Clustering of H2SO4 with BX3 (X = H, F, Cl, Br, CN, OH) compounds creates strong acids and superacids

Abstract

The interaction of H2SO4 with boron compounds including BH3, BF3, BCl3, BBr3, B(CN)(3) and B(OH)(3) was studied computationally using the omega B97xD density functional. All the BX3 compounds except B(OH)(3) bind to H2SO4 via both SOH center dot center dot center dot X hydrogen bonds, and interactions between the B atoms and the S=O oxygen atoms. B(OH)(3) interacts with H2SO4 solely through hydrogen bonds. B(CN)(3) and BCl3 exhibit the strongest and weakest interactions with H2SO4, respectively. Natural bond orbital (NBO) analysis shows that the relative weakness of the H2SO4- BCl3 interaction may be due to pi-bonding between the B and Cl atoms, and the occupation of the p(z) orbital of the B atom. The strong electron withdrawing groups CN in B(CN)(3) intensify electron deficiency of B atom and promote its tendency to capture electrons of oxygen atom of O=S group. Atoms in molecules (AIM) calculations show bond critical points (BCP) between the X groups of BX3 and the hydrogen atoms of H2SO4 for all cases except X = OH. Enthalpies and Gibbs free energies of deprotonation in the gas phase (Delta H-acid, Delta G(acid)) were calculated for (BX3)H2SO4 and (BX3)(2)H2SO4 complexes. These data revealed that clustering of BX3 with H2SO4 enhances the acidity of H2SO4 by about 9-58 kcal mol(-1). The (B(CN)(3))(2)H2SO4 cluster had Delta H-acid and Delta G(acid) values of 255.0 and 246.7 kcal mol(-1), respectively, and is the strongest Bronsted acids among the (BX3)(2)H2SO4 clusters.