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Gaseous Ion Activation Dynamics: The Role of the Bulk Gas in the Racemization of Chiral Oxonium Ions
Author(s) -
Filippi Antonello,
Speranza Maurizio
Publication year - 2004
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200400249
Subject(s) - oxonium ion , chemistry , ion , crystallography , organic chemistry
The kinetics of the inversion of configuration of a family of chiral oxonium ions, that is, O‐protonated 1‐aryl‐1‐methoxyethanes [Y Me + ], were investigated in two different gaseous media (in CH 3 X with X=F and X=Cl) at 720 torr of pressure and in the temperature range: 25–140 °C. The activation parameters of the [Y Me + ] inversion reaction were found to obey two different isokinetic relationships (IKR), depending on the nature and the position of the substituents in the oxonium ions and on the nature of the bulk gas employed. The observation of two IKR for the same family of reactions was related to a switchover in the resonant vibrational energy exchange between the reactants' critical mode, active in the transition state (ω), and the discrete vibrational levels ν of the bulk gas. In CH 3 F, this vibrational–vibrational coupling switchover concerns the out‐of‐plane CF⋅⋅⋅HO bending (the Φ family) and the H 3 CF stretching (the Γ family) modes in the proton‐bound [CH 3 F⋅Y Me+ ] complex. In CH 3 Cl, the coupling switchover concerns the out‐of‐plane CCl⋅⋅⋅HO bending (the Φ family) and the H 3 CCl methyl group rocking (the Γ family) modes in the proton‐bound [CH 3 Cl⋅Y Me+ ] complex. The [Y Me + ] activation dynamics also determine the inversion dynamics. The [Y Me + ] ret ⇄[Y Me + ] inv isomerization for the Φ family involves the same “thermodynamically most favorable” transition state in both the CH 3 F and the CH 3 Cl media, whereas the same process for the Γ family proceeds through different, dynamically favored transition states.