Solvent-separated Pairs Research Articles

The Interplay of Solvation and Polarization Effects on Ion Pairing in Nanoconfined Electrolytes.

The nature of ion-ion interactions in electrolytes confined to nanoscale pores has important implications for energy storage and separation technologies. However, the physical effects dictating the structure of nanoconfined electrolytes remain debated. Here we employ machine-learning-based molecular dynamics simulations to investigate ion-ion interactions with density functional theory level accuracy in a prototypical confined electrolyte, aqueous NaCl within graphene slit pores. We find that the free energy of ion pairing in highly confined electrolytes deviates substantially from that in bulk solutions, observing a decrease in contact ion pairing but an increase in solvent-separated ion pairing. These changes arise from an interplay of ion solvation effects and graphene's electronic structure. Notably, the behavior observed from our first-principles-level simulations is not reproduced even qualitatively with the classical force fields conventionally used to model these systems. The insight provided in this work opens new avenues for predicting and controlling the structure of nanoconfined electrolytes.

Partially Ion-Paired Solvation Structure Design for Lithium-Sulfur Batteries under Extreme Operating Conditions.

Achieving increased energy density under extreme operating conditions remains a major challenge in rechargeable batteries. Herein, we demonstrate an all-fluorinated ester-based electrolyte comprising partially fluorinated carboxylate and carbonate esters. This electrolyte exhibits temperature-resilient physicochemical properties and moderate ion-paired solvation, leading to a half solvent-separated and half contact-ion pair in a sole electrolyte. As a result, facile desolvation and preferential reduction of anions/fluorinated co-solvents for LiF-dominated interphases are achieved without compromising ionic conductivity (>1 mS cm-1 even at -40 °C). These advantageous features were found to apply to both lithium metal and sulfur-based electrodes even under extreme operating conditions, allowing stable cycling of Li || sulfurized polyacrylonitrile (SPAN) full cells with high SPAN loading (>3.5 mAh cm-2 ) and thin Li anode (50 μm) at -40, 23 and 50 °C. This work offers a promising path for designing temperature-resilient electrolytes to support high energy density Li metal batteries operating in extreme conditions.

Ion-pair reorganization regulates reactivity in photoredox catalysts.

Cyclometalated and polypyridyl complexes of d6 metals are promising photoredox catalysts, using light to drive reactions with high kinetic or thermodynamic barriers via the generation of reactive radical intermediates. However, while tuning of their redox potentials, absorption energy, excited-state lifetime and quantum yield are well-known criteria for modifying activity, other factors could be important. Here we show that dynamic ion-pair reorganization controls the reactivity of a photoredox catalyst, [Ir[dF(CF3)ppy]2(dtbpy)]X. Time-resolved dielectric-loss experiments show how counter-ion identity influences excited-state charge distribution, evincing large differences in both the ground- and excited-state dipole moment depending on whether X is a small associating anion (PF6-) that forms a contact-ion pair versus a large one that either dissociates or forms a solvent-separated pair (BArF4-). These differences correlate with the reactivity of the photocatalyst toward both reductive and oxidative electron transfer, amounting to a 4-fold change in selectivity toward oxidation versus reduction. These results suggest that ion pairing could be an underappreciated factor that modulates reactivity in ionic photoredox catalysts.

Crowding effects on water-mediated hydrophobic interactions.

Understanding the fundamental forces such as hydrophobic interactions in a crowded intracellular environment is necessary to comprehensively decipher the mechanisms of protein folding and biomolecular self-assemblies. The widely accepted entropic depletion view of crowding effects primarily attributes biomolecular compaction to the solvent excluded volume effects exerted by the "inert" crowders, neglecting their soft interactions with the biomolecule. In this study, we examine the effects of chemical nature and soft attractive energy of crowders on the water-mediated hydrophobic interaction between two non-polar neopentane solutes using molecular dynamics simulations. The crowded environment is modeled using dipeptides composed of polar and non-polar amino acids of varying sizes. The results show that amongst the non-polar crowders, Leu2 strengthens the hydrophobic interactions significantly, whereas the polar and small-sized non-polar crowders do not show significant strengthening. Distinct underlying thermodynamic driving forces are illustrated where the small-sized crowders drive hydrophobic interaction via a classic entropic depletion effect and the bulky crowders strengthen it by preferential interaction with the solute. A crossover from energy-stabilized solvent-separated pair to entropy-stabilized contact pair state is observed in the case of bulky non-polar (Leu2) and polar (Lys2) crowders. The influence of solute-crowder energy in affecting the dehydration energy penalty is found to be crucial for determining the neopentane association. The findings demonstrate that along with the entropic (size) effects, the energetic effects also play a crucial role in determining hydrophobic association. The results can be extended and have implications in understanding the impact of protein crowding with varying chemistry in modulating the protein free energy landscapes.

