Intermolecular interactions and proton transfer in the hydrogen halide–superoxide anion complexes

Abstract

The superoxide radical anion O₂⁻ is involved in many important chemical processes spanning different scientific disciplines (e.g., environmental and biological sciences). Characterizing its interaction with various substrates to help elucidate its rich chemistry may have far reaching implications. Herein, we investigate the interaction between O₂⁻ ( ²Π_g) and the hydrogen halides ( ¹Σ) with coupled-cluster theory. In contrast to the short (1.324 Å) hydrogen bond formed between the HF and O₂⁻ monomers, a barrierless proton transfer occurs for the heavier hydrogen halides with the resulting complexes characterized as long (>1.89 Å) hydrogen bonds between halide anions and the HO₂ radical. The dissociation energy with harmonic zero-point vibrational energy (ZPVE) for FH⋯O₂⁻ ( ²A′′) → HF ( ¹Σ) + O₂⁻ ( ²Π_g) is 31.2 kcal mol⁻¹. The other dissociation energies with ZPVE for X⁻⋯HO₂ ( ²A′′) → X⁻ ( ¹Σ) + HO₂ ( ²A′′) are 25.7 kcal mol⁻¹ for X = Cl, 21.9 kcal mol⁻¹ for X = Br, and 17.9 kcal mol⁻¹ for X = I. Additionally, the heavier hydrogen halides can form weak halogen bonds H–X⋯O₂⁻ ( ²A′′) with interaction energies including ZPVE of −2.3 kcal mol⁻¹ for HCl, −8.3 kcal mol⁻¹ for HBr, and −16.7 kcal mol⁻¹ for HI.