NON-CONCERTED UNIMOLECULAR REACTIONS OF IONS IN THE GAS-PHASE - IMPORTANCE OF ION-DIPOLE INTERACTIONS IN CARBONIUM-ION ISOMERIZATIONS

The behaviour of six members of a homologous series of oxonium ions, C3H7O+CR1R2 (R1, R2 = H or CH3) are discussed. For the two lowest members (R1 = R2 = H) isomerization of the propyl side-chain from n- to iso-structure is possible at energies lower than those required to promote decomposition of either ion. For higher members, rearrangement of the propyl side-chain from n- to iso-structure constitutes the rate-determining step in the dissociation of the ion in question. Extension of the analysis, including a consideration of plausible reactions which are found not to occur, permits estimates to be made for the heats of formation of the intermediates required for rearrangement of the propyl side-chain of C3H7O+CR1R2. It is suggested that these intermediates are stabilized by an ion-dipole interaction between the incipient propyl cation and the potential carbonyl compound, OCR1R2. The magnitude of this stabilization appears to lie in the range 15-25 kcal mol-1. © 1979.