Stable Ion Pairs Research Articles

Direct observation of the complex S(IV) equilibria at the liquid-vapor interface

The multi-phase oxidation of S(IV) plays a crucial role in the atmosphere, leading to the formation of haze and severe pollution episodes. We here contribute to its understanding on a molecular level by reporting experimentally determined pKa values of the various S(IV) tautomers and reaction barriers for SO2 formation pathways. Complementary state-of-the-art molecular-dynamics simulations reveal a depletion of bisulfite at low pH at the liquid-vapor interface, resulting in a different tautomer ratio at the interface compared to the bulk. On a molecular-scale level, we explain this with the formation of a stable contact ion pair between sulfonate and hydronium ions, and with the higher energetic barrier for the dehydration of sulfonic acid at the liquid-vapor interface. Our findings highlight the contrasting physicochemical behavior of interfacial versus bulk environments, where the pH dependence of the tautomer ratio reported here has a significant impact on both SO2 uptake kinetics and reactions involving NOx and H2O2 at aqueous aerosol interfaces.

Theoretical Study of an Authentic Hydrocarbon Ion Pair.

For many years researchers believed that hydrocarbons only contain covalent bonds. However, since 1985 Okamoto et al. demonstrated the formation of hydrocarbon salts in several systems, demolishing the structural principle that hydrocarbons only contain covalent bonds. Despite the great importance of this outcome to the study of chemical bonds, quantum chemical calculations on these systems are essentially nonexistent. The stability of the hydrocarbon ions along with the steric hindrance associated with the formation of the covalent bond contribute to their occurrence either in solution (dissociated) or in the solid state. These facts along with the common formation of ion pairs in solvents of low polarity motivated us to search for hydrocarbon ion pairs in the gas phase. Its energetics has also been studied in four nonprotic solvents, through a continuum solvation model (CPCM). DFT and CASSCF calculations indicate a metastable and highly polar ion pair between the tricyclopropylcyclopropenylium cation and a simplified Kuhn's anion. The barrier to the covalent structure varies from ∼4.8 to 14.4 kcal/mol, while the energy difference between the ion pair and the covalent form varies from ∼4.3 to 25.4 kcal/mol. The obtained theoretical results along with previous experimental results suggest the following strategy to obtain kinetically and thermodynamically stable hydrocarbon ion pairs: choose very stable hydrocarbon ions and systematically increase the steric hindrance between them.

Reply to “Artificially high stability of the sodium hydromolybdate ion pair in aqueous solution” by Gleb S. Pokrovski, Max Wilke and Maria A. Kokh

Reply to “Artificially high stability of the sodium hydromolybdate ion pair in aqueous solution” by Gleb S. Pokrovski, Max Wilke and Maria A. Kokh

Artificially high stability of the sodium hydromolybdate ion pair in aqueous solution. Comment on Guan et al. (2023) “Different metal coordination in sub- and super-critical fluids: Do molybdenum (IV) chloride complexes contribute to mass transfer in magmatic systems?”, Geochim. Cosmochim. Acta 354, 240–251

Artificially high stability of the sodium hydromolybdate ion pair in aqueous solution. Comment on Guan et al. (2023) “Different metal coordination in sub- and super-critical fluids: Do molybdenum (IV) chloride complexes contribute to mass transfer in magmatic systems?”, Geochim. Cosmochim. Acta 354, 240–251

Specificity of Photochemical Energy Conversion in PhotosystemI from the Green Microalga Chlorella ohadii.

