DFT modeling on the suitable crown ether architecture for complexation with Cs⁺ and Sr²⁺ metal ions

Study on Dynamic Column Behavior and Complexation Mechanism of DBS-Modified Crown Ether-Based Silica to 90Sr

A crown ether-loaded hybrid adsorbent suitable for the separation and enrichment of strontium from high-level liquid waste was synthesized. 4',4'(5″)-di(tert-butylcyclohexano)-18-crown-6 (DtBuCH18C6) and its modifiers dodecyl benzenesulfonic acid (DBS) and 1-dodecanol were impregnated into silica-based polymer support. The hybrid adsorbent exhibited excellent Sr(II) selectivity ability, and effective chromatographic separation and recovery of Sr(II) from simulated high-level liquid waste could be achieved with a (DtBuCH18C6 + DBS + dodec)/SiO2-P packed column. The recovery rate of Sr(II) calculated based on the mass balance was approximately 99% and over 80% for the other coexisting metal ions. An appropriate increase in the concentration of Na-DTPA eluent was favorable to improve the efficiency of the elution process because of the increased complexation capacity of [DTPA]5- to Sr(II). The developed theoretical model can simulate the dynamic breakthrough curves of the material on the basis of short column data, thereby predicting the scale-up column of the practical operation. Density functional theory calculation was used to explore the action mechanism of DBS modifiers on the Sr(II) complexation process of crown ether groups. Two Sr(II) complexation isomeric models of DtBuCH18C6 were established, and the calculation results revealed a similar complexation ability. DtBuCH18C6 could form a stable Sr(II) complexation structure with DBS coordination, which further indicated that DBS could be a ligand to promote the Sr(II) adsorption ability of crown ether materials.

Computational insight towards the binding affinity and participation of aliphatic unsaturated sidearms of aza-18-crown-6 extractants for Sr2+ encapsulation in different solvent medium

CONTEXT

Macrocyclic extractants capture cations in solution phase; therefore, their structures and energetics in different solvents are worth investigating. In this computational research work, the optimized geometry of three aza-18-crown-6 extractants (1-3) has been obtained in suitable solvent mediums to work out the influence of the solvation on their binding affinity with Sr2+. The designed macrocyclic extractants are remarkable as the N-substituted side chain bears the rarely examined simple aliphatic unsaturated double and triple bond. All significant structural perturbations of the macro ring wherein Sr2+ binds are examined. Prediction of the auxiliary effect of the simple aliphatic unsaturated side arm on the binding of Sr2+ through solvent competition or cation-pi interaction is undertaken. The binding affinity of the complex formed in the solvent and gas phases is calculated. Results obtained in this study favor the utilization of solvents of low dielectric constant (CHCl3 and DCM) for effective binding of Sr2+ ions by the extractants.

METHOD

All DFT calculations were performed using the Gaussian 09 program. The optimized geometries and their structural features were visualized by GaussView. Density functional calculation involving B3LYP functional and LANL2DZ basis set was employed to obtain optimized geometry and energy of the extractants and their Sr2+ complex. The solvation effects were considered by employing the calculations with the polarized continuum model (PCM). The computational method's reliability was assured by comparing the optimized structure with that of X-ray reported structure of a similar type of complex.

Improved alkali metal ion capturing utilizing crown ether-based diblock copolymers in a sandwich-type complexation

The compexation behavior of metals with free crown ethers (CE) and diblock copolymer-based CE is investigated. The latter shows at least 10 000 times stronger complexation than free CEs. On this basis, a highly stable CE complex within the polymer for efficient extraction of metal ions from low concentrations, e.g. lithium in seawater, is presented.

