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Gas‐Phase Activation and Reaction Dynamics of Chiral Ion–Dipole Complexes
Author(s) -
Filippi Antonello,
Speranza Maurizio
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200304850
Subject(s) - oxonium ion , chemistry , ion , activation energy , dissociation (chemistry) , reaction dynamics , protonation , kinetic energy , reaction coordinate , photochemistry , computational chemistry , crystallography , molecule , organic chemistry , physics , quantum mechanics
A family of enantiomerically pure oxonium ions, that is O‐protonated 1‐aryl‐1‐methoxyethanes, has been generated in the gas phase by the (CH 3 ) 2 Cl + 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 dissociation. 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 dissociation reaction obey a single isokinetic relationship. The inversion and dissociation rate constants do not follow simple linear free‐energy relationships. This complicated kinetic picture has been rationalized in terms of different activation dynamics in gaseous CH 3 Cl, which, in turn, determine the reaction dynamics of the oxonium ion. When the predominant activation of the oxonium ion involves resonant energy exchange from the 1015 cm −1 CH 3 rocking mode of unperturbed CH 3 Cl, the inversion reaction proceeds through the dynamically most favored TS, characterized by the unassisted C α O bond elongation. When, instead, the activation of the oxonium ions requires the formation of an intimate encounter complex with CH 3 Cl, the inversion reaction takes place via the energetically most favored TS, characterized by multiple coordination of the CH 3 OH moiety with the H α and H ortho atoms of the benzylic residue. The activation dynamics operating in the intimate encounter complex with CH 3 Cl is also responsible for the dissociation of most selected oxonium ions.