Chiral Ions in the Gas Phase. 1. Intramolecular Racemization and Isomerization of O-Protonated (S)-trans-4-Hexen-3-ol

The acid-catalyzed racemization and regioisomerization of (S)-trans-4-hexen-3-ol (1S) has been investigated in gaseous CH4 and C3H8 at 720 Torr and in the 40−120 °C temperature range. The contribution to the racemization and isomerization products by free 1-methyl-3-ethylallyl cations, arising from unimolecular fragmentation of excited O-protonated (S)-trans-4-hexen-3-ol (IS), was evaluated by generating them from protonation of isomeric 2,4-hexadienes and by investigating their behavior toward H218O under the same experimental conditions. The rate constant of the gas phase racemization of IS (1.4−21.3 × 106 s-1) was found to exceed that of its isomerization (1.0−9.9 × 106 s-1) over the entire temperature range. The experimental results, combined with ab initio theoretical calculations on the model [C3H5+/H2O] system, are consistent with a gas phase intramolecular IS racemization and isomerization involving the intermediacy of structured ion−molecule complexes, wherein the H2O molecule is coplanarly coord...