Primary excitation energy transfer and charge separation in photosystemI(PSI) from the extremophile desert green alga Chlorella ohadii grown in low light were studied using broadband femtosecond pump-probe spectroscopy in the spectral range from 400 to 850nm and in the time range from 50fs to 500ps. Photochemical reactions were induced by the excitation into the blue and red edges of the chlorophyll Qy absorption band and compared with similar processes in PSI from the cyanobacterium Synechocystissp. PCC6803. When PSI from C.ohadii was excited at 660nm, the processes of energy redistribution in the light-harvesting antenna complex were observed within a time interval of up to 25ps, while formation of the stable radical ion pair P700+A1- was kinetically heterogeneous with characteristic times of 25 and 120ps. When PSI was excited into the red edge of the Qy band at 715nm, primary charge separation reactions occurred within the time range of 7ps in half of the complexes. In the remaining complexes, formation of the radical ion pair P700+A1- was limited by the energy transfer and occurred with a characteristic time of 70ps. Similar photochemical reactions in PSI from Synechocystis6803 were significantly faster: upon excitation at 680nm, formation of the primary radical ion pairs occurred with a time of 3ps in ~30% complexes. Excitation at 720nm resulted in kinetically unresolvable ultrafast primary charge separation in 50% complexes, and subsequent formation of P700+A1- was observed within 25ps. The photodynamics of PSI from C.ohadii was noticeably similar to the excitation energy transfer and charge separation in PSI from the microalga Chlamydomonas reinhardtii; however, the dynamics of energy transfer in C.ohadii PSI also included slower components.

Cholesterol sulfate-mediated ion-pairing facilitates the self-nanoassembly of hydrophilic cationic mitoxantrone

HypothesisHydrophilic cationic drugs such as mitoxantrone hydrochloride (MTO) pose a significant delivery challenge to the development of nanodrug systems. Herein, we report the use of a hydrophobic ion-pairing strategy to enhance the nano-assembly of MTO. ExperimentsWe employed biocompatible sodium cholesteryl sulfate (SCS) as a modification module to form stable ion pairs with MTO, which balanced the intermolecular forces and facilitated nano-assembly. PEGylated MTO-SCS nanoassemblies (pMS NAs) were prepared via nanoprecipitation. We systematically evaluated the effect of the ratio of the drug module (MTO) to the modification module (SCS) on the nanoassemblies. FindingsThe increased lipophilicity of MTO-SCS ion pair could significantly improve the encapsulation efficiency (∼97 %) and cellular uptake efficiency of MTO. The pMS NAs showed prolonged blood circulation, maintained the same level of tumor antiproliferative activity, and exhibited reduced toxicity compared with the free MTO solution. It is noteworthy that the stability, cellular uptake, cytotoxicity, and in vivo pharmacokinetic behavior of the pMS NAs increased in proportion to the molar ratio of SCS to MTO. This study presents a self-assembly strategy mediated by ion pairing to overcome the challenges commonly associated with the poor assembly ability of hydrophilic cationic drugs.

Anion Competition at Positively Charged Surfactant Monolayers.

Interactions of anions with hydrophobic surfaces of proteins and water-soluble polymers depend on the ability of the ions to shed their hydration shells. At positively charged surfactant monolayers, the interactions of anions are less well understood. Due to the interplay of electrostatic surface forces, hydration effects, and ion-ion interactions in the electrostatic double layer, a comprehensive microscopic picture remains elusive. Herein, we study the interactions of chloride, bromide, and a mixture of these two anions at the aqueous interface of dihexadecyldimethylammonium (DHDA+) and dioctadecyldimethylammonium (DODA+) cationic monolayers. Using molecular dynamics simulations and three surface-sensitive X-ray scattering techniques, we demonstrate that bromide interacts preferentially over chloride with both monolayers. The structure of the two monolayers and their interfacial electron density profiles obtained from the simulations quantitatively reproduce the experimental data. We observe that chloride and bromide form contact ion pairs with the quaternary ammonium groups on both monolayers. However, ion pairing with bromide leads to a greater reduction in the number of water molecules hydrating the anion, resulting in more energetically stable ion pairs. This leads to long-range (>3 nm) lateral correlations between bromide ions on the structured DODA+ monolayer. These observations indicate that ion hydration is the dominant factor determining the interfacial electrolyte structure.

Ionic Pairing and Selective Solvation of Butylmethylimidazolium Chloride Ion Pairs in DMSO-Water Mixtures: A Comprehensive Examination via Molecular Dynamics Simulations and Potentials of Mean Force Analysis.

