Stereodynamical Origin of Anti-Arrhenius Kinetics: Negative Activation Energy and Roaming for a Four-Atom Reaction.
The OH + HBr → H2O + Br reaction, prototypical of halogen-atom liberating processes relevant to mechanisms for atmospheric ozone destruction, attracted frequent attention of experimental chemical kinetics: the nature of the unusual reactivity drop from low to high temperatures eluded a variety of theoretical efforts, ranking this one among the most studied four-atom reactions. Here, inspired by oriented molecular-beams experiments, we develop a first-principles stereodynamical approach. Thermalized sets of trajectories, evolving on a multidimensional potential energy surface quantum mechanically generated on-the-fly, provide a map of most visited regions at each temperature. Visualizations of rearrangements of bonds along trajectories and of the role of specific angles of reactants' mutual approach elucidate the mechanistic change from the low kinetic energy regime (where incident reactants reorient to find the propitious alignment leading to reaction) to high temperature (where speed hinders adjustment of directionality and roaming delays reactivity).
†Unidade Universitária de Ciências Exatas e Tecnológicas, 75001-970, Anápolis, and Unidade de Ipameri, Ipameri, Universidade Estadual de Goiás, 75780-000 Goiás, Brazil.|‡Instituto de Química, Universidade de Brasília, Caixa Postal 4478, 70904-970 Brasília, Brazil.
Nayara D
ND
Coutinho
†Unidade Universitária de Ciências Exatas e Tecnológicas, 75001-970, Anápolis, and Unidade de Ipameri, Ipameri, Universidade Estadual de Goiás, 75780-000 Goiás, Brazil.
Valter H C
VH
Silva
‡Instituto de Química, Universidade de Brasília, Caixa Postal 4478, 70904-970 Brasília, Brazil.
Heibbe C B
HC
de Oliveira
†Unidade Universitária de Ciências Exatas e Tecnológicas, 75001-970, Anápolis, and Unidade de Ipameri, Ipameri, Universidade Estadual de Goiás, 75780-000 Goiás, Brazil.
Ademir J
AJ
Camargo
‡Instituto de Química, Universidade de Brasília, Caixa Postal 4478, 70904-970 Brasília, Brazil.
Kleber C
KC
Mundim
§Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy.
Vincenzo
V
Aquilanti
Angle-of-approach selectivity; Born−Oppenheimer molecular dynamics; Canonical Trajectories; Density Functional Theory; OH + HBr; On-the-fly free energy; d-Arrhenius