Ion Pairing and Dielectric Decrement in Glycosaminoglycan Brushes.

Cell-surface polysaccharides are essential to many aspects of physiology, serving as a highly conserved evolutionary feature of life and as an important part of the innate immune system in mammals. Here, as simplified biophysical models of these sugar coatings, we present results of molecular dynamics simulations of hyaluronic acid and heparin brushes that show important effects of ion pairing, water dielectric decrease, and coion exclusion. As in prior studies of macromolecular crowding under physiologically relevant salt concentrations, our results show equilibria with electroneutrality attained through screening and pairing of brush anionic charges by monovalent cations at the atomistic detail. Most surprising is the reversal of the Donnan potential obtained from both nonpolarizable and Drude polarizable force fields, in contrast to what would be expected based on electrostatic Boltzmann partitioning alone. Water dielectric decrement within the brush domain is also associated with Born hydration-driven cation exclusion from the brush. We observe that the primary partition energy attracting cations to attain brush electroneutrality is the ion pairing or salt-bridge energy. Potassium and sodium pairings to glycosaminoglycan carboxylates and sulfates show similar abundance of contact-pairing and solvent-separated pairing. We conclude that in these crowded macromolecular brushes, ion-pairing, Born-hydration, and electrostatic potential energies all contribute to attain electroneutrality and should therefore contribute in mean-field models to accurately represent brush electrostatics.

Алгоритмы численного моделирования сверхбыстрого фотоиндуцированного межмолекулярного переноса заряда в жидкостях

Предложены подходы к численному решению систем уравнений, описывающих кинетику двухстадийной фотохимической реакции в вязком полярном растворителе. Математическая модель построена на основе расширенной интегральной теории встреч и учитывает диффузионную подвижность молекул-реагентов в жидкости, неравновесность среды и внутримолекулярных степеней свободы, удаленный перенос электрона в донорно-акцепторных парах, разделенных растворителем. В рамках метода броуновского моделирования разработаны алгоритмы расчета безреакционных стохастических траекторий частиц на поверхностях свободной энергии, соответствующих различным состояниям реагентов и продуктов, схемы детектирования реакционных событий и генерации электронных прыжков, а также алгоритмы расчета нестационарных потоков частиц между электронными состояниями и вычисления интегральных ядер кинетических уравнений. Представлены результаты тестовых расчетов, демонстрирующие корректность численного решения и воспроизводящие известные особенности реакций электронного переноса в полярных жидкостях. Efficient approaches to the numerical solution of equations describing the kinetics of two-stage photochemical reactions in a viscous polar solvent are proposed. The mathematical model is based on the extended integral encounter theory and takes into account diffusive mobility of reactants in solution, nonequilibrium of solvent and intramolecular degrees of freedom, and remote electron transfer in solvent-separated donor-acceptor pairs. In the framework of the Brownian simulation technique, a number of numerical algorithms for calculating unreactive stochastic trajectories of particles on free energy surfaces corresponding to different electronic states of reactants and products are suggested, some computational schemes for the detection of reaction events and the generation of electronic hops are developed, and algorithms for calculating the time-dependent reaction fluxes between the electronic states and integral kernels of the kinetic equations are implemented. The results of test simulations demonstrating the validity of the numerical solutions and reproducing well-known features of electron transfer reactions in polar solvents are discussed.