Ionic liquids (ILs) with dimethyl sulfoxide (DMSO) and water act as a promising solvent medium for the dissolution of cellulose in an efficient manner. To develop a proper solvent system, it is really important to understand the thermodynamics of the molecular solutions consisting of ILs, DMSO, and water. The ion-pairing propensity of the ILs in the presence of DMSO and water plays a crucial role in governing the property of the solvent mixtures. Employing all-atom molecular dynamics simulations, we estimate the potentials of mean force between BMIM+ and Cl- ions in DMSO-water mixtures. Analysis reveals a significant increase in the thermodynamic stability of both contact ion pair (CIP) and solvent-assisted ion pair (SAIP) states with a rising DMSO mole fraction. Thermodynamic assessments highlight the entropic stabilization of CIP states and SAIP states in pure water, in DMSO-water mixtures, and in pure DMSO. The structural analysis reveals that in comparison to the DMSO local density, the local water density is relatively very high around ion pairs, more specifically in the solvation shell of a chloride ion. Preferential binding coefficients also consistently indicate exclusion of DMSO from the ion pair in DMSO-water mixtures. To enhance our understanding regarding the solvent molecules kinetics around the ion pairs, the survival probabilities of DMSO and water are computed. The calculations reveal that the water molecules prefer a prolonged stay in the solvation shell of Cl- ions.

Stabilization of Naphthalene Diimide Anions by Ion Pair Formation in Nonaqueous Organic Redox Flow Batteries.

In redox flow batteries, a compelling strategy for enhancing the charge capacity of redox-active organic molecules involves storing multiple electrons within a single molecule. However, this approach poses unique challenges such as chemical instability by forming radicals, elevated energy requirements, and unsustainable charge concentration. Ion pairing is a possible solution to achieve charge neutrality and engineer redox potential shifts but has received limited attention. In this study, we demonstrate that Li+ can stabilize naphthalene diimide (NDI) anions dissolved in acetonitrile and significantly shift the second cathodic potential close to the first. Our findings, supported by density functional theory calculations and Fourier transform infrared spectroscopy, indicate that dimeric NDI species form stable ion pairs with Li+. Conversely, K+ ions exhibit weak interactions, and cyclic voltammograms confirm significant potential shifts when stronger Lewis acids and solvents with lower donor numbers are employed. Galvanostatic examinations reveal a single voltage plateau with Li+, which indicates a rapid redox process involving doubly charged NDI2- with Li+. These aggregated ion pairs offer the additional benefits of hindering crossover events, contributing to excellent cyclability, and suppressing undesirable side reactions even after 1000 redox cycles.

Structures of Th4+ aqueous solutions: insights from AIMD and metadynamics simulations.

Solution chemistry of actinide ions is critical to understanding the solvation behaviors and hydrolysis process. Using tetravalent thorium ion Th4+ as a representative example, we investigate the local structures and dynamic behaviors of hydrated Th4+ ions by ab initio molecular dynamics (AIMD) simulations using the recently developed norm-conserving pseudopotentials and basis sets optimized for actinides (J.-B. Lu et al., J. Chem. Theory Comput. 2021, 17, 3360-3371). AIMD simulations reveal two distinct solvation shells, with the first shell comprising 9 water molecules at approximately rTh-O = 2.50 Å and exhibiting a tricapped trigonal prism geometry. These conclusions are confirmed through metadynamics simulations and further structural analysis. AIMD simulations also show the slight effect of temperature and counterions on the structure of the solution. The structured solvation shells of the highly charged Th4+ ion with the specific geometry, distinct from the structure of liquid water, lead to corresponding structural changes in the hydrogen bond network in water. Additionally, beyond the solvent-shared ion pair (SIP) state observed in the unbiased AIMD simulations, the metadynamics simulations reconstruct a two-dimensional free energy surface that clearly indicates the potential stability of the contact ion pair (CIP) state in the system with Cl- as a counterion. The findings in this work provide insights into the solution chemistry of actinides and serve as a reference for studying other actinide solution systems.

