Solvation Cage Research Articles

The deconstruction of a polymeric solvation cage: a critical promotion strategy for PEO-based all-solid polymer electrolytes

PEO could be regarded as a macromolecular version of G4, Li+-transport is hindered by its multi-scale polymer structure. A strategy loosening the chain entanglement and tight (EO)n–Li+ interaction is established to improve the Li+-transport of PEO.

Infrared Spectroscopy of Li+ Solvation in Diglyme: Ab Initio Molecular Dynamics and Experiment.

Infrared (IR) spectra of solutions of the lithium salt LiBF4 in diglyme, CH3O(CH2CH2O)2CH3, are studied via IR spectroscopy and ab initio molecular dynamics (AIMD) simulations. Experiments show that the major effects of LiBF4, compared to neat diglyme, are the appearance of a new broad band in the 250-500 cm-1 frequency region and a broadening and intensity enhancement of the diglyme band in the 900-1150 cm-1 region accompanied by a red-shift. Computational analysis indicates that hindered translational motions of Li+ in its solvation cage are mainly responsible for the new far-IR band, while the changes in the mid-IR are due to Li+-coordination-dependent B-F stretching vibrations of BF4- anions coupled with diglyme vibrations. Molecular motions in these and lower frequency regions are generally correlated, revealing the collective nature of the vibrational dynamics, which involve multiple ions/molecules. Herein, a detailed analysis of these features via AIMD simulations of the spectrum and its components, combined with analysis of the generalized normal modes of the solution components, is presented. Other minor spectral changes as well as diglyme conformational changes induced by the lithium salt are also discussed.

Ion Solvation Cage Structure in Polymer Electrolytes Determined by Combining X-ray Scattering and Simulations.

Solvation structure plays a crucial role in determining ion transport in electrolytes. We combine wide-angle X-ray scattering (WAXS) and molecular dynamics (MD) simulation to identify the solvation cage structure in two polymer electrolytes, poly(pentyl malonate) (PPM) and poly(ethylene oxide) (PEO) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. As the salt concentration increases, the amorphous halo in the pure polymers is augmented by an additional peak at low scattering angles. The location of this peak and its height are, however, different in the two electrolytes. By decoupling the total intensity into species contributions and mapping scattering peaks to position-space molecular correlations, we elucidate distinct origins of the additional peak. In PPM, it arises from long-range charge-ordering between solvation cages and anions, while in PEO it is dominated by correlations between anions surrounding the same cage. TFSI- ions are present in the PPM solvation cage, but expelled from the PEO solvation cage.

Molecular Origin of High Cation Transference in Mixtures of Poly(pentyl malonate) and Lithium Salt

The rational development of new electrolytes for lithium batteries rests on the molecular-level understanding of ion transport. We use molecular dynamics simulations to study the differences between a recently developed promising polymer electrolyte based on poly(pentyl malonate) (PPM) and the well-established poly(ethylene oxide) (PEO) electrolyte; LiTFSI is the salt used in both electrolytes. Cation transference is calculated by tracking the correlated motion of different species. The PEO solvation cage primarily contains 1 chain, resulting in strong correlations between Li+ and the polymer. In contrast, the PPM solvation cage contains multiple chains, resulting in weak correlations between Li+ and the polymer. This difference results in a high cation transference in PPM relative to PEO. Our comparative study suggests possible designs of polymer electrolytes with ion transport properties better than both PPM and PEO. The solvation cage of such a hypothetical polymer electrolyte is proposed based on insights from our simulations.

Infrared Spectroscopy of Li+ Solvation in EmimBF4 and in Propylene Carbonate: Ab Initio Molecular Dynamics and Experiment.

