Cs+–π interactions and the design of macrocycles for the capture of environmental radiocesium (Cs-137): DFT, QTAIM, and CSD studies

Association Complexes of Calix[6]arenes with Amino Acids Explained by Energy-Partitioning Methods

Intermolecular complexes with calixarenes are intriguing because of multiple possibilities of noncovalent binding for both polar and nonpolar molecules, including docking in the calixarene cavity. In this contribution calix[6]arenes interacting with amino acids are studied with an additional aim to show that tools such as symmetry-adapted perturbation theory (SAPT), functional-group SAPT (F-SAPT), and systematic molecular fragmentation (SMF) methods may provide explanations for different numbers of noncovalent bonds and of their varying strength for various calixarene conformers and guest molecules. The partitioning of the interaction energy provides an easy way to identify hydrogen bonds, including those with unconventional hydrogen acceptors, as well as other noncovalent bonds, and to find repulsive destabilizing interactions between functional groups. Various other features can be explained by energy partitioning, such as the red shift of an IR stretching frequency for some hydroxy groups, which arises from their attraction to the phenyl ring of calixarene. Pairs of hydrogen bonds and other noncovalent bonds of similar magnitude found by F-SAPT explain an increase in the stability of both inclusion and outer complexes.

Hyper-crosslinked tetraphenylborate as a regenerable sorbent for Cs+ sequestration in aqueous media through cation-π interactions

Practical adsorbents that could efficiently collect radioactive Cesium (Cs⁺) are critically important in achieving proper management and treatment measures for nuclear wastes. Herein, a hyper-crosslinked tetraphenylborate-based adsorbent (TPB-X) was prepared by reacting TPB anions as Cs⁺ binding sites with dimethoxymethane (DMM) as crosslinker. The most efficient TPB-X synthesis was attained at 1:4 TPB/DMM mole ratio with sorbent yield of 81.75%. Various techniques such as FTIR, TGA-DTG, N₂ adsorption/desorption and SEM-EDS reveal that TPB-X is a water-insoluble, thermally stable and highly porous granular sorbent. Its hierarchical pore structure explains its very high BET surface area (1030 m² g^-1). Sequestration of Cs⁺ by TPB-X involves its exchange with H⁺ followed by its binding with the phenyl rings of TPB through cation-π interactions. The Cs⁺ adsorption in TPB-X is endothermic and spontaneous, which adheres to the Hill isotherm model (q_m = 140.58 mg g^-1) and follows pseudo-second order kinetics (k₂ = 0.063 g mg^-1 h^-1). Calculations from the density functional theory reveal that the binding of TPB anion is strongest for Cs⁺. Thus, TPB-X was able to selectively capture Cs⁺ in simulated surface water containing Na⁺, K⁺, Mg²⁺, and Ca²⁺ and in HLLW containing Na⁺, Rb⁺, Sr²⁺, and Ba²⁺. Hyper-crosslinking was found beneficial in rendering TPB-X reusable as the sorbent was easily retrieved from the feed after Cs⁺ capture and was able to withstand the acid treatment for its regeneration. TPB-X exhibited consistent performance with no sign of chemical or physical deterioration. TPB-X offers a practical approach in handling Cs⁺ contaminated streams as it can be repeatedly used to enrich Cs⁺ in smaller volume of media, which can then be purified for Cs⁺ reuse or stored for long-term natural Cs⁺ decay process.