The mass spectra of pyrimidines: 2(1H)-pyrimidinone and some N(1)-substituted derivatives

The EI induced fragmentation of 2-pyrimidinone (1) and several N(1)-substituted derivatives has been studied. Principal fragmentation pathways have been identified using 2H labelling, metastable defocussing, and exact mass measurements.The fragmentation of 1 parallels that of cytosine. The hydrogen atom at C(4), and not the tautomeric hydrogen, is involved in the formation of the prominent [M – H]+ peak in the spectrum of 1; subsequent fragmentations of the [M – H]+ moiety contribute significantly to the spectrum.Side chain eliminations predominate in the mass spectra of the N(1)-substituted derivatives, but peaks characteristic of the pyrimidinone nucleus are observable in every case. Alternative structures for the [M – H]+ entity formed by H• loss from the substituent are proposed for the N-phenyl and N-benzyl compounds.