Infrared (IR) spectra of solutions of the lithium salt LiBF4 in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) and in the organic solvent propylene carbonate (PC) are studied via infrared spectroscopy and ab initio molecular dynamics (AIMD) simulations. The measurements show that the major effects of LiBF4 in both solutions, compared to their neat counterparts, are the appearance of a new broad band in the 300-450 cm-1 frequency region and a broadening of the IR structure in the 900-1200 cm-1 region with the development of a new peak at 980 cm-1. Computational analysis indicates that hindered translational motions of Li+ in its solvation cage are mainly responsible for the former, while the latter is due to Li+-induced structural changes and accompanying vibrational frequency shifts of constituent ions and molecules of the solutions. In addition, molecular motions in these and lower-frequency regions are generally correlated, disclosing the collective nature of the vibrational dynamics, which involve multiple ions/molecules. Herein, a detailed analysis of these features via AIMD simulations of the spectrum and its components arising from auto- and cross-correlations of motions of constituent molecular species, combined with generalized normal modes of the solutions and normal modes of small Li+-containing clusters, is presented. Other minor spectral changes caused by the lithium salt as well as the interaction-induced effect on IR spectra are also discussed.

Time-Resolved Observation of the Solvation Dynamics of a Rydberg Excited Molecule Deposited on an Argon Cluster. II. DABCO☆ at Long Time Delays.

The real-time dynamics of DABCO-argon clusters is investigated in a femtosecond pump-probe experiment where the pump excites DABCO to the S1 state within the argon cluster. The probe operates by photoionization and documents the energy and angular distributions of the resulting photoelectrons. The present work complements a previous work from our group [Awali Phys. Chem. Chem. Phys., 2014, 16, 516-526] where this dynamics was probed at short time, up to 4 ps after the pump pulse. Here, the dynamics is followed up to 500 ps. A multiscale dynamics is observed. It includes a jump between two solvation sites (time scale 0.27 ps) followed by the relaxation of the solvation cage excess vibrational energy (time scale 14 ps) and then by that of DABCO (time scale >150 ps). Polarization anisotropy, double polarization, and angular anisotropy effects are reported also. They are interpreted (quantitatively for the former effect) in terms of decoherence of rotational alignment, driven by the overall rotation of the DABCO-argon clusters. A tomographic view of the DABCO excited orbital, provided by the double anisotropy effect, is discussed on a qualitative basis.

Preferential solvation of quercetin in aqueous aprotic solvent mixtures

Solvatochromism of quercetin was studied in binary mixtures of water with dimethyl sulfoxide, N,N-dimethylformamide and N,N-dimethylacetamide at 25 ?C by UV?Vis measurements. For all mixtures, a non-linear trend was observed in spectral shifts plotted against the bulk mole fractions. Deviation from ideal behaviour indicates that the solvation shell of quercetin differs in composition from the bulk because of preferential solvation. The solvent exchange model was applied in the analysis of solvatochromic data in order to quantify the extent of preferential solvation in the case of solute?solvent and solvent?solvent intermolecular interactions. The results show that the solvation shell of quercetin is enriched in aprotic solvent and the complex that was formed by the interaction between water and an aprotic solvent, over the whole composition range. The distribution of the solvent species in the solvation cage was obtained from the calculation of the local mole fractions as a function the bulk composition. It shows that the solvent?solvent interactions have great influence on the solvation behaviour of quercetin in aqueous aprotic solvent mixtures.

Voronoi polyhedra probing of hydrated OH radical

Voronoi polyhedron method is employed to extract the smallest volume shared by ˙OH radical in liquid water at the biologically important temperature (37 °C). The 3D-visualization and the probability distributions of the metric and topological properties of ˙OH solvation cage are provided.

Filming the Birth of Molecules and Accompanying Solvent Rearrangement

Molecules are often born with high energy and large-amplitude vibrations. In solution, a newly formed molecule cools down by transferring energy to the surrounding solvent molecules. The progression of the molecular and solute-solvent cage structure during this fundamental process has been elusive, and spectroscopic data generally do not provide such structural information. Here, we use picosecond X-ray liquidography (solution scattering) to visualize time-dependent structural changes associated with the vibrational relaxation of I(2) molecules in two different solvents, CCl(4) and cyclohexane. The birth and vibrational relaxation of I(2) molecules and the associated rearrangement of solvent molecules are mapped out in the form of a temporally varying interatomic distance distribution. The I-I distance increases up to ~4 Å and returns to the equilibrium distance (2.67 Å) in the ground state, and the first solvation cage expands by ~1.5 Å along the I-I axis and then shrinks back accompanying the structural change of the I(2) molecule.