In operando NMR investigations of the aqueous electrolyte chemistry during electrolytic CO2 reduction

The electrolytic reduction of CO2 in aqueous media promises a pathway for the utilization of the green house gas by converting it to base chemicals or building blocks thereof. However, the technology is currently not economically feasible, where one reason lies in insufficient reaction rates and selectivities. Current research of CO2 electrolysis is becoming aware of the importance of the local environment and reactions at the electrodes and their proximity, which can be only assessed under true catalytic conditions, i.e. by in operando techniques. In this work, multinuclear in operando NMR techniques were applied in order to investigate the evolution of the electrolyte chemistry during CO2 electrolysis. The CO2 electroreduction was performed in aqueous NaHCO3 or KHCO3 electrolytes at silver electrodes. Based on 13C and 23Na NMR studies at different magnetic fields, it was found that the dynamic equilibrium of the electrolyte salt in solution, existing as ion pairs and free ions, decelerates with increasingly negative potential. In turn, this equilibrium affects the resupply rate of CO2 to the electrolysis reaction from the electrolyte. Substantiated by relaxation measurements, a mechanism was proposed where stable ion pairs in solution catalyze the bicarbonate dehydration reaction, which may provide a new pathway for improving educt resupply during CO2 electrolysis.

Development of mabuterol transdermal patch: Molecular mechanism study of ion-pair improving patch stability

This paper aimed to prepare a Mabuterol (MAB) patch for treating asthma by ion-pair strategy to overcome the drug's thermal instability and elucidate the molecular mechanisms of the stabilization effect. The formulation factor, including counter-ion and pressure-sensitive adhesive (PSA), was optimized by the stability and in vitro skin permeation studies. The molecular mechanism of ion-pair stability was characterized using TGA, Raman, FT-IR, NMR, XPS, and molecular modeling. The optimized patch comprised MAB-Lactic acid (MAB-LA) and hydroxyl adhesive (AAOH) as the matrix, with Q = 126.47 ± 9.75 μg/cm2 and Fabs = 75.27%. The increased TGA (213.11 °C), disproportionation energy (ΔG = 97.44 KJ), and ion-pair lifetime (Tlife = 2.21 × 103) indicated that the counter-ion improved MAB stability through strong ionic and hydrogen bonds with LA. The remaining drug content in the MAB-LA patch was 15% higher than that of the pure MAB patch after storage for 12 months at room temperature, which was visualized by Raman imaging. The interaction between MAB-LA and AAOH PSA via hydrogen bond decreased the diffusion rate and increased the drug stability further. This study successfully developed the MAB patch, which provided a reference for applying ion-pairing strategies to improve the stability of transdermal patches.

Outstanding Performance of the Deep Eutectic Solvent-Based Aqueous Biphasic System Constructed with Sodium Citrate for a Green Gold Separation.

Aqueous biphasic systems (ABSs) that are based on deep eutectic solvents (DESs) are environmentally benign systems to use for metal ion separation. In this work, a series of DESs was synthesized for the first time with PEG 400 as hydrogen bond donors and tetrabutylphonium bromide (P4Br), tetrabutylammonium bromide (N4Br), or tetrabutylammonium chloride (N4Cl) as hydrogen bond acceptors, and then they were combined with citrate (Na3C6H5O7), which is eco-friendly, to construct an ABS for use in the separation of Au(I) from an aurocyanide solution. Phase diagrams of DESs + Na3C6H5O7 + H2O systems were constructed using the experimentally measured data. Multiple factors that affect the efficiency of the gold extraction were studied; these factors were the species of salt or DES and their content, the equilibrium pH, the oscillation time, and the initial gold concentration. Gold(I) is preferentially retained in the DES-rich phase, and the P4Br:PEG 1:2 + Na3C6H5O7 + H2O system has a high extraction efficiency of 100.0% under optimized conditions. FT-IR, NMR, and TEM characterizations and DFT calculations show that the migration of Au(I) from the salt-rich to the DES-rich phase follows an ion exchange mechanism. Specifically, Au(CN)2- replaces Br- in the original P4Br and generates a stable ion pair with the quaternary phosphonium salt cation, P+, and this replacement is driven by electrostatic attractions. A new strong hydrogen bond network simultaneously forms between the anionic Au(CN)2- and the -OH group in the PEG 400 component. Finally, the gold of Au(I)-loaded P4Br:PEG 1:2 can be successfully reduced by sodium borohydride with an efficiency of 100.0%. The strategy to extract gold(I) from alkaline cyanide solutions using an ABS based on DESs as proposed in this work provides a potential platform for developing green technology for recovering gold.

