Ab Initio Investigation of the Microspecies and Energy in Hydrated Strontium Ion Clusters

Ion hydration and association in aqueous potassium tetrahydroxyborate solutions

Potassium tetrahydroxyborate solution is a significant material in the borate solution family, but there is limited knowledge about hydration structures and interactions of K⁺, [B(OH)₄^-], and water. In this study, the X-ray diffraction measurements of potassium tetrahydroxyborate solutions have been made. The experimental structure factors are subjected to Empirical Potential Structure Refinement (EPSR) modeling to reveal the details of ion hydration and association in the aqueous solutions. This study shows that the O(W)-O(W) distance of water in the studied solutions ranges from 2.82 to 2.76 Å with a coordination number that ranges from 4.7 ± 1.4 to 3.1 ± 1.3 when the value of the water-salt molar ratio (WSR) is decreased from 30 to 6. The addition of ions slightly affects the tetrahedral structure of water even when the concentration of ions is high. The first hydration distance of K⁺ remained at ∼2.67 Å, whereas the value of the coordination number (CN) decreased from 5.4 ± 1.3 to 3.9 ± 1.5 when the concentration of the borate solution was increased. The hydration ability of K⁺ was weak and almost did not have a fixed local hydration structure. The pair distribution function (PDF) of g_B-O(W)(r) shows that [B(OH)₄^-] has a broad hydration distance from 2.9 to 5.4 Å because of the complex interactive relationship between K⁺, [B(OH)₄^-] and water. There is a competitive hydration between K⁺ and [B(OH)₄^-]. Both the X-ray diffraction and DFT-based calculations confirm that the main species is monodentate contact ion pairs when WSR = 30, bidentate contact ion pairs when WSR = 14, and triple contact ion pairs when WSR = 6. These results will provide a new understanding about potassium tetrahydroxyborate solution.

Micro-Raman and density functional theory analyses of ion pairs in concentrated sodium tetrahydroxyborate droplets

In this study, ion pairs in a single sodium tetrahydroxyborate [Na [B(OH)₄] droplet were analyzed using an "in-situ strategy" in which a sample-droplet of nanogram mass was deposited on a hydrophobic substrate and droplet was forced to enter into a supersaturated state by decreasing the relative humidity (RH) of the environment. The structure of the solvated [B(OH)₄^-] ionic moiety with various molar water-to-solute ratios (WSR) was analyzed using Raman spectroscopy. To confirm the structural changes in the droplet, electronic structure calculations were carried out using density functional theory (DFT). The frequencies calculated for the totally symmetric BO stretching vibration (v_sym(BO)) of the [B(OH)₄^-] moiety were compared with those of the fundamental bands observed in the Raman spectra recorded of the droplets. The following results have been obtained: (i) when WSR is reduced from 9 to 0.1, the frequency of the band that corresponds to v_sym(BO) shifts from 745 to 746 cm^-1, and its full-width at half-maximum value increases from 19.7 to 20.5 cm^-1; (ii) when WSR ≥7, the solvent-shared ion pair (SIP) is predominantly present in the solution, whereas in the case of WSR < 7, SIP transforms into a contact ion pair (CIP) formed by Na⁺ and [B(OH)₄^-] in bidentate coordination; (iii) when WSR = 3, most of the CIPs transform into a cationic type of triple ion pair (TIP) composed of two Na⁺ and one [B(OH)₄^-] in bidentate coordination; (iv) when WSR is further reduced, most TIP continually associate to form a more complex structure and with a small amount of six-membered ring complex also formed. These results will help us understand the ion association mechanism during dehydration process of Na[B(OH)₄] droplets.

Ab-initio investigation on ion-associated species and association process in Li[B(OH)4] solution

In this paper, the factors determining the spectroscopic characteristics of v_sym-B(OH)₄^- band including coupling effect, hydrogen bonding effect, and direct contact effect in Li[B(OH)₄] solutions are investigated by using ab initio calculation. The coupling effect between the liberations of water and [B(OH)₄^-] has a larger effect on v_sym-B(OH)₄^- in solvent-shared ion pair (SIP) and monodentate contact ion pair (MCIP), but the smaller effect in bidentate contact ion pair (BCIP). Water molecule tends to hydrate to the middle position between the first sphere of B(OH)₄^- and outer-sphere of [Li(H₂O)₄⁺] and has a different effect on v_sym-B(OH)₄^- in ion pairs. The direct contact effect and polarization effect lead to 19.7 cm^-1 red shift of v_sym-B(OH)₄^- in MCIP, and 0.4 cm^-1 blue shift in BCIP. The association process in Li[B(OH)₄] solution was also introduced by using Raman spectral evolution of v_sym -B(OH)₄^- in the dehydration process.