Electron impact mass spectrometry in a series of methyl-2-pyrimidones

The fragmentation under electron impact of various methyl-2-pyrimidones is investigated. The fragmentation is markedly influenced by the methyl substituent in the 4- or 5-position and the main fragmentation pathways of the molecular ion involve the loss of carbon monoxide or methyl radical. The latter fragmentation process shows that the methyl groups are more labile in the 4- and(or) 6-position than in the 5-position. This is corroborated by the study of trideuteriomethylated analogues. One can thus distinguish by mass spectrometry the different isomers of position, 4-methyl and 5-methyl or 4,5-dimethyl and 4,6-dimethyl-2-pyrimidones.