Excitation functions from quasiclassical trajectory calculations on the H + H2O --> OH + H2, H + HF --> F + H2, and H + H'F --> H' + HF reactions indicate a different behavior at low and high vibrational excitation of the breaking bond. When the reactant tri- or diatomic molecule is in vibrational ground state or in a low vibrationally excited state, all these reactions are activated; i.e., there is a nonzero threshold energy below which there is no reaction. In contrast, at high-stretch excited-states capture-type behavior is observed; i.e., with decreasing translational energy the reactive cross-section diverges. The latter induces extreme vibrational enhancement of the thermal rate consistent with the experiments. The results indicate that the speed-up observed at high vibrational excitation is beyond the applicability of Polanyi's rules in their common form; instead, it can be interpreted in terms of an attractive potential acting on the attacking H atom when it approaches the reactant with a stretched X-H bond.
Experimental and theoretical investigations of bistability occurring in the nitric acid-hydroxylamine CSTR system are presented. Features of the stationary concentration vs. inlet concentration diagram and some mechanistic implications are discussed.
The only known example of chiral autocatalysis is the alkylation of N-heterocyclic aldehydes with iPr(2)Zn (Soai reaction). The mechanism and some details of this reaction are not yet clear. An empirical formula is proposed here for the description of this chiral autocatalytic reaction. This formula allows the calculation of some very informative parameters.
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