Behaviour of hydrogen bonding and structure of poly(acrylic acid) in water–ethanol solution investigated by explicit ion molecular dynamics simulations

Molecular dynamics simulations of polyelectrolyte poly(acrylic acid) at infinitely dilute conditions in aqueous solutions containing up to 6 vol% ethanol were performed with explicit solvent and counter-ion description for varying degree of neutralisation. In the range of ethanol concentration employed, this study shows a swelling in the case of non-ionised chain, chain collapse for 25% ionised system, minimal swelling for 50% ionised chain and no significant change in conformation for 75–100% ionised chains. Ethanol interacts with non-ionised residues via hydrogen bonding and is found to be dominant in the solvation cage surrounding them, whereas no such interaction is seen with ionised residues. Thus, chains with higher charge densities show a preference for the aqueous environment in comparison to ethanol and hydrogen bonding to water is unaltered. For chains with lower charge density, a reduction in the hydrogen bonding with water is seen with an increase in ethanol concentration.

Hydrogen Bond Dynamics of Methyl Acetate in Methanol

The H-bond interactions of methyl acetate in methanol have been studied by means of ab initio molecular dynamics simulations within the Car−Parrinello approach. It has been observed that the C═O stretching band of methyl acetate splits into a doublet as a consequence of the interaction with the solvent. The H-bond effects on the spectroscopic properties of methyl acetate in methanol have been interpreted by wavelet transform analysis in conjunction with a structural and dynamic characterization of the solvation cage. Localizing a vibrational mode in time and frequency during the simulations has allowed association of the different interactions with the solvent to the vibrational properties. This represents an important development in the capability of molecular dynamics simulations to explain experimental data obtained by time-resolved spectroscopic methods.

Thermolytic decomposition of benzylic dialkoxy disulfides

Dialkoxy disulfides have been used as an alkoxy radical source under photolytic conditions. In addition, this class of disulfide thermally decomposes to deliver S 2 to dienes. We examined benzylic dialkoxy disulfides (X–Ph–CH 2–O–S–S–O–CH 2–Ph–X) under thermolytic conditions and observed that the rates of decomposition are related to Swain and Lupton’s field constant, F . In addition, the observed non 1:1 ratio of alcohol to aldehyde reaffirms Harpp’s-postulated cage mechanism. We have shown that the ratio is dependant upon the substituent present which can enhance the pi-stacking ability with the solvent, and thus favor diffusion out of the solvation cage yielding the non 1:1 ratio observed.

Sodium Diffusion through Aluminum-Doped Zeolite BEA System: Effect of Water Solvation

To investigate the effect of hydration on the diffusion of sodium ions through the aluminum-doped zeolite BEA system (Si/Al = 30), we used the grand canonical Monte Carlo (GCMC) method to predict the water absorption into aluminosilicate zeolite structure under various conditions of vapor pressure and temperature, followed by molecular dynamics (MD) simulations to investigate how the sodium diffusion depends on the concentration of water molecules. The predicted absorption isotherm shows first-order-like transition, which is commonly observed in hydrophobic porous systems. The MD trajectories indicate that the sodium ions diffuse through zeolite porous structures via hopping mechanism, as previously discussed for similar solid electrolyte systems. These results show that above 15 wt % hydration (good solvation regime) the formation of the solvation cage dramatically increases sodium diffusion by reducing the hopping energy barrier by 25% from the value of 3.8 kcal/mol observed in the poor solvation regime.

