molecules HNCCCNH are also shown to be stable when formed in the low-pressure gas phase of the mass spectrometer by using the technique of neutralization-reionization mass spectrometry. The arguments developed for the characterization of 3a +' have also been extended to the methylated and phenylated analogues 3b+' and 3c+' [ RN=C=C=C=NH+', R = Cfi, or C,H,I. On flash-vacuum pyrolysis at 7OO0C, 1 also loses HN=C=O producing the cumulene 3a, which is in turn readily tautomerized into malononitrile via wall collisions.