The structural elucidation of aqueous H3BO3 solutions by DFT and neutron scattering studies

Abstract

The micro-structure of aqueous boric acid (H₃BO₃) solutions is of broad interest in earth sciences, geochemistry, material science, as well as chemical engineering. In the present study, the structure of aqueous H₃BO₃ solutions was studied via neutron scattering with ²H and ¹¹B isotope labelling combined with empirical potential structure refinement (EPSR) modelling. In aqueous H₃BO₃ solutions, B(OH)₃ is the dominant borate species. Density function theory (DFT) calculations show that the boron hydroxyl has a lower electrostatic potential (ESP), which makes B(OH)₃ a relatively weakly hydrated, compared with the bulk water. In the 0.95 mol L⁻¹ H₃BO₃ solution at 298 K (saturated), ∼18 water molecules enter the hydration sphere of B(OH)₃ with the hydration distance (B–O(W)) of 3.75 Å, while only 4.23 of them hydrate with H₃BO₃ as the hydrogen bond (H-bond) acceptor or H-bond donor. Both neutron scattering and DFT calculations for 2B(OH)₃·6H₂O clusters at the ωB97XD/6-311++g(3df,3pd) basis level show that B(OH)₃ forms molecular clusters in bidentate contact molecular pairs (BCMP), mono-dentate molecular pairs (MCMP), solvent-shared molecular pairs (SMP), and parallel solvent-shared molecular pairs (PSMP) in aqueous solutions. Their relative contents are both concentration- and temperature-sensitive. BCMP with the B–B distance of ∼4.1 Å is the dominant molecular pair in the aqueous solutions. Relatively less content and van der Waals interactions stabilized PSMP, with a B–B distance of ∼3.6 Å between the two parallel layers, which is a crucial species for the crystallization of H₃BO₃ from aqueous solution.