FORMATION AND IDENTIFICATION OF IONIZED AND NEUTRAL CUMULENES, RN=C=C=C=NH, BY TANDEM MASS-SPECTROMETRY

Using a combination of mass spectrometric techniques, it is shown that 5-amino-4-cyanoisoxazole molecular ions (1+.), lose isocyanic acid HNC=O, not fulminic acid, HC=N --> 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 = CH3 or C6H5]. On flash-vacuum pyrolysis at 700-degrees-C, 1 also loses HN=C=O producing the cumulene 3a, which is in turn readily tautomerized into malononitrile via wall collisions.