Charge-transfer mass spectra of isomers. III. cis- and trans-2-butene

Abstract

Charge-transfer mass spectra of the two 2-butenes were obtained after ionization with slow positive ions in a tandem mass spectrometer of perpendicular type. The breakdown graph of cis-2-butene, i.e. the mass spectrum as a function of the energy absorbed by the molecule on ionization, was constructed. The fragmentation pathways, metastable ions and doubly-charged ions are discussed. The 2-butene isomers gave indistinguishable mass spectra with incident ions having single and well defined recombination energies. This agrees with the fact that the breakdown graphs are the same. However, significantly different mass spectra were obtained with incident COS +, Kr +, N 2 + and F + ions. This is qualitatively explained by differences in the photoelectron spectra of the isomers, if it is assumed that these spectra also reflect the variation of charge-transfer cross sections with energy. The result shows that it is possible to distinguish isomers by charge-transfer mass spectrometry, even if the molecular ions isomerize prior to fragmentation, by exploiting differences in the ionization probabilities. Proton and hydride ion transfers were observed with a number of slow positive ions.