Sulfur dioxide and hypohalous acids (HOX, X = F, Cl, Br, I) are ubiquitous molecules in the atmosphere that are central to important processes like seasonal ozone depletion, acid rain, and cloud nucleation. Recent studies have presented evidence that the interactions between SO₂ and HOX may be relevant to certain atmospheric chemical pathways. We present the first theoretical examinations of the HOX-SO₂ binary complexes. Reliable geometries were optimized at the CCSD(T)/aug-cc-pV(T+d)Z level of theory for HOF and HOCl complexes. The HOBr and HOI complexes were optimized at the CCSD(T)/aug-cc-pV(D+d)Z level of theory with the exception of the halogen atoms which were modeled with a aug-cc-pwCVDZ-PP pseudopotential. 25 HOX-SO₂ complexes were characterized and the interaction energies were extrapolated using the focal point method to produce highly reliable CCSDT(Q)/CBS + corrections results. The 25 ab initio complexes were grouped into 8 unique conformer classes independent of halogen substitution. Natural Bond Orbital Analysis and Symmetry Adapted Perturbation Theory were used to classify the nature of each noncovalent interaction in each class. All hydrogen bonded complexes were found to have greater dissociation energies than their halogen bonded counterparts. The interaction energies of the hydrogen bonded complexes spanned a range of 4.21 to 5.34 kcal mol^-1 considering each halogen substitution. Likewise, the halogen bonded complexes spanned a range of 1.84 to 3.66 kcal mol^-1. For each halogen substitution, the strongest interaction energies were: 5.34, 5.24, 5.18, and 5.00 for HOF, HOCl, HOBr, and HOI, respectively. A key finding is that the difference in interaction energies for halogen and hydrogen bonded complexes ranges from 0.69 to 2.15 kcals mol^-1, providing evidence that both may be quite relevant to atmospheric chemistry.

Prof. Henry F Schaefer

Graham Perdue Professor, University of Georgia

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