Studies in organic mass spectrometry‐XIII: Investigation of electron‐induced isomerisation of αβ‐ and βγ‐unsaturated esters

The mass spectral fragmentation of methyl esters of α,β‐ and β,γ‐unsaturated carboxylic acids has been studied. Deuterium labelling, metastable ion analysis and high resolution mass spectrometry have been utilised to elucidate the mechanism of a number of fragmentations and to check possible double bond migration prior to fragmentation. Some breakdown modes were found to occur through double bond migration. Additional support was provided by application of the ‘Metastable Ion Characteristics’ method. Partial isomerisation of molecular ions must therefore be accepted.