Chloride ions as integral parts of hydrogen bonded networks in aqueous salt solutions: the appearance of solvent separated anion pairs

Hydrogen bonding to chloride ions has been frequently discussed over the past 5 decades. Still, the possible role of such secondary intermolecular bonding interactions in hydrogen bonded networks has not been investigated in any detail. Here we consider computer models of concentrated aqueous LiCl solutions and compute the usual hydrogen bond network characteristics, such as distributions of cluster sizes and of cyclic entities, both for models that take and do not take chloride ions into account. During the analysis of hydrogen bonded rings, a significant amount of 'solvent separated anion pairs' have been detected at high LiCl concentrations. It is demonstrated that taking halide anions into account as organic constituents of the hydrogen bonded network does make the interpretation of structural details significantly more meaningful than when considering water molecules only. Finally, we compare simulated structures generated by 'good' and 'bad' potential sets on the basis of the tools developed here, and show that this novel concept is, indeed, also helpful for distinguishing between reasonable and meaningless structural models.

Progressive Hydrophobicity of Fluorobenzenes.

The potentials of mean force for the dimers of fluorobenzenes sample both π-stacked and T-shaped structures for partially fluorinated benzenes, namely, 1,4-difluorobenzene, 1,3,5-trifluorobenzene, and 1,2,4,5-tetrafluorobenzene, and sample only the T-shaped structures for benzene and hexafluorobenzene. While the free energy for the dimerization in water is very weakly dependent on the number of fluorine atoms, the formation of π-stacked structures is entropy-driven and the T-shaped structures appear due to an enthalpic minimum. Interestingly, the solvation behavior suggests that the accumulation of water around the contact and solvent-separated pairs decreases with the increase in the number of fluorine atoms, which signifies progressive hydrophobicity of fluorobenzenes.

Pairwise Hydrophobicity at Low Temperature: Appearance of a Stable Second Solvent-Separated Minimum with Possible Implication in Cold Denaturation.

The hydrophobic effect appears to be a key driving force for many chemical and biological processes, such as protein folding, protein-protein interactions, membrane bilayer self-assembly, and so forth. In this study, we calculated the potential of mean force (PMF) using umbrella sampling technique between different model hydrophobes (methane-methane, cyclobutane-cyclobutane, and between two rodlike hydrophobes) at lower than ambient temperatures (300, 260, and 240 K). We find the appearance of a second solvent-separated minimum at ∼1.0 nm apart from the usual contact and first solvent-separated minimum in the PMF profile of the methane pair at low temperature. In the PMF between both cyclobutane and the rodlike hydrophobe pairs, the second solvent-separated pair (SSSP) becomes even more stable than the first solvent-separated pair (FSSP) at 240 K. Analysis of the water structure shows that, at 240 K, the core water of SSSP for the rodlike hydrophobe pair is more strongly hydrogen bonded and more tetrahedrally oriented than that of the FSSP. Strongly hydrogen-bonded ordered water molecules implicate strong water-water interactions, which are responsible for stabilization of SSSP at low temperature. This weakening of hydrophobic interactions through stabilization of SSSP may play a key role in the cold denaturation of protein.

Effects of Concentration on Like-Charge Pairing of Guanidinium Ions and on the Structure of Water: An All-Atom Molecular Dynamics Simulation Study.

Like-charge ion-pair formation in an aqueous solution of guanidinium chloride (GdmCl) has two important facets. On one hand, it describes the role of the arginine (ARG) side chain in aggregation and dimer formation in proteins, and on the other hand, it lends support for the direct mechanism of protein denaturation by GdmCl. We employ all-atom molecular dynamics simulations to investigate the effect of GdmCl concentration on the like-charge ion-pair formation of guanidinium ions (Gdm(+)). From analyses of the radial distribution function (RDF) between the carbon atoms of two guanidinium moieties, the existence of both contact pairs and solvent-separated pairs has been observed. Although the peak height corresponding to the contact-pair state decreases, the number of Gdm(+) ions in the contact-pair state actually increases with increasing GdmCl concentration. We have also investigated the effect of the concentration of Gdm(+) on the structure of water. The effect of GdmCl concentration on the radial and tetrahedral structures of water is found to be negligibly small; however, GdmCl concentration has a considerable effect on the hydrogen-bonding structure of water. It is demonstrated that the presence of chloride ions, not Gdm(+), in the first solvation shell of water causes the distortion in the hydrogen-bonding network of water. In order to establish that Gdm(+) not only stacks against another Gdm(+) but also directly attacks the ARG residue of a protein or peptide, simulation of an ARG-rich peptide in 6 M aqueous solution of GdmCl has been performed. The analyses of RDFs and orientation distributions reveal that the Gdm(+) moiety of the GdmCl attacks the same moiety in the ARG side chain with a parallel stacking orientation.

