ENERGY BARRIERS FOR ISOMERIZATION OF GASEOUS C3H5+ IONS

It is shown from collisional activation studies on the C3H5+ion in the mass spectra of allyl and 2-propenyl bromides that low-energy, nondecomposing allyl and 2-propenyl cations do not interconvert. In contrast, similar studies on cyclopropyl and 1-propenyl bromides reveal that the C3H5+ions in the mass spectra of these compounds have the allyl structure. These results are consistent with a small or negligible energy barrier to rearrangement of gaseous 1-propenyl or cyclopropyl cations to allyl cation. Complementary conclusions may be deduced from a consideration of the kinetic energy release which accompanies Br-loss from the molecular ions of the C3H5Br isomers. Loss of Br-from ionized allyl or 2-propenyl bromides is accompanied by only a small release of kinetic energy; this is consistent with these reactions being simple bond cleavages occurring without rearrangement of the incipient C3H5+ion. Considerably more kinetic energy release is associated with elimination of Br-from ionized cyclopropyl or 1-propenyl bromides, thus suggesting that rate-determining rearrangement of the potential C3H5+cations to a more stable structure(s) occurs prior to dissociation. © 1979, American Chemical Society. All rights reserved.