Raman and ab initio analyses of ion pairs in concentrated K[B(OH)₄] droplets

In this study, microscopic Raman spectroscopy and Ab initio quantum chemical calculation were used to determine the structural details of ion pairs and their transformation in concentrated K[B(OH)₄] droplets. The Raman experiment shows that the v_sym-B(OH)₄^- undergoes a downward shift with the decrease of WSR. The contact ion pairs (CIPs) change to solvent shared ion pairs when the molar water-to-solute ratio (WSR) is bigger than 6; CIPs are the dominant species when 1.33 < WSR < 6, where K⁺ bonds to [B(OH)₄^-] in bidentate form (CIP-II); the CIPs quickly dehydrate and associate to triple ion pairs (TIPs) when WSR < 5. Raman experiment and ab initio quantum chemical calculation show that TIPs are mainly present in "anionic" type such as {[B(OH)₄^-]K⁺[B(OH)₄^-](H₂O)_n}, where K⁺ bonds to two [B(OH)₄^-] in bidentate or/and tridentate form (TIP-a-II or/and TIP-a-III). When WSR <1.33, most TIPs convert to complex clusters such as chain-like structure. The remaining TIPs associate to six-membered ring structure [B₃O₃(OH)₄^-] and the relative content increases from 0 to 20% when the WSR ranges from 1.33 to 0.55.