TheoreticalReaction Kinetics Astride the Transition between Moderate and DeepTunneling Regimes: The F + HD Case.

Forthe reaction between F and HD, giving HF + D and DF + H, the rateconstants, obtained from rigorous quantum scattering calculationsat temperatures ranging from 350 K down to 100 K, show deviationsfrom the Arrhenius behavior that have been interpreted in terms oftunneling of either H or D atoms through a potential energy barrier.The interval of temperature investigated extends from above to belowa crossover value Tc, a transition temperatureseparating the moderate and deep quantum tunneling regimes. Below Tc, the rate of the H or D exchange reactionis controlled by the prevalence of tunneling over the thermal activationmechanism. In this temperature range, Bell’s early treatmentof quantum tunneling, based on a semiclassical approximation for thebarrier permeability, provides a reliable tool to quantitatively accountfor the contribution of the tunneling effect. This treatment is hereapplied for extracting from rate constants properties of the effectivetunneling path, such as the activation barrier height and width. Weshow that this is a way of parametrizing the dependence of the apparentactivation energy on temperature useful for both calculated and experimentalrate constants in an ample interval of temperature, from above tobelow Tc, relevant for modelization ofastrophysical and in general very low-temperature environments. [ABSTRACT FROM AUTHOR]