The impact of oxide impurity on the structure of FLiNaK and FLiNaK–CeF3 melts: A simulation study

Oxide impurities affect the performance of the molten mixtures applicable in molten salt reactors. To reveal the underlying reasons of changes in physicochemical properties, the in-depth investigations on microscopic structure of melts are demanded. In this study, the FLiNaK and FLiNaK–CeF3 molten mixtures with small addition of Li2O were simulated within the ab initio theory to obtain the local structure patterns. The emphasis is placed on oxygen-based groupings since these are expected to be the most stable. In order to estimate the stability, the calculation of ion pair lifetimes was performed. It is shown that the most probable ion groupings existing in the FLiNaK–Li2O melt are [OLi5K], [OLi5K2] and [OLi4NaK2], while the FLiNaK–Li2O–CeF3 melt contain [OCe2Li2], [OCe2Li2K], [OCe2Li3] and [OCe3Li] units predominantly. Within the groupings, the most stable ion pairs are O-Li or O-Ce, depending on the melt composition.

Superbasicity of imines with bicyclo[5.1.0]octa-1,3,5,7-tetraene scaffold due to electron delocalization in the conjugated acids

Gas phase basicity of organic superbases with bicyclo[5.1.0]octa-1,3,5,7-tetraene (fused cyclopropene and cycloheptatriene) scaffold and an imine group as proton acceptor site was computationally assessed using B3LYP/6-311++G(d,p) method. The basicity of these imines originates from the electron delocalization and consequently delocalization of the positive charge upon protonation leading to stability of the conjugated acid. Also, the effects of different electron donating groups (EDGs) and hydrogen bond formation on the basicity enhancement were studied. The calculated PAs were 1000–1307 kJ mol−1 indicating superbasicity and hyperbasicity of the studied organic bases. These organic bases can deprotonate H2SO4 and FHSO4 in the gas phase and produce stable ion pair clusters.

Exploring the structural and dynamic differences between human carnosinase I (CN1) and II (CN2).

Human carnosinases (CNs) are dimeric dipeptidases in the metallopeptidase M20 family. Two isoforms of carnosinases (Zn2+ -containing carnosinase 1 (CN1) found in serum and Mn2+ -carnosinase 2 (CN2) in tissue) were identified. Both CNs cleave histidine-containing (Xaa-His) dipeptides such as carnosine where CN2 was found to accept a broader spectrum of substrates. A loss of CN function, resulting in a high carnosine concentration, reduces risk for diabetes and neurological disorders. Although several studies on CN activities and its Michaelis complex were conducted, all shed the light on CN1 activity where the CN2 data is limited. Also, the molecular details on CN1 and CN2 similarity and dissimilarity in structure and function remain unclear. Thus, in this work, molecular dynamics (MD) simulations were employed to study structure and dynamics of human CN1 and CN2 in comparison. The results show that the different catalytic ability of both CNs is due to their pocket size and environment. CN2 can accept a wider range of substrate due to the wider mouth of a binding pocket. The L1 loop seems to play a role in gating activity. Comparing to CN2, CN1 provides more electronegative entrance, more wettability, and higher stability of catalytic metal ion-pair in the active site which allow more efficient water-mediated catalysis. The microscopic understanding obtained here can serve as a basis for CN inhibition strategies resulting in higher carnosine levels and consequently mitigating complications associated with diseases such as diabetes and neurological disorder.

A chiral pentanidium and pyridinyl-sulphonamide ion pair as an enantioselective organocatalyst for Steglich rearrangement.

