Reaction dynamics on barrierless reaction surfaces: a model for isoergic gas-phase proton-transfer reactions

The kinetics of some gas-phase alcohol/alkoxide proton-transfer reactions is slower than predicted by simple Rice–Ramsberger–Kassel–Marcus (RRKM) rate theory modeling on the near-barrierless reaction surfaces. Reaction dynamics can be investigated in isolation from nonequilibrium and/or thermodynamic considerations through the study of a generic isoergic ion–molecule system X-+X-H. Monte Carlo quasiclassical trajectory simulations on barrierless reaction surfaces show that the slow experimental kinetics is consistent with both (i) locking of the external rotations of the reactants and (ii) passage over the (orbital angular momentum) centrifugal barrier being the rate-determining steps in bimolecular association, rather than only the latter process. In addition, there may be non-RRKM product selectivity.