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.