Enantioselective ion pair catalysis has gained significant attention due to its ability to exert selectivity control in various reactions. Achiral counterions have been found to play crucial roles in modulating reactivity and selectivity. The modular nature of an ion pair catalyst allows rapid alterations of the achiral counterion to achieve optimal outcomes, without the need to modify the more onerous chiral component. In this study, we report the successful development of a stable chiral pentanidium pyridinyl-sulphonamide ion pair as a nucleophilic organocatalyst for asymmetric Steglich rearrangement. The ion pair catalyst demonstrated excellent performance, leading to enantioenriched products with up to 99% ee through simple alterations of the achiral anions. We conducted extensive ROESY experiments and concluded that the reactivity and enantioselectivity were correlated to the formation of a tight ion pair in solution. Further computational analyses provided greater clarity to the structure of the ion pair catalyst in solution. Our findings reveal the critical roles of NMR experiments and computational analyses in the design and optimisation of ion pair catalysts.

Stable Radical Ion Pairs Induced by Single Electron Transfer: Frustrated Versus Nonfrustrated

Stable Radical Ion Pairs Induced by Single Electron Transfer: Frustrated Versus Nonfrustrated

Development of Spectrophotometric Procedures for the Simultaneous Determination of Voxilaprevir, Sofosbuvir and Velpatasvir in Pure Form and in Tablets Ion-Association Complex Method

Two spectrophotometric procedures are developed for the estimation of three antihepatitic drugs namely Voxilaprevir (VOXL), Sofosbuvir (SOFOS) and Velpatasvir (VELP) in pure form and in different pharmaceutical formulations. The first method is based on the oxidation of velpatasvir and sofosbuvir with potassium iodate in an acidic medium followed by extracting the liberated iodine with cyclohexane and measurement at λ= 525 nm. Beer’s law is valid in the concentration ranges from 10–400 and 5–400 µg/ml for velpatasvir and sofosbuvir, respectively. The apparent molar absorptivity of the resulting-colored complex was found to be 1.34 × 10 for velpatasvir and sofosbuvir, respectively. The second method is based on the interaction of the antithepatatis drugs, Voxilaprevir, Velpatasvir and sofosbuvir in 1,2-dichloroethane with bromophenol blue (BPB), bromocresol green (BCG) and bromocresol purple (BCP) in the same solvent to produce stable colored ion pairs with maximum absorbance at 385–405 nm. Regression analysis of Beer’s plots showed good correlation in the concentration ranges 10–60, 10–60 and 5–40 µg/mL for voxilaprevir and 1.64 × 103 l.mol−1.cm−1 for velpatasvir, 10–150, 10–150 and 10–60 µg/mL−for sofosbuvir with BPB, BCG and BCP reagents, respectively. The limit of detection is 0.46–7.69 µgml11 for sofosbuvir and 10–250, 10–150 and 10–100 µgml−1 and limits of quantitation range between 1.52–8.53 µgml1. The optimum assay conditions were investigated and the recovery of the drugs from their dosage forms ranged from 99.33% to 100.5%. Intraday relative standard deviations (RSD) were 0.029–1.397% and the correlation coefficients ranged from 0.9992 to 1. The two methods can be applied successfully for the determination of these drugs in tablets. The results of analysis were validated statistically through recovery studies.

Microscopic Insights Into the Formation of Methanesulfonic Acid–Methylamine–Ammonia Particles Under Acid-Rich Conditions

Understanding the microscopic mechanisms of new particle formation under acid-rich conditions is of significance in atmospheric science. Using quantum chemistry calculations, we investigated the microscopic formation mechanism of methanesulfonic acid (MSA)–methylamine (MA)–ammonia (NH3) clusters. We focused on the binary (MSA)2n-(MA)n and ternary (MSA)3n-(MA)n-(NH3)n, (n = 1–4) systems which contain more acid than base molecules. We found that the lowest-energy isomers in each system possess considerable thermodynamic and dynamic stabilities. In studied cluster structures, all bases are protonated, and they form stable ion pairs with MSA, which contribute to the charge transfer and the stability of clusters. MA and NH3 have a synergistic effect on NPF under acid-rich conditions, and the role of NH3 becomes more remarkable as cluster size increases. The excess of MSA molecules does not only enhance the stability of clusters, but provides potential sites for further growth.