Bis-nickel-bridged p-terphenyl dianion – Synthesis and structures

In this paper we present the synthesis and characterisation of novel bisnickelacyclic complexes. We carried out the reaction between 2,2′,2″,5′-tetralithio-1,1′,4′,1″-terphenyl and nickelocene in which bisnickelacyclic–dilithium complex was formed. It represented the contact ion pair that consisted of nickelacyclic dianion (the first of it's kind) and lithium cations. Exchange of solvent from diethyl ether to DME (1,2-dimethoxyethane) afforded solvent separated ionic pair. Both of these complexes exhibited paramagnetic behaviour, so structural investigations had to be carried out by means of X-ray diffraction. DFT calculations were employed for better understanding of electronic structure of these complexes.

Responses of polar organic compounds to different ionic environments in aqueous media are interrelated.

Solubilities of 17 polar organic compounds in aqueous solutions of Na2SO4, NaCl, NaClO4, and NaSCN at the salt concentrations of up to 1.0-2.0 M were determined and the Setschenow constant, ksalt, values were estimated. It was found that NaClO4 may display both salting-in and salting-out effects depending on the particular compound structure. The Setschenow constant values for all the polar compounds examined in different salt solutions are found to be interrelated. Similar relationships were observed for partition coefficients of nonionic organic compounds in aqueous polyethylene glycol-sodium sulfate two-phase systems in the presence of different salt additives reported previously [Ferreira et al., J. Chromatogr. A, 2011, 1218, 5031], and for the effects of different salts on optical rotation of amino acids reported by Rossi et al. [J. Phys. Chem. B, 2007, 111, 10510]. In order to explain the observed relationships it is suggested that all the effects observed originate as responses of the compounds to the presence of a given ionic environment and its interaction with the compounds by forming direct or solvent-separated ionic pairs. The response is compound-specific and its strength is determined by the compound structure and the type (and concentration) of ions inducing the response.

Concentration-dependent like-charge pairing of guanidinium ions and effect of guanidinium chloride on the structure and dynamics of water from all-atom molecular dynamics simulation

An all-atom molecular dynamics simulation shows concentration-dependent like-charge ion pairing of the guanidinium ion in an aqueous solution of guanidinium chloride. We have observed two types of like-charge ion pairing for guanidinium ions, namely, stacked ion pairs and solvent-separated ion pairs. Interestingly, both of these like-charge ion-pair formations are dependent on the concentration of guanidinium chloride in water. The probability of stacked like-charge ion-pair formation decreases, whereas, the probability of solvent-separated like-charge pairing increases as the concentration of guanidinium chloride increases, which is shown from radial distribution functions and is confirmed from the energy calculations. Besides like-charge ion-pair formation, we also investigated guanidinium chloride induced changes in water structure. Hydrogen-bond analysis indicates that guanidinium chloride does not alter the strict-hydrogen-bonding patterns of water, whereas, it breaks the bend-hydrogen bond and the non-hydrogen-bonding patterns. Tetrahedral order, nearest neighbor orientation, and distance distribution of water molecules around a probe water molecule show the extent of water structure distortion.

Beyond the Diffuse Layer Theory: A Molecular Analysis of the Structural, Dynamical, and Mechanical Properties of Charged Solid/Liquid Interfaces

