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

The mass spectrum of propene‐2‐[ 13 C] shows 81% retention of C‐2 in the [C 2 H 3 ] + fragment ion at 70 eV electron energy, decreasing to 75% C‐2 retention at low electron energies. The mass spectra of propene‐2‐ d 1 , propene‐1,1,3,3,3‐ d 5 , propene‐1,1,2‐ d 3 and propene‐3,3,3‐ d 3 also have been examined at a resolution sufficient to resolve H 2 ‐D doublets. The results at 70 eV electron energy show complete H/D randomization prior to fragmentation to form [C 3 (H, D) 5 ] + but, in agreement with the 13 C labelling data, incomplete H/D interchange prior to fragmentation to form [C 2 (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.