Synthesis, Biological Evaluation, and Molecular Modeling of Aza-Crown Ethers

Synthetic and natural ionophores have been developed to catalyze ion transport and have been shown to exhibit a variety of biological effects. We synthesized 24 aza- and diaza-crown ethers containing adamantyl, adamantylalkyl, aminomethylbenzoyl, and ε-aminocaproyl substituents and analyzed their biological effects in vitro. Ten of the compounds (8, 10–17, and 21) increased intracellular calcium ([Ca²⁺]_i) in human neutrophils, with the most potent being compound 15 (N,N’-bis[2-(1-adamantyl)acetyl]-4,10-diaza-15-crown-5), suggesting that these compounds could alter normal neutrophil [Ca²⁺]_i flux. Indeed, a number of these compounds (i.e., 8, 10–17, and 21) inhibited [Ca²⁺]_i flux in human neutrophils activated by N-formyl peptide (fMLF). Some of these compounds also inhibited chemotactic peptide-induced [Ca²⁺]_i flux in HL60 cells transfected with N-formyl peptide receptor 1 or 2 (FPR1 or FPR2). In addition, several of the active compounds inhibited neutrophil reactive oxygen species production induced by phorbol 12-myristate 13-acetate (PMA) and neutrophil chemotaxis toward fMLF, as both of these processes are highly dependent on regulated [Ca²⁺]_i flux. Quantum chemical calculations were performed on five structure-related diaza-crown ethers and their complexes with Ca²⁺, Na⁺, and K⁺ to obtain a set of molecular electronic properties and to correlate these properties with biological activity. According to density-functional theory (DFT) modeling, Ca²⁺ ions were more effectively bound by these compounds versus Na⁺ and K⁺. The DFT-optimized structures of the ligand-Ca²⁺ complexes and quantitative structure-activity relationship (QSAR) analysis showed that the carbonyl oxygen atoms of the N,N’-diacylated diaza-crown ethers participated in cation binding and could play an important role in Ca²⁺ transfer. Thus, our modeling experiments provide a molecular basis to explain at least part of the ionophore mechanism of biological action of aza-crown ethers.

Complexation of thorium with pyridine monocarboxylate-N-oxides: Thermodynamic and computational studies

The feed wastes and waste water treatment plants are the major sources for the entry of N-oxides into the soils then to aquatic life. The complexation of actinides with potentially stable anthropogenic ligands facilitate the transportation and migration of the actinides from the source confinement. The present study describes the determination of thermodynamic parameters for the complexation of Th(IV) with the three isomeric pyridine monocarboxylates (PCNO) namely picolinic acid-N-oxide (PANO), nicotinic acid-N-oxide (NANO) and isonicotinic acid-N-oxide (IANO). The potentiometric and isothermal calorimetric titrations were carried out to determine the stability and enthalpy of the formations for all the Th(IV)-PCNO complexes. Th-PANO complexes are more stable than Th-NANO and Th-IANO complexes which can be attributed to chelate formation in the former complexes. Formation of all the Th-PCNO complexes are endothermic and are entropy driven. The geometries for all the predicted complexes are optimized the energies, bond distances and charges on individual atoms are obtained using TURBOMOLE software. The theoretical calculation corroborated the experimental determinations.

Structure, Dynamics, and Adsorption of Charged Guest within the Nanocavity of Polymer-Functionalized Neutral Macrocyclic Host

Host-guest encapsulation has been widely applied for purification and seizing of the metal ions. Macrocyclic crown ethers are one of the most popular hosts in the field of host-guest chemistry, which on functionalization with polymers are employed as an effective adsorbent. In spite of their vast applications, the microscopic information about their sensing mechanism toward cations/molecules is very scarce. Therefore, the present study is focused on the molecular insights of ion-exchange mechanism within the cavity of crown ether-functionalized polymers using molecular dynamics (MD) simulations. This present study investigates the molecular-level events of chloromethylated polystyrene (CMPS) bearing dibenzo-18-crown-6 (DB18C6) in the aqueous and acidic environment, which has been found to be particularly successful in sensing of various alkali and alkali earth metal ions. A strategy has been envisaged to design a crown ether-based functionalized polymeric resin, which exhibits good match of properties with the in-house-synthesized resin. The MD studies well capture the experimentally observed Langmuir-type adsorption isotherms of Li⁺ ions on crown ether-grafted polymer resins. The presence of acid reduces the adsorption of Li⁺ ions due to the competition with H₃O⁺ ions. In addition, the results revealed that the "adsorption in crown cavity" follows a dual residence time function. To the best of our knowledge, this is the first report on the adsorption isotherm of functionalized crown ether using MD simulations. The structure and dynamics of binding sites were explored using radial distribution functions and diffusion coefficients. All of these effects have been studied for different Li⁺-ion concentrations, acid concentrations, and counterions as well as different lengths of polymer chains and degrees of polymerization. Overall, the present study provides insights into and quantitative information about adsorption on the CMPS-DB18C6 resin, which might be useful in myriads of host-guest-based adsorption experiments.

