Hindered Inversion of Chiral Ion-Dipole Pairs

O-Protonated S-(-)-1-phenyl-1-methoxyethane (IS) has been generated in the gas phase by (CH3)2Cl+ methylation of S-(-)-1-phenylethanol (1S). Detailed information on the reorganization dynamics of the intimate ion-dipole pair (IIS), arising from IS by C-O bond dissocn., is inferred from the kinetic study of the intramol. inversion of configuration of IS vs its dissocn. to .alpha.-methylbenzyl cation and CH3OH. The behavior of IIS in the gas phase is compared to that obsd. in aq. solns., where the loss of optical activity of IS is prevented by exchange of the leaving CH3OH with the solvent shell. Hindered inversion of IS in soln. is attributed to the operation of attractive interactions between the moving CH3OH moiety and the solvent cage which inhibit internal return in the intimate ion-dipole pair IIS. Similar interactions do not operate in the solvolysis of 18O-labeled 1S in aq. acids, whose loss of optical activity efficiently competes with exchange of the leaving H218O with the solvent shell.