Mass spectrometry of heterocyclic compounds VIIIâelectronâimpactâinduced fragmentation of 1,2âdiphenylâpyrazolidineâ3,5âdione and some 4âsubstituted derivatives

The mass spectra of 1,2-diphenylâpyrazolidine-3,5-dione and twentyâone 4âsubstituted derivatives are reported. Their fragmentation patterns have been studied by deuterium labelling, exact mass measurements, metastable studies by the defocusing technique and low energy spectra. Hydrogen rearrangements from the 4âposition of the heterocycle and/or from the Ãâposition of the 4âsubstituent groups, lead to the main primary fragment ions [C12H11N2]+ (m/e 183) as shown by the metastables. The 4,4âd2 derivative shows an appreciable isotope effect even for molecular ions decomposing in the ion source. By comparison with the metastable abundances of competitive reactions, the molecular ions (m/e 252) of the 4âunsubstituted compound appear to be structurally different from the corresponding m/e 252 fragment ions formed from 4âderivatives by the loss of 4âsubstituent with H rearrangement. If only vinylic or aromatic hydrogen atoms are present, primary cleavage of the heterocyclic ring occurs with loss of OH·, C3O2 and C3HO2. Important rearrangements leading to elimination of C6H6N and C6H7N are typical for unsaturated substituents on position four having allylic hydrogen atoms. Fragment ions, identical to molecular ions of some compounds discussed here, are obtained by electronâimpact and/or thermal decompostion of some complex compounds containing more than one 1,2âdiphenylâpyrazolidineâ3,5âdione system. The [C6H5N2]+ (m/e 105) and [C6H5]+ (m/e 77) ions are common fragments of all the title compounds. Any hydrogen scrambling reactions between phenyl and heterocycle or 4âsubstituent groups can be excluded. Copyright © 1975 Heyden & Son Ltd