Ion-specificity for hydrogen-bonding hydration of polymer: an approach by ab initio molecular orbital calculations II

摘要

Ab initio molecular orbital calculations at the Hartree-Fock level were carried out to investigate stability of complexes between some ion-water clusters (F~-, Cl~-, Na~+, Li~+ and Mg~(2+)) and organic molecules (N-methylpyrrolidone, methanol, phenol, methylamine nd aniline) used as monomer analogs of polymers. The calculations suggested that, in most cases, the hydrogen-bonding hydration of the ion-water clusters to the organic molecules is destabilized by anions (F~- and Cl~-) and stabilized by cations (Na~+, Li~+ and Mg~(2+)) via ionic hydration. This order strongly correlated with the hydrogen-bond length between ion-water cluster and organic molecule, and atomic charges of water oxygen and hydrogen bound to polar groups of the organic molecules. The latter supported our hydrogen-bonding hydration model as a mechanism of the ion-specific gel swelling: changes in water electron-pair donation (EPD) and acceptance (EPA) abilities through ionic hydration are responsible for the ion-specific swelling behaviors of uncharged polymer gels in aqueous systems. An interesting exception in the presently obtained relative order for cations and anions was found for complexes with phenol, which were most stabilized by F~-. This was attributed to the acidity of phenol, because hydrogen-bonding hydration to proton of phenol OH, which has a higher positive charge than e.g. methanol OH, is more favorable with water having enhanced EPD through hydration to F~-.