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

Abstract

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 vsym-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.