The effect of ether-functionalisation in ionic liquids analysed by DFT calculation, infrared spectra, and Kamlet-Taft parameters

The effect of ether-functionalisation on ionic liquids (ILs) is discussed based on Kamlet-Taft parameters and the infrared (IR) spectra of N-ethoxyethyl-N-methylpiperidinium bis(trifluoromethanesulfonyl)imide ([P_1,2O2][TFSI]) and N-ethoxyethyl-N-methylmorpholinium bis(trifluoromethanesulfonyl)imide ([M_1,2O2][TFSI]). The results are analysed taking into consideration their ion conformers, electronegativity and hardness, and the IR active vibrations obtained by means of DFT calculations. From the evaluation of Kamlet-Taft parameters, the ether-functionalisation in the cationic ring is found to improve the polarity and hydrogen bond acidity of the ILs. This correlates with the computational result which designates that the oxygen atom in the cationic ring increases the electronegativity of the cation. The comparison with the IR spectra, which were obtained experimentally and computationally, revealed that trans-[TFSI] was preferably formed in [M_1,2O2][TFSI] compared to [P_1,2O2][TFSI]. Although the Kamlet-Taft parameters indicate that [M_1,2O2][TFSI] has a higher polarity, this IL preferably adopts trans-[TFSI], which is normally stabilised with the cations having a lower polarity. This may be due to the presence of the oxygen in the cationic ring which delocalises the electron density of the lowest unoccupied molecular orbitals (LUMO) and increases the conformational freedom of the hydrogen bonds between cations and anions. Moreover, the mixtures of pure ILs with a suitable Li-salt were also investigated to analyze the effect of the Li salt on the polarity and the ion conformers.

Structural, luminescence, thermodynamic and theoretical studies on mononuclear complexes of Eu(III) with pyridine monocarboxylate-N-oxides in aqueous solution

The mononuclear complexes formed by Eu(III) with three isomeric pyridine monocarboxylate-N-oxides namely picolinic acid-N-oxide (PANO), nicotinic acid-N-oxide (NANO) and isonicotinic acid-N-oxide (IANO) in aqueous solutions were studied by potentiometry, luminescence spectroscopy and isothermal titration calorimetry (ITC) to determine the speciation, coordination, luminescence properties and thermodynamic parameters of the complexes formed during the course of the reaction. More stable six membered chelate complexes with stoichiometry (ML_i, i=1-4) are formed by Eu(III) with PANO while non chelating ML and ML₂ complexes are formed by NANO and IANO. The stability of Eu(III) complexes follow the order PANO>IANO>NANO. The ITC studies inferred an endothermic and innersphere complex formation of Eu(III)-PANO and Eu(III)-IANO whereas an exothermic and outer-sphere complex formation for Eu(III)-NANO. The luminescence life time data further supported the ITC results. Density functional theoretical calculations were carried out to optimize geometries of the complexes and to estimate the energies, structural parameters (bond distances, bond angles) and charges on individual atoms of the same. Theoretical approximations are found to be in good agreement with the experimental observations.

