Gas-phase activation and reaction dynamics of chiral ion - Dipole complexes

A family of enantiomerically pure oxonium ions, that is O-protonated 1-aryl-1-methoxyethanes, has been generated in the gas phase by the (CH3)2Cl+ methylation of the corresponding 1-arylethanols. Some information on their reaction dynamics was obtained from a detailed kinetic study of their inversion of configuration and dissocn. The activation parameters of the inversion reaction are found to obey two different isokinetic relationships depending upon the nature and the position of the substituents in the oxonium ions. In contrast, the activation parameters of the dissocn. reaction obey a single isokinetic relationship. The inversion and dissocn. rate consts. do not follow simple linear free-energy relationships. This complicated kinetic picture has been rationalized in terms of different activation dynamics in gaseous CH3Cl, which, in turn, det. the reaction dynamics of the oxonium ion. When the predominant activation of the oxonium ion involves resonant energy exchange from the 1015 cm-1 CH3 rocking mode of unperturbed CH3Cl, the inversion reaction proceeds through the dynamically most favored TS, characterized by the unassisted C.alpha.-O bond elongation. When, instead, the activation of the oxonium ions requires the formation of an intimate encounter complex with CH3Cl, the inversion reaction takes place via the energetically most favored TS, characterized by multiple coordination of the CH3OH moiety with the H.alpha. and Horiho atoms of the benzylic residue. The activation dynamics operating in the intimate encounter complex with CH3Cl is also responsible for the dissocn. of most selected oxonium ions.