Skeletal and hydrogen rearrangements in the electron impact mass spectrum of propene

The mass spectrum of propene-2-¹³C] shows 81% retention of C-2 in the C₂H₃]⁺ fragment ion at 70 eV electron energy, decreasing to 75% C-2 retention at low electron energies. The mass spectra of propene-2-d₁, propene-1,1,3,3,3-d₅, propene-1,1,2-d₃ and propene-3,3,3-d₃ also have been examined at a resolution sufficient to resolve H₂-D doublets. The results at 70 eV electron energy show complete H/D randomization prior to fragmentation to form C₃(H, D)₅]⁺ but, in agreement with the ¹³C labelling data, incomplete H/D interchange prior to fragmentation to form C₂(H, D)₃]⁺. The results are interpreted in terms of a reversible isomerization of the propene molecular ion to a cyclopropane structure in competition with fragmentation.