Insight into selectivity: uptake studies of radionuclides 90Sr2+, 137Cs+, and 233UO22+ with bis-amidoxime polymers

Density functional theory and separation experiments combine to demonstrate the selectivity of bis-amidoxime polymer for uranyl extraction from aqueous solution.

Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl alcohol) as Specialized Microfibrous Li+ Adsorbents

Crown ether (CE)-based Li⁺ adsorbent microfibers (MFs) were successfully fabricated through a combined use of CE diols, electrospinning, and aerosol cross-linking. The 14- to 16-membered CEs, with varied ring subunits and cavity dimensions, have two hydroxyl groups for covalent attachments to poly(vinyl alcohol) (PVA) as the chosen matrix. The CE diols were blended with PVA and transformed into microfibers via electrospinning, a highly effective technique in minimizing CE loss during MF fabrication. Subsequent aerosol glutaraldehyde (GA) cross-linking of the electrospun CE/PVA MFs stabilized the adsorbents in water. The aerosol technique is highly effective in cross-linking the MFs at short time (5 h) with minimal volume requirement of GA solution (2.4 mL g^-1 MF). GA cross-linking alleviated CE leakage from the fibers as the CEs were securely attached with PVA through covalent CE-GA-PVA linkages. Three types of CE/PVA MFs were fabricated and characterized through Fourier transform infrared-attenuated total reflection, ¹³C cross-polarization magic angle spinning NMR, field emission scanning electron microscope, N₂ adsorption/desorption, and universal testing machine. The MFs exhibited pseudo-second-order rate and Langmuir-type Li⁺ adsorption. At their saturated states, the MFs were able to use 90-99% CEs for 1:1 Li⁺ complexation, suggesting favorability of their microfibrous structures for CE accessibility to Li⁺. The MFs were highly Li⁺-selective in seawater. Neopentyl-bearing CE was most effective in blocking larger monovalents Na⁺ and K⁺, whereas the dibenzo CE was best in discriminating divalents Mg²⁺ and Ca²⁺. Experimental selectivity trends concur with the reaction enthalpies from density functional theory calculations, confirming the influence of CE structures and cavity dimensions in their "size-match" Li⁺ selectivity.

Discriminating single-molecule sensing by crown-ether-based molecular junctions

Crown-ether molecules are well known to selectively bind alkali atoms, so by incorporating these within wires, any change in electrical conductance of the wire upon binding leads to discriminating sensing. Using a density functional theory-based approach to quantum transport, we investigate the potential sensing capabilities of single-molecule junctions formed from crown ethers attached to anthraquinone units, which are in turn attached to gold electrodes via alkyl chains. We calculate the change in electrical conductance for binding of three different alkali ions (lithium, sodium, and potassium). Depending on the nature of the ionic analyte, the conductance is enhanced by different amounts. This change in electrical conductance is due to charge transfer from the ion to molecular wire causing the molecular resonances to shift closer to the electrode Fermi energy.

DFT-B3LYP study of interactions between host biphenyl-1-aza-18-crown-6 ether derivatives and guest Cd(2+): NBO, NEDA, and QTAIM analyses

This report present the results of natural energy decomposition analysis (NEDA), natural bond orbital (NBO), and quantum theory of atoms in molecules (QTAIM) calculations of three derivatives of biphenyl-1-aza-18-crown-6 ether and their 1:1 complexes with Cd(2+). All calculations used the B3LYP density functional theory in combination with the 6-311G and WTBS basis sets for ligands and Cd(2+) ion, respectively. Ligands 1 and 3 have a single 1-aza-18-crown-6, substituent; ligand 2 has two such substituents. The results show that, in the optimized geometries of the complexes, the distance between N and Cd(2+) is greater than the distance between O and Cd(2+). NBO and QTAIM data confirm these results. There was no stabilization energy or bond critical point for N · · · Cd(2+) in NBO or QTAIM, respectively. Data show that the O · · · Cd(2+) interaction is a kind of closed shell interaction. The trend of the calculated stabilization energy was similar to the experimental data. Different contributions of interaction energies for complex formation were analyzed by NEDA, and the results show that the main component of the interactions is accounted for by polarization.

