Computational study of cesium cation-humic substance interactions. A neutral analogue ligand molecules approach.

Interactions between the cesium cation (Cs⁺) and humic substances were studied by the calculations of complexes between Cs⁺ and small ligand molecules mimicking different moieties in humic substances. The cesium cation gas-phase affinities (CsCAs) and basicities (CsCBs) of 52 model compounds (mainly aromatic) were calculated using the density functional method. The most common binding patterns of Cs⁺ are discussed. The CsCAs are mainly governed by ion-induced dipole p-type interaction (CsCA around 11 kcal/mol) in unsubstituted fused aromatic systems. The cesium cation tends to form a more localized type of bond in substituted aromatic compounds via the lone pairs of electronegative atoms (O, N) of substituents resulting in enhanced CsCA (around 20 kcal/mol). If the model compound is able to form chelate-type complexes with the Cs cation, CsCA may be even higher (20-28 kcal/mol). [ABSTRACT FROM AUTHOR]