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

Ab initio molecular orbital calculations at the Hartree-Fock level were car ried out to investigate stability of complexes between some ion-water clust ers (F-, Cl-, Na+, Li+ and Mg2+) and organic molecules (N-methylpyrrolidone , methanol, phenol, methylamine and aniline) used as monomer analogs of pol ymers. The calculations suggested that, in most cases, the hydrogen-bonding hydration of the ion-water clusters to the organic molecules is destabiliz ed by anions (F- and Cl-) and stabilized by cations (Na+, Li+ and Mg2+) via ionic hydration. This order strongly correlated with the hydrogen-bond len gth between ion-water cluster and organic molecule, and atomic charges of w ater oxygen and hydrogen bound to polar groups of the organic molecules. Th e latter supported our hydrogen-bonding hydration model as a mechanism of t he 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 s ystems. An interesting exception in the presently obtained relative order f or cations and anions was found for complexes with phenol, which were most stabilized by F-. This was attributed to the acidity of phenol, because hyd rogen-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-. (C) 2001 Elsevier Science B.V. All rights res erved.