Reactive intermediates play a key role in a great many metabolic pathways and organic reactions.
Therefore, information regarding their intrinsic stability and reactivity is of considerable importance. Despite the enormous progress made in the study of such elusive species in condensed phases, there are often advantages to resort to the rarefied gas phase of the mass spectrometer where intermolecular reactions are virtually absent. One of the advantages of NR mass spectrometry is that the neutral species are accessed by a Franck-Condon type electron transfer to their ionic counterparts, which may be available through the dissociative ionization of a carefully chosen precursor molecule. However, in order to unambiguously establish the neutral's identity in a NR experiment, it may be necessary to demonstrate that the initial ion beam is free from interfering ions and that the incipient neutral does not isomerize. A case in point are the various ylide and carbene type hydrogen shift isomers of the five membered species thiazole and imidazole as well as the six membered species pyridine, pyrazine and pyrimidine.