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

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.