Tuning a Potassium Channel—The Caress of the Surroundings

Rapid permeation of ions through membrane-spanning channels is only possible if the free-energy penalty for dehydration is effectively balanced by the free energy of binding within the channel. Channel selectivity arises if different ions have significantly different partitioning free energies. Potassium channels of muscle and nerve are exquisitely designed to select for potassium over sodium (1). This has traditionally been explained by noting that ligand coordination at the K+ binding sites mimics that in water and that channel binding sites form a rigid scaffold providing a “snug fit” for K+ but not the smaller Na+ (2,3). Recent work has called these ideas into question. K+ has six waters in its canonical inner solvation shell, a structure markedly different from the eightfold carbonyl coordination of K-channels (4). Proteins are dynamic and their thermal fluctuations obscure size differences between K+ and Na+ (here coordination and selectivity are intimately dependent on electrostatic interactions among the binding sites' fluctuating carbonyl groups) (5), an idea with origins in Eisenman's explanation of the properties of glass electrodes (6). The same eightfold coordination environment that leads to high selectivity in nerve and muscle K-channels is essentially nondiscriminatory in NaK channels (7). In this issue, Varma and Rempe provide a novel explanation for these observations (8). Using quantum chemical methods, they analyze K+ and Na+ binding to clusters of water and of a few other ligands. Octa-coordinate channel binding sites are modeled as clusters of four glycine dipeptide moieties, thus mimicking the bidentate geometry that channel architecture imposes on neighboring intrastrand carbonyls while simultaneously allowing the interstrand carbonyl pairs to move freely. To discriminate between longer-range interactions contributing to bulk hydration (or channel solvation), they then embed these small clusters in different environments and carry out a series of gedanken experiments. They find that preferred solvation structures, whether in water or in channel surroundings, are sensitive to their environment, providing new insight into factors controlling selectivity (5). In bulk water, with its high dielectric constant (ɛ), both K+ and Na+ prefer low coordination. However, if cation-water clusters could be embedded in low ɛ-surroundings, behavior would alter substantially, and higher coordination numbers would be favored. Since the surroundings can compete with the ion for ligation (by water in this case), provided they have hydrogen-bonding capacity, they can alter the structure of the solvent cage. Interestingly, the octa-coordinate hydration cage does not match the channel's preferred ion ligation structure; to achieve selectivity K+ is overcoordinated. Although the local ɛ in the vicinity of a channel binding site cannot be “tuned”, the hydrogen bonding proclivity of its surroundings can be (and is, in physiologically relevant situations). In highly selective K-channels, there are no proximal H-bond donors to disrupt a binding site's solvation cage. In weakly selective K-channels, bioinformatic analysis shows that all have a common feature: H-bond donors near the selectivity filter (9). In fact, with sufficiently disruptive (high H-bonding capacity) surroundings, like those found adjacent to all the octa-coordinating sites in the NaK channel (7), Varma and Rempe demonstrate that ion ligation at the channel binding site would change dramatically, accounting for hydration effects observed in the NaK system (10). The ion would bind five or six carbonyl oxygens in its inner solvation shell and K/Na selectivity would be lost. What emerges is a hybrid between the “snug fit” and the “fluctuating structure” pictures. Ion binding sites in potassium channels are dynamic and thermal fluctuations do swamp ionic size differences; in octa-coordinate surroundings, electric field strength considerations from carbonyl ligands account for K/Na selectivity (5). However there must be sufficient structural rigidity to maintain the high (>6) carbonyl ligation environment. If this is lost, selectivity is lost as well. These ideas can be directly tested, and recent experimental data already lend support (11). They imply that K-channel selectivity behavior can be designed to order by site-directed mutations that introduce (or delete) H-bond donors near a K-channel selectivity filter.

Multivariate statistical treatment of solvent effects in the photoreduction of CoIII(En)2(Br)(RC6H4NH2)2+ complexes in aqua-organic solvent media

Ultraviolet photolysis (λ = 254 nm) studies were carried out for a series of cobalt(III) complexes, CoIII(En)2(Br)(RC6H4NH2)2+, where R = m-OMe, p-F, H, m-Me, p-Me, p-OEt, and p-OMe, in various compositions of water-methanol/1,4-dioxane mixtures (0, 5, 10, 15, 20, 25, and 30 vol % organic cosolvent) at two different temperatures (287 and 300 K). The ligand-to-metal charge-transfer excited state produced in the excitation of the complex initially generated a solvent-caged {CoII; ligand radical} pair, which eventually undergoes recombination/separation into products. The quantum yield sharply increased from a mixture containing a lower mole fraction of organic cosolvent (x org) to higher one. In other words, when x org in the mixture increases, a steady increase in the quantum yield is observed. The quantum yield of CoIII(En)2(Br)(RC6H4NH2)2+ in various solvent mixtures is found to exhibit a linear (logΦCo(II) − 1/er) dependence. This is consistent with solvation or solvent cage effect, which may be nonspecific, specific, or both. In order to throw light on these effects, a phenomenological model of solvent effects was applied. Therefore, the quantum yield values have been correlated statistically with some success containing different solvent parameters. The solvent parameters considered in this work are Grunwald-Winstein’s Y, Krygowski-Fawcett’s E T N and DN N along with Kamlet-Taft’s α, β, π* parameters. The regression model proposed is Y S = Y 0 + Σ i = 1 n a i X i , where YS is the solvent-dependent property (here logΦCo(II)) in a given solvent; Y 0 is the statistical quantity corresponding to the value of property in the reference solvent; X 1, X 2, X 3 … are explanatory variables, the solvent parameters, which can explain the various solvation effects on reactants/{CoII; ligand radical} pair, and a 1, a 2, a 3… etc., are the regression coefficients. The coefficient values can be quantified to measure the relative importance of solvent effects on the physicochemical quantity, that is, the photoreduction yields in the present investigation.

