Theoretical exploration of the potential energy surface of the HOI  HIO isomerization reaction

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Abstract

The isomerization reaction HOI  HIO has been investigated using quantum mechanical techniques. The activation energy of the reaction at the CCSD(T) level of theory is 77.0 kcal/mol. Molecular parameters and relative energies of HOI, HIO, five HOI–H2O complexes, three HIO–H2O complexes and four transition states have been calculated by the B3LYP method. For isomerization assisted by a single water molecule, the activation energy whereby the water molecule directly facilitates proton transfer during isomerization, reduces to 48.4 kcal/mol. Where the water molecule interacts with the HOI/HIO system by forming hydrogen-bond and/or van der Waals complexes, the activation energy is not significantly reduced.

Graphical abstract

For one water molecule assisted HOI  HIO isomerization reaction, the activation energy via a mechanism which involves a single water molecule reduces to 48.4 kcal/mol.

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Introduction

Iodine chemistry has been considered for almost two decades to play a role in tropospheric ozone loss [1], [2], [3], [4]. The principal gaseous sources of iodine in the troposphere are from alkyl halides, which are emitted from macroalgae and phytoplankton, found in coastal regions and in the open ocean [5], [6], [7], [8]. In addition, there are recent studies reporting elevated concentrations of iodinated methanes in seawater and marine air samples [9], [10], [11], [12]. For iodine-containing species, HOI is considered to be one of the most important iodine reservoirs in the troposphere. Additionally, there is one possible isomer of HOI, i.e., HIO, in which the iodine atom bonded to both hydrogen and oxygen atoms, with HOI being more stable than HIO by about 49.3 kcal/mol [13]. To our best of knowledge, no experimental study of the HIO isomer has been published, though it is possible to produce HIO from the isomerization of HOI. In this Letter we report a theoretical study of the reaction properties, including the heat of reaction and the activation energy barrier of the isomerization reaction of HOI to HIO.

Moreover, as there is abundant water in the troposphere, we consider the effect of water on the HOI/HIO isomerization reaction. As the interaction between H2O and HOI/HIO is not strongly dependent on cluster size [14], the coupling modes of HOI and HIO with a single water molecule are examined in this Letter. It is also known that, in many similar systems, the energy barrier of the isomerization reaction can be reduced under the assistance of water [15]. Thus, we investigate the isomerization reaction of HOI under conditions of one water molecule, to determine the extent of water-assisted isomerization. This study can serve as a guide for future experimental investigations of the HOI–H2O and HIO–H2O complexes and the resultant isomerization reaction of HOI to HIO.

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Computational details

Density Functional Theory (DFT) is an established theoretical technique, capable of successfully predicting the structural and energetic properties of weakly bonded systems [16], [17], [18]. In addition, the Lanl2DZspdf + ECP type Gaussian function basis set has been successfully applied for previous studies of the reactions of iodine-containing species [13], [19]. In this Letter we use the DFT method, with the Lanl2DZspdf + ECP basis set level for iodine, to calculate the geometric properties,

Isomerization reaction of HOI  HIO and subsequent decomposition pathways

The reaction profile and the energy properties of isomerization reaction HOI  HIO and their decomposition pathways at the CCSD(T)//B3LYP are depicted in Fig. 1 and associated energies are summarized in Table 1. The transition state or saddle point along the potential energy surface is labelled TS. In Fig. 1, we have listed the calculational geometrical parameters transition state (TS) and HIO calculated at the B3LYP level. No experimental data for HIO or TS is available. The calculated

Conclusions

The structural properties and binding energies of the five HOI–H2O complexes and the three HIO–H2O complexes have been investigated using the B3LYP method. For the isomerization reaction of HOI  HIO in the absence of water, the activation energy is 77.0 kcal/mol. Additionally, we also studied the one water assisting isomerization reaction of HOI into HIO. The changes of the activation energies of isomerization reactions via TS2 and TS3, in which the one water interaction with HOI and HIO by

Acknowledgements

This research was supported in part by ‘The Independent Investigator Project Grant Scheme, No. IIP14’ from the University of Newcastle, Australia, to Q. Sun, and she also thanks the Australian Research Council for support of her postdoctoral fellowship.

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