This work focuses on the long-range gap detected during the swelling of charged solid/liquid interfaces neutralized by monovalent counterions. A molecular model is used to investigate the influence of the long-range electrostatic coupling on the thermodynamical properties of charged solid/liquid interfaces at separations varying between 20 and 40 Å, corresponding to the transition between discontinuous crystalline swelling, detected below 20 Å, and the continuous osmotic swelling, detected at separations larger than 40 Å. While the primitive model perfectly reproduces the stability of these charged interfaces in the osmotic swelling regime, it becomes inappropriate in the crystalline swelling regime, since it neglects the organization of the confined water molecules which is responsible for the discontinuous crystalline swelling. In that context, we study the influence of the valence of the neutralizing counterions and the location of the electric charges embedded within the charged lamellae on the structural, dynamical, and mechanical properties of these interfacial systems. This molecular approach clearly identifies two different environments accessible to the neutralizing sodium counterions, corresponding respectively to solvent separated and contact ionic pairing. The mobility of the confined counterions and water molecules is quantified by numerical simulations of molecular dynamics. Finally, the interfacial free energy provides, without introducing any additional parameter, the first theoretical interpretation of the coexistence between swollen and collapsed charged interfaces neutralized by monovalent counterions, in agreement with experimental evidence.

Probabilistic Approach to the Length-Scale Dependence of the Effect of Water Hydrogen Bonding on Hydrophobic Hydration

We present a probabilistic approach to water-water hydrogen bonding that allows one to obtain an analytic expression for the number of bonds per water molecule as a function of both its distance to a hydrophobic particle and hydrophobe radius. This approach can be used in density functional theory (DFT) and computer simulations to examine particle size effects on the hydration of particles and on their solvent-mediated interaction. For example, it allows one to explicitly identify a water hydrogen bond contribution to the external potential, whereto a water molecule is subjected near a hydrophobe. The DFT implementation of the model predicts the hydration free energy per unit area of a spherical hydrophobe to be sharply sensitive to the hydrophobe radius for small radii and weakly sensitive thereto for large ones; this corroborates the vision of the hydration of small and large length-scale particles as occurring via different mechanisms. On the other hand, the model predicts that the hydration of even apolar particles of small enough radii may become thermodynamically favorable owing to the interplay of the energies of pairwise (dispersion) water-water and water-hydrophobe interactions. This sheds light on previous counterintuitive observations (both theoretical and simulational) that two inert gas molecules would prefer to form a solvent-separated pair rather than a contact one.

A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

Bond cleavage and bond formation are central to organic chemistry. Carbocations play a key role in our understanding of nucleophilic substitution reactions that involve both processes. The precise understanding of the mechanism and dynamics of the photogeneration of carbocations and carbon radicals is therefore an important quest. In particular, the role of electron transfer for the generation of carbocations from the radical pair is still unclear. A quantitative femtosecond absorption study is presented, with ultrabroad probing on selected donor and acceptor substituted benzhydryl chlorides irradiated with 270 nm (35 fs) pulses. The ultrafast bond cleavage within 300 fs is almost exclusively homolytic, thus leading to a radical pair. The carbocations observable in the nanosecond regime are generated from these radicals by electron transfer from the benzhydryl to the chlorine radical within the first tens of picoseconds. Their concentration is reduced by geminate recombination within hundreds of picoseconds. In moderately polar solvents this depletion almost extinguishes the cation population; in highly polar solvents free ions are still observable on the nanosecond timescale. The explanation of the experimental findings requires the microscopic realm of the intermediates to be accounted for, including their spatial and environmental distributions. The distance dependent electron transfer described by Marcus theory is combined with Smoluchowski diffusion. The depletion of the radical pair distribution at small distances causes a temporal increase of the mean distance and the observed stretched exponential electron transfer. A close accord with experiment can only be reached for a broad distribution of the nascent radical pairs. The increase in the inter-radical and inter-ion pair distance is measured directly as a shift of the UV/Vis absorption of the products. The results demonstrate that, at least for aprotic solvents, traditional descriptions of reaction mechanisms based on the concept of contact and solvent-separated pairs have to be reassessed.

