Sobre a origem da energia de ativação negativa na taxa de reação do oh + hbr: Estudo do comportamento não-arrhenius usando dinâmica molecular de born-oppenheimer

Abstract

In recent years there were several studies of gas-phase reactions, which are believed to be elementary. These studies have been shown that these reactions present negative temperature dependence in the Arrhenius plot (negative activation energy). The clarification of the origin of these features is of interest in recent debates of the foundation of chemical kinetics and its applications. The reaction of the OH radical with HBr molecule is a relevant example of an elementary process presenting negative activation energy behavior. The apparent negative activation energy of this reaction shows up below 150K. Additional, this reaction is known to play a key role in stratospheric chemistry because it produces Br atoms which destroy the ozone layer very effectively. With these motivations, understand the reaction mechanism for this process becomes very important and, thus, the Borh-Oppenheimer molecular dynamics happens to be a great tool to understand the dynamics of the process. During the simulations multiple paths of reactions were found. Only for lower energies were found the mechanism direct hydrogen abstraction from HBr through intermediate complex; and the transition state has lower energy than the reactants. At higher energies, in all paths analyzed, there is no formation of intermediate complex and the transition state energy has higher energy than the reactants. This result seems to be consistent with recent crossed molecular beam studies of the steric effect for this reaction. Still, at lower energies the molecules can be oriented more easily, facilitating complex formation. However, for higher energies, the molecules cannot be oriented and disfavor complex formation. Thus, the negative activation energy obtained in this process can be attributed to two factors: complex formation and orientation of the reagents needed to produce effective collisions. Since, there was an increasing in the reaction rate at lower temperatures and a decreasing at higher temperatures. Thus, the simulations observed in this study provide a better understanding on the origin of the negative activation energy for OH + HBr reaction.