Film drying and complexation effects in the simultaneous skin permeation of ketoprofen and propylene glycol from simple gel formulations

This work investigated the simultaneous permeation of ketoprofen and propylene glycol (PG) across pig ear skin from simple gel formulations administered under simulated in-use conditions. The aims were to quantify rates of permeation of both solvent and active, probe the effects of formulation drying and gain insight into drag/complexation interactions. Simple 3-component gels were formulated using a fixed amount of ketoprofen and hydroxypropyl cellulose thickener with decreasing content of solvent propylene glycol. Multiple finite (5 mg × 15 mg) doses were massaged over 24 h into full thickness pig ear skin in vertical Franz-type diffusion cells. The permeation of ketoprofen was inversely proportional to the content of PG, whereas the permeation of PG was directly proportional, although the amount of PG permeated was always greater than ketoprofen, even from the driest gel practically achievable. In this state, the molar ratio of PG/ketoprofen was ∼12, suggesting that this number of PG molecules constitutes the solvation cage of ketoprofen. Dragging/pulling effect extends throughout the skin and into the receptor compartment and probably the system, in an in vivo situation. Although PG may represent a worse case scenario given its well-documented skin permeation enhancement properties, it is probable that other solvents exert a similar effect on solutes across skin. A drying film will behave in different ways depending on the nature of both the thickener and solvent, where the outcomes are not readily predictable. It is important to account for the fate of all species administered from a topical formulation.

A molecular dynamics study of the hydroxyl radical in solution applying self-interaction-corrected density functional methods

We have performed density functional theory based molecular dynamics (MD) simulations of the *OH radical in solution using self-interaction corrected (SIC) methods. We use a scheme recently proposed by M. d'Avezac, M. Calandra and F. Mauri [arXiv:cond-mat/0407750] in which a correction is only applied to the spin density within a restricted open shell formulation. In addition to two correction formulas employed within this scheme by M. d'Avezac, M. Calandra and F. Mauri, we propose and test an new empirical form which only introduces a scaled Coulomb term. This new functional leads to good agreement with reference calculations on radical cation dimers and on the hydroxyl water dimer in the gas phase. Applied in ab initio MD simulations, these three SIC methods provide a picture of the *OH solvation that differs qualitatively from the one obtained using the standard generalised gradient approximation (GGA). Hemibonded water, observed in GGA simulations and believed to be an artefact due to self-interaction error, is not present. We find that the *OH acts as a good hydrogen bond donor, but accepts less than two hydrogen bonds on average. These hydrogen bonds are part of a mobile, otherwise quasi-hydrophobic solvation cage. Our results show the potential of this computationally expedient scheme, which might extend the range of problems that can be modelled adequately with density functional theory.

Vibrational analysis of the chorismate rearrangement: relaxed force constants, isotope effects and activation entropies calculated for reaction in vacuum, water and the active site of chorismate mutase