Multifunctional flexible free-standing titanate nanobelt membranes as efficient sorbents for the removal of radioactive (90)Sr(2+) and (137)Cs(+) ions and oils

For the increasing attention focused on saving endangered environments, there is a growing need for developing membrane materials able to perform complex functions such as removing radioactive pollutants and oil spills from water. A major challenge is the scalable fabrication of membranes with good mechanical and thermal stability, superior resistance to radiation, and excellent recyclability. In this study, we constructed a multifunctional flexible free-standing sodium titanate nanobelt (Na-TNB) membrane that was assembled as advanced radiation-tainted water treatment and oil uptake. We compared the adsorption behavior of ¹³⁷Cs⁺ and ⁹⁰Sr²⁺ on Na-TNB membranes under various environmental conditions. The maximum adsorption coefficient value (K_d) for Sr²⁺ reaches 10⁷ mL g⁻¹. The structural collapse of the exchange materials were confirmed by XRD, FTIR and XPS spectroscopy as well as Raman analysis. The adsorption mechanism of Na-TNB membrane is clarified by forming a stable solid with the radioactive cations permanently trapped inside. Besides, the engineered multilayer membrane is exceptionally capable in selectively and rapidly adsorbing oils up to 23 times the adsorbent weight when coated with a thin layer of hydrophobic molecules. This multifunctional membrane has exceptional potential as a suitable material for next generation water treatment and separation technologies.

An efficient ab initio DFT and PCM assessment of the potentiometric selectivity of a salophen type Schiff base

As a neutral carrier component for the preparation of a potentiometric membrane sensor, the affinity and selectivity of the salophen type Schiff base ligand obtained by 1:2 condensation of 2.3-diaminopyridine with salicylaldehyde toward a series of common cations has been fully examined by DFT/B3LYP and integral equation formalism polarizable continum model (IEF-PCM or only given with PCM as default input in the computations) in combination with the experimental data. Both the potentiometric measurements and DFT calculations have exhibited that the ionophore shows appreciable selectivity for Cu(2+) ion over other cations. Four different approaches where the last three are the modified version of each other have been evaluated and compared with potentiometric data. Based upon the results of comparison among the approaches suggested to verify the selective behavior of ionophore toward Cu(2+), PCM implemented approach having a whole computational groundwork has given well-matched results with the observed data and with the method augmented with experimental hydration energies. The foremost interferences were detected by determining potentiometric selectivity coefficients for each metal ion relative to Cu(2+) and compared to the results obtained by the DFT calculations.

Ab initio and density functional theoretical design and screening of model crown ether based ligand (host) for extraction of lithium metal ion (guest): effect of donor and electronic induction

The structures, energetic and thermodynamic parameters of model crown ethers with different donor, cavity and electron donating/ withdrawing functional group have been determined with ab initio MP2 and density functional theory in gas and solvent phase. The calculated values of binding energy/ enthalpy for lithium ion complexation are marginally higher for hard donor based aza and oxa crown compared to soft donor based thia and phospha crown. The calculated values of binding enthalpy for lithium metal ion with 12C4 at MP2 level of theory is in good agreement with the available experimental result. The binding energy is altered due to the inductive effect imparted by the electron donating/ withdrawing group in crown ether, which is well correlated with the values of electron transfer. The role of entropy for extraction of hydrated lithium metal ion by different donor and functional group based ligand has been demonstrated. The HOMO-LUMO gap is decreased and dipole moment of the ligand is increased from gas phase to organic phase because of the dielectric constant of the solvent. The gas phase binding energy is reduced in solvent phase as the solvent molecules weaken the metal-ligand binding. The theoretical values of extraction energy for LiCl salt from aqueous solution in different organic solvent is validated by the experimental trend. The study presented here should contribute to the design of model host ligand and screening of solvent for metal ion recognition and thus can contribute in planning the experiments.