Elsevier

Chemical Physics Letters

Volume 346, Issues 5–6, 12 October 2001, Pages 497-502
Chemical Physics Letters

An investigation of the formation of OBrO radicals from the HO+HOBrO reaction

https://doi.org/10.1016/S0009-2614(01)00796-5Get rights and content

Abstract

The geometries, vibrational spectra, and relative energetics of the formation of OBrO radicals during the reaction between hydroxyl radicals and HOBrO have been examined by using various ab initio methods. The heat of reaction for the formation of OBrO radicals from the reaction between HO and HOBrO is −37.0kcalmol−1, while the energy barrier for activation of the reaction is 3.9kcalmol−1. Results suggest that the reaction between HO and HOBrO species could serve as a minor source of OBrO radicals in the atmosphere.

Introduction

It has been recognized that reactions involving bromine in the stratosphere could perturb ozone in a manner analogous to the reactions involving chlorine [1], [2]. The contributions to ozone loss from bromine reactions were found to be the largest below 20 km in the atmosphere. Even though bromine species are much less abundant than chlorine, reactions involving bromine have been found to play a very important role in stratospheric ozone depletion. On a per-atom basis, bromine is estimated to be about 50 times more efficient than chlorine in destroying the ozone layer in the stratosphere [3]. Stratospheric modeling has suggested that the predominant form of bromine in the stratosphere is bromine monoxide (BrO), which, as an `active' form of bromine, can participate in various catalytic cycles leading to the destruction of ozone. Bromine also exists in an active form in the atmosphere as bromine atoms (Br).

The coupling of the BrOx radical families with HOx have been examined thoroughly by various experimental and theoretical studies. HOx radical species such as HO (hydroxyl radical) and HO2 (hydroperoxy radical) are present in abundance in the atmosphere. The coupling of bromine oxides with HOx species to destroy ozone has been of particular importance. A critical reaction that couples BrOx and HOx species in the above cycle is the reaction of BrO and OH. This process should increase the recycling of bromine radicals, and could be efficient in regions which have significant OH concentration profiles. Bogan et al. [4] conducted experiments, at 300 K, on the BrO–OH reaction and postulated that the pathway 1 is the predominant and perhaps exclusive product channel which proceeds via the short-lived [HOOBr] vibrationally excited addition complex:BrO+OH→[HOOBr]Br+HO2The HOOBr complex and its isomeric form HOBrO are the two structural forms that exist as stable intermediates during the reaction between HO and BrO radicals, as determined by theoretical studies performed by Lee [5] and Guha and Francisco [6]. The HOBrO structural form tends to bind the bromine atom in it serving as a stable reservoir of inorganic bromine during the night-time. This process is very important, since the binding of the bromine atom in the stable structural framework of HOBrO prevents bromine from being easily released to react with and destroy the ozone layer. Once formed, the HOBrO intermediate will not isomerize to the HOOBr structural form due to the high energy barrier for isomerization between HOBrO and HOOBr [7].

Although the HOBrO intermediate has been suggested to be stable at night, it should photolyze in the presence of sunlight during the daytime. It is also possible that OBrO would undergo reaction with hydroxyl radicals in the atmosphere to produce bromine dioxide (OBrO) radicals:HO+HOBrOH2O+OBrOThe formation of OBrO radicals is of importance in the context of atmospheric chemistry. since OBrO has been suggested to be the principal bromine species at night in the stratosphere at mid-latitudes, according to the observations of Renard et al. [8]. During the daytime, OBrO undergoes photolysis to release bromine atoms which play critical roles in the catalytic destruction of ozone [9], similar to the roles played by the release of chlorine atoms from the OClO radical species [10]. Due to the importance of the existence of OBrO in the atmosphere, it is critical to evaluate the reactions by which it is produced. Thus, the reaction between HO radicals and HOBrO, which could potentially produce OBrO radicals, could be of atmospheric importance.

In this Letter, we present ab initio molecular orbital calculation results of the structures, vibrational spectrum, and energetics of the transition state of the HO+HOBrO→H2O+OBrO reaction (see Fig. 1). Such a study enables us to determine whether there exists a significant energy barrier for the formation of OBrO radicals in the atmosphere. Knowledge of the energy barrier is extremely important in ascertaining whether the reaction of HOBrO species with hydroxyl radicals could be a significant source of OBrO radicals in the atmosphere.

Section snippets

Computational methods

Ab initio molecular orbital calculations were performed using the Gaussian 98 program [11]. The equilibrium geometries of the reactants and products for the HO+HOBrO→H2O+OBrO reaction as well as the transition state for the reaction were fully optimized to better than 0.001 Å for bond distances and 0.1° for bond angles, with a self-consistent field convergence of at least 10−9 on the density matrix. The unrestricted second-order Møller–Plesset (UMP2) method was used with the 6-31G(d) basis set

Geometries and vibrational frequencies

Computations on the transition state for the HO+HOBrO→H2O+OBrO reaction were performed using the UMP2 method in conjunction with the 6-31G(d) and 6-311G(2d,2p) basis sets. In general, the structural parameters optimized using the two basis sets are found to be in good agreement. The structural parameters for the HO+HOBrO→H2O+OBrO transition state are provided in Table 1. The optimized structural parameters of the reactants and products of the reaction between HO radicals and HOBrO are also

Conclusion

The equilibrium structures, vibrational and electronic spectra, and relative energetics of the transition state for the HO+HOBrO→H2O+OBrO reaction have been investigated. The formation of bromine dioxide from the reaction between HO radicals and HOBrO is facilitated by hydrogen abstraction from HOBrO by the hydroxyl radicals after overcoming a small energy barrier of 3.9kcalmol−1. Our results suggest that the reaction between HO and HOBrO species may serve as a minor source of production of

References (11)

  • J.G. Anderson et al.

    Science

    (1991)
  • T.J. Lee

    Chem. Phys. Lett.

    (1996)
  • S. Guha et al.

    Chem. Phys. Lett.

    (2000)
  • J.B. Renard et al.

    Compt. Ren. Acad. Sci.

    (1997)
  • R.J. Salawitch et al.

    Geophys. Res. Lett.

    (1990)
There are more references available in the full text version of this article.
View full text