Clustering of HClO4 with Brønsted (H2SO4, HClO4, HNO3) and Lewis acids BX3 (X = H, F, Cl, Br, OH): a DFT study

Abstract

HClO₄ is an important catalyst in organic chemistry, and also acts as a reservoir or sink species in atmospheric chlorine chemistry. In this study, we computationally investigate the interactions of Brønsted (H₂SO₄, HClO₄, HNO₃) and Lewis acids (BH₃, BF₃, BCl₃, BBr₃, B(OH)₃) with HClO₄ using the ωB97xD method and the aug-cc-pVDZ basis set. Different isomers of clusters with up to 4 molecules (tetramer) were optimized, and the most stable structures were determined. The enthalpies, ΔH, and Gibbs free energies, ΔG, of cluster formation were calculated in the gas phase at 298 K. Atoms in molecules (AIM) calculations find B–O bond critical points only in the (BH₃)_nHClO₄ clusters, while formation of other clusters was based on hydrogen bonding interactions. (H₂SO₄)HClO₄ and (B(OH)₃)HClO₄, with formation enthalpies of −14.1 and −12.0 kcal mol⁻¹, were the most stable, and (BCl₃)HClO₄ with a formation enthalpy of −2.9 kcal mol⁻¹, was the least stable cluster among the dimers. Clustering of the Lewis and Brønsted acids with HClO₄ enhanced its acidity, so that clustering of four HClO₄ molecules and formation of (HClO₄)₄ increases the acidity of HClO₄ by about 35 kcal mol⁻¹. The most acidic dimer cluster found in the study was (BBr₃)HClO₄, with ΔH_acid of 275 kcal mol⁻¹; 26 kcal mol⁻¹ stronger than that of the HClO₄ monomer.