Hindered inversion of chiral ion-dipole pairs

O-Protonated S-(-)-1-phenyl-1-methoxyethane (I-S) has been generated in the gas phase by (CH3)(2)- Cl+ methylation of S-(-)-1-phenylethanol (I-S). Det ailed information on the reorganization dynamics of the intimate ion-dipole pair (IIS), arising from I-S by C-O bond dissociation, is inferred from th e kinetic study of the intramolecular inversion of configuration of I-S vs its dissociation to alpha -methylbenzylcation (III) and CH3OH. The behavior of IIS in the gas phase is compared to that observed in aqueous solutions, where the loss of optical activity of Is is prevented by exchange of the l eaving (CH3H)-H-O with the solvent shell. Hindered inversion of I-S in solu tion is attributed to the operation of attractive interactions between the moving CH3OH moiety and the solvent cage which inhibit internal return in t he intimate ion-dipole pair ns. similar interactions do not operate in the solvolysis of O-18-labeled Is in aqueous acids, whose loss of optical activ ity efficiently competes with exchange of the leaving (H2O)-O-18 with the s olvent shell.