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

Abstract 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 (vsym(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 vsym(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