Gaseous Ion Activation Dynamics: the Role of the BUlk Gas in the Racemization of Chiral Oxonium Ions.

The kinetics of the inversion of configuration of a family of chiral oxonium ions, i.e., O-protonated 1-aryl-1-methoxyethanes [YMe+], were investigated in two different gaseous media (in CH3X with X = F and X = Cl) at 720 torr of pressure and in the temp. range: 25-140C. The activation parameters of the [YMe+] 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' crit. mode, active in the transition state (.omega.), and the discrete vibrational levels .nu. of the bulk gas. In CH3F, this vibrational-vibrational coupling switchover concerns the out-of-plane C-FH-O bending (the family) and the H3C-F stretching (the family) modes in the proton-bound [CH3FYMe+] complex. In CH3Cl, the coupling switchover concerns the out-of-plane C-ClH-O bending (the family) and the H3C-Cl Me group rocking (the family) modes in the proton-bound [CH3ClYMe+] complex. The [YMe+] activation dynamics also det. the inversion dynamics. The [YMe+]ret.dblarw.[YMe+]inv isomerization for the family involves the same "thermodynamically most favorable" transition state in both the CH3F and the CH3Cl media, whereas the same process for the family proceeds through different, dynamically favored transition states.