AbstractWe present results derived from vibrational Hessians calculated for the reactant complex, transition structure and product complex of the rearrangement of chorismate to prephenate under different conditions. The AM1 semiempirical MO and B3LYP/6–31G* density functional methods were employed for calculations in vacuum, whereas a hybrid QM/MM method AM1/CHARMM/TIP3P was used for calculations in water and within the active site of B. subtilis chorismate mutase. Kinetic and equilibrium isotope effects and entropies of activation and reaction were investigated as a function of the increasing size of the Hessian, as the system is expanded to include not only the atoms of chorismate/prephenate itself but also an increasing number of surrounding water molecules (up to 99) or active‐site residues (up to 225 atoms). Primary 13C and 18O isotope effects are not sensitive to the size of the Hessian, but secondary 3H–C5 and 3H2–C9 effects require the inclusion of at least those atoms directly involved in hydrogen bonds to the substrate or, better, a complete first solvation shell or cage of active‐site amino acid residues. Pauling bond orders for the breaking CO and making CC bonds are remarkably similar for the transition structures in all three media. Relaxed force constants for stretching of these bonds (which allow meaningful comparisons to be made along a reaction path) give a significantly different picture of the bonding changes in the transition structures. The ratio of logarithms of kinetic and equilibrium isotope effects does not agree with measures of transition‐state structure derived from Pauling bond orders or from relaxed force constants. There is no simple relationship between kinetic isotope effects and transition‐state structure for this Claisen rearrangement. The calculated vibrational entropy of activation for the enzymic reaction agrees well with an experimental value for E. coli chorismate mutase. Vibrational entropy reduces the free energy barrier for the catalysed reaction by about 1 kJ mol−1 at 333 K. Copyright © 2004 John Wiley & Sons, Ltd.

Simultaneous permeation of tamoxifen and γ linolenic acid across excised human skin. Further evidence of the permeation of solvated complexes

Tamoxifen is the hormonal treatment of choice in women who have hormone-dependent breast cancer and its efficacy in those women considered to have a high risk of developing breast cancer, has also been established. γ linolenic acid (GLA) has been shown to decrease the invasion of breast cancer and recent studies have demonstrated that GLA can enhance the oestrogen receptor down-regulation induced by tamoxifen. However, tamoxifen is associated with serious side-effects due mainly to systemic delivery, and targeted delivery of both tamoxifen and GLA would be highly beneficial. This work was a preliminary study for the development of a transcutaneous system to simultaneously deliver both tamoxifen and GLA directly to the breast. Full thickness human skin was dosed with 500 μl saturated solution of tamoxifen in borage oil (25% GLA) and the simultaneous permeation of the two actives determined. There was rapid flux with minimal lag time, the cumulative permeation at 24 h was 764.3±94.2 μg cm −2 for GLA and 5.44±0.67 μg cm −2 for tamoxifen: the latter being comparable to the amount of tamoxifen associated with cancerous breast tissue from a 20 mg oral dose. The ratio of GLA/tamoxifen permeated at different timepoints was quite consistent, both in terms of mass (mean 138, S.D. 15.1) and mols (mean 184, S.D. 20.3). It was determined that 2.5 molecules of GLA were associated with each molecule of tamoxifen in the permeation process, equating to a solvation cage of three molecules of triacylglycerol. This study has demonstrated the feasibility of administering simultaneously tamoxifen and GLA using borage oil as vehicle, which warrants further investigation as a novel topical two-component system in relation to or prophylaxis of those perceived at high risk of developing breast cancer. The study also provides further evidence of the permeation of solvated complexes across skin, rather than discrete penetrant molecules.

A theoretical study of the mechanism of selective fluorination of saturated hydrocarbons by molecular fluorine. Participation of CHCl3 solvent molecules in the ionic process.

The fluorination reaction of methane and isobutane by molecular F(2) with and without CHCl(3) solvent was studied with the ab initio and ONIOM theoretical calculations. The electrophilic pathway for the RH + F(2) reaction in general is a two-step process, consisting of hydride abstraction leading to an intermediate of the type R(+)(delta)...HF...F(-)(delta), followed by complicated rearrangement to give the electrophilic substitution product, RF + HF. In the case of methane, the overall barrier for this reaction is too high for the reaction to take place under mild conditions even in the presence of CHCl(3) solvent molecules. In the case of isobutane without CHCl(3), the electrophilic pathway has a high rate-determining barrier of 25 kcal/mol, and is not likely to take place; the radical process forming t-Bu* + HF + F* may be preferred. However, the t-Bu(+)(delta)...HF...F(-)(delta) intermediate and, in particular, the transition state TS2 for rearrangement of the intermediate are highly ionic, and are stabilized dramatically when a few CHCl(3) solvent molecules form a solvation cage. The electrophilic reaction for isobutane + F(2) has a low overall barrier when at least three CHCl(3) molecules are present and can take place under mild conditions with full retention of configuration.