Cations Bind Only Weakly to Amides in Aqueous Solutions

We investigated salt interactions with butyramide as a simple mimic of cation interactions with protein backbones. The experiments were performed in aqueous metal chloride solutions using two spectroscopic techniques. In the first, which provided information about contact pair formation, the response of the amide I band to the nature and concentration of salt was monitored in bulk aqueous solutions via attenuated total reflection Fourier transform infrared spectroscopy. It was found that molar concentrations of well-hydrated metal cations (Ca(2+), Mg(2+), Li(+)) led to the rise of a peak assigned to metal cation-bound amides (1645 cm(-1)) and a decrease in the peak associated with purely water-bound amides (1620 cm(-1)). In a complementary set of experiments, the effect of cation identity and concentration was investigated at the air/butyramide/water interface via vibrational sum frequency spectroscopy. In these studies, metal ion-amide binding led to the ordering of the adjacent water layer. Such experiments were sensitive to the interfacial partitioning of cations in either a contact pair with the amide or as a solvent separated pair. In both experiments, the ordering of the interactions of the cations was: Ca(2+) > Mg(2+) > Li(+) > Na(+) ≈ K(+). This is a direct cationic Hofmeister series. Even for Ca(2+), however, the apparent equilibrium dissociation constant of the cation with the amide carbonyl oxygen was no tighter than ∼8.5 M. For Na(+) and K(+), no evidence was found for any binding. As such, the interactions of metal cations with amides are far weaker than the analogous binding of weakly hydrated anions.

Helium diffusion in aqueous sodium chloride solution at high pressures

A molecular dynamic investigation of self-diffusion coefficients of helium and water in aqueous NaCl solution is performed. To understand the nonlinear behavior of the self-diffusion coefficients, a number of structural characteristics (radial distribution functions, hydrogen bond network topology) are calculated. The electrolyte concentration range is discussed in which a local increase in helium self-diffusion coefficient is observed. It is shown that this behavior is caused by a breakdown of the network of hydrogen bonds due to competition between ion positions in the states of solvent-separated and contact pairs.

Magnetic Field Effects on Photochemical Reactions in Ionic Liquids with Short Alkyl Chains

Magnetic field effects (MFEs) on the photoinduced hydrogen abstraction reactions of benzophenone and thiobenzophenone from thiophenol were investigated in several ionic liquids with short alkyl chains by a nanosecond laser flash photolysis technique. In each ionic liquid, escaped radical yields of benzophenone and thiobenzophenone ketyl radicals gradually decreased with increasing magnetic field strength. The observed results were analyzed by using the stochastic Liouville equation (SLE), employing a solvent separated radical pair (SSRP) model where the SSRP with a specific radical–radical distance is stable and has a long lifetime. The SLE analysis revealed that the lifetime of the SSRP and the radical rotation in the SSRP states are strongly correlated with the macroviscosity of the ionic liquids.

Photochemistry of Hydrogen Halides on Water Clusters: Simulations of Electronic Spectra and Photodynamics, and Comparison with Photodissociation Experiments

The photochemistry of small HX·(H(2)O)(n), n = 4 and 5 and X = F, Cl, and Br, clusters has been modeled by means of ab initio-based molecular simulations. The theoretical results were utilized to support our interpretation of photodissociation experiments with hydrogen halides on ice nanoparticles HX·(H(2)O)(n), n ≈ 10(2)-10(3). We have investigated the HX·(H(2)O)(n) photochemistry for three structural types: covalently bound structures (CBS) and acidically dissociated structures in a form of contact ion pair (CIP) and solvent separated pair (SSP). For all structures, we have modeled the electronic absorption spectra using the reflection principle combined with a path integral molecular dynamics (PIMD) estimate of the ground state density. In addition, we have investigated the solvent effect of water on the absorption spectra within the nonequilibrium polarizable continuum model (PCM) scheme. The major conclusion from these calculations is that the spectra for ionic structures CIP and SSP are significantly red-shifted with respect to the spectra of CBS structures. We have also studied the photodynamics of HX·(H(2)O)(n) clusters using the Full Multiple Spawning method. In the CBS structures, the excitation led to almost immediate release of the hydrogen atom with high kinetic energy. The light absorption in ionically dissociated species generates the hydronium radical (H(3)O) and halogen radical (X) within a charge-transfer-to-solvent (CTTS) excitation process. The hydronium radical ultimately decays into a water molecule and hydrogen atom with a characteristic kinetic energy irrespective of the hydrogen halide. We have also investigated the dynamics of an isolated and water-solvated H(3)O radical that we view as a central species in water radiation chemistry. The theoretical findings support the following picture of the HX photochemistry on ice nanoparticles investigated in our molecular beam experiments: HX is acidically dissociated in the ground state on ice nanoparticles, generating the CIP structure, which is then excited by the UV laser light into the CTTS states, followed by the H(3)O radical formation.