Characterization of iminopropadienone ions and neutrals in a tandem mass spectrometer

Abstract

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...

Iminopropadienones, RN:C:C:C:O. Theory and experiment

“…The calculated stability of HNC3O is consistent with its experimental observation in the gas phase. 2 We are currently extending this fruitful interplay between theory and experiment to the characterization of bisimines RN=C=C=C=NR'.3…”

Characterization of New Cumulenes C2NX2 (X = O or S): Tandem Mass Spectrometry and ab Initio Studies

“…Moreover, cumulenes containing these atoms are of both theoretical and synthetic importance 1b. We have recently described a number of such systems, including SC n S ( n = 1−3), SCCS, RNCCO, RNCCS, RNCCCO, N 2 S, and the nitrile sulfides RCNS . We now report a new series of cumulenic cations, SNCC=X + and SCNCX + (X = O or S), formed by dissociative ionization of suitably substituted 1,2,5-thiadiazoles and other precursors.…”

Characterization of New Sulfide Ions (C2S3•+) from Ethenedithione by Ion−Molecule Reactions

“…The loss of CHN ( m / z 70) must be the result of a rearrangement process. Iminopropadiene thione ions have not yet been reported in the literature, while iminopropadienone ions are well-established species. ,

3 CA (linked scan E/B/E mode) spectrum of the m / z 97 ions produced in the ion−molecule reaction of C 2 S 3 •+ ions (from 3 ) with methyl isocyanide.

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“…Iminopropadiene thione ions have not yet been reported in the literature, while iminopropadienone ions are well-established species. 17,18 Summarizing the experimental results, dissociative ionization of the heterocyclic dithiolodithioles 1-3 generates mixtures of isomeric C 2 S 3 •+ radical cations ascribed as S-and C-sulfide ions derived from ethenedithione (5 and 4, respectively). These species cannot be differentiated with their collisional activation spectra in the high or the low kinetic energy regime nor with the neutralization-reionization methodology.…”