Dipolar Solvents Research Articles

Langevin-Poisson-EQT: A dipolar solvent based quasi-continuum approach for electric double layers.

Water is a highly polar solvent. As a result, electrostatic interactions of interfacial water molecules play a dominant role in determining the distribution of ions in electric double layers (EDLs). Near a surface, an inhomogeneous and anisotropic arrangement of water molecules gives rise to pronounced variations in the electrostatic and hydration energies of ions. Therefore, a detailed description of the structural and dielectric properties of water is important to study EDLs. However, most theoretical models ignore the molecular effects of water and treat water as a background continuum with a uniform dielectric permittivity. Explicit consideration of water polarization and hydration of ions is both theoretically and numerically challenging. In this work, we present an empirical potential-based quasi-continuum theory (EQT) for EDL, which incorporates the polarization and hydration effects of water explicitly. In EQT, water molecules are modeled as Langevin point dipoles and a point dipole based coarse-grained model for water is developed systematically. The space dependence of the dielectric permittivity of water is included in the Poisson equation to compute the electrostatic potential. In addition, to reproduce hydration of ions, ion-water coarse-grained potentials are developed. We demonstrate the EQT framework for EDL by simulating NaCl aqueous electrolyte confined inside slit-like capacitor channels at various ion concentrations and surface charge densities. We show that the ion and water density predictions from EQT agree well with the reference molecular dynamics simulations.

Mild, Efficient and Catalyst‐Free Hydroxylation of Alkyl Halides in Water: Significant Enhancement of Water Nucleophilicity in Dipolar Solvents

AbstractA new and efficient method for the hydroxylation of heteroarylalkyl, arylalkyl and alkyl halides has been developed by exploiting the nucleophilicity of water. The method being operated under neutral conditions, avoids acid/base or metal assisted catalysis. The protocol has been optimized for a range of aqueous aprotic/protic solvents. The new operation is very simple, highly economical, ecofriendly, safest and suitable for acid/base sensitive motifs in yielding the products at high purity.

Collective excitations in liquid dimethyl sulfoxide (DMSO): FIR spectrum, low frequency vibrational density of states, and ultrafast dipolar solvation dynamics.

Valuable dynamical and structural information about neat liquid DMSO at ambient conditions can be obtained through a study of low frequency vibrations in the far infrared (FIR), that is, terahertz regime. For DMSO, collective excitations as well as single molecule stretches and bends have been measured by different kinds of experiments such as OHD-RIKES and terahertz spectroscopy. In the present work, we investigate the intermolecular vibrational spectrum of DMSO through three different computational techniques namely (i) the far-infrared spectrum obtained through the Fourier transform of total dipole moment auto-time correlation function, (ii) from the Fourier transform of the translational and angular velocity time autocorrelation functions, and (iii) a quenched normal mode analysis of the parent liquid at 300 K. The three spectra, although exhibit differences among each other, reveal similar features which are in good, semi-quantitative, agreement with experimental results. The study of participation ratio of the density of states obtained from the normal mode analysis shows that the broad spectrum around 100 cm-1 involves collective oscillations of 300-400 molecules. Dipolar solvation dynamics exhibit ultrafast energy relaxation with an initial time constant around 157 fs which can be attributed to the coupling to the collective excitations. We compare the properties of DMSO with those of water vis-a-vis the existence of the low frequency collective modes. Last, we find that the collective excitation spectrum exhibits strong temperature dependence.

Solvent Polarity Governs Ion Interactions and Transport in a Solvated Room-Temperature Ionic Liquid.

We explore the influence of the solvent dipole moment on cation-anion interactions and transport in 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl), [BMIM+][Tf2N-]. Free energy profiles derived from atomistic molecular dynamics (MD) simulations show a correlation of the cation-anion separation and the equilibrium depth of the potential of mean force with the dipole moment of the solvent. Correlations of the ion diffusivity with the dipole moment and the concentration of the solvent were further demonstrated by classical MD simulations. Quasi-elastic neutron scattering experiments with deuterated solvents reveal a complex picture of nanophase separation into the ionic liquid-rich and solvent-rich phases. The experiment corroborates the trend of concentration- and dipole moment-dependent enhancement of ion mobility by the solvent, as suggested by the simulations. Despite the considerable structural complexity of ionic liquid-solvent mixtures, we can rationalize and generalize the trends governing ionic transport in these complex electrolytes.

Solvent, temperature and concentration effects on the optical rotatory dispersion of (R)-3-methylcyclohexanone

Optical rotatory dispersion (ORD) spectra are reported for isolated and solvated (R)-3-methylcyclohexanone (R-3MCH) in 10 solvents, of wide polarity range, and over the spectral range 350–650 nm. Sample concentration effects on ORD spectra of R-3MCH were also recorded and investigated over widely varying concentrations from 2.5 × 10−3 to 2.5 × 10−1 g/mL where an observed sensitivity of optical rotation (OR) to incident light wavelength at low concentrations is correlated to solvent effects. Temperature effects were also studied by recording ORD spectra over the temperature range 0–65 °C in toluene. Recorded specific OR was plotted against various solvent parameters, namely, dipole moment, polarity, refractive index and polarizability to probe solvent effects. Furthermore, solvent effects were studied by incorporating Kamlet's and Taft's solvent parameters in the multi-parametric linear fitting. Theoretically, ORD spectra and populations of optimized geometries of equatorial and axial conformers of R-3MCH were calculated in the gas and solvated phases. All theoretical calculations were performed employing the polarizable continuum model using density functional theoretical and composite scheme (G4) methods with aug-cc-pVTZ and aug-cc-pVDZ basis sets. Net ORD spectra of R-3MCH were generated by the Boltzmann-weighted sum of the contributions of the dominant conformers. Upon comparing theoretical and experimental ORD spectra, a very good agreement is observed for the ORD spectra in the gas phase and high polarity solvents compared to relatively lesser agreement in low polarity solvents.

DMSO enhanced conformational switch of an interfacial enzyme.

Interfacial proteins function in unique heterogeneous solvent environments, such as water-oil interfaces. One important example is microbial lipase, which is activated in an oil-water emulsion phase and has many important enzymatic functions. A unique aprotic dipolar organic solvent, dimethyl sulfoxide (DMSO), has been shown to increase the activity of lipases, but the mechanism behind this enhancement is still unknown. Here, all-atom molecular dynamics simulations of lipase in a binary solution were performed to examine the effects of DMSO on the dynamics of the gating mechanism. The amphiphilic α5 region of the lipase was a focal point for the analysis, since the structural ordering of α5 has been shown to be important for gating under other perturbations. Compared to the closed-gorge ensemble in an aqueous environment, the conformational ensemble shifts towards open-gorge structures in the presence of DMSO solvents. Increased width of the access channel is particularly prevalent in 45% and 60% DMSO concentrations (w/w). As the amount of DMSO increases, the α5 region of the lipase becomes more α-helical, as we previously observed in studies that address water-oil interfacial and high pressure activation. We believe that the structural ordering of α5 plays an essential role on gating and lipase activity.

Nile blue shows its true colors in gas-phase absorption and luminescence ion spectroscopy.

Nile blue is used extensively in biology as a histological stain and fluorescent probe. Its absorption and emission spectra are strongly solvent dependent, with variations larger than 100 nm. The molecule is charged due to an iminium group, and it is therefore an obvious target for gas-phase ion spectroscopy. Here we report the absorption and emission spectra of the mass-selected bare ions isolated in vacuo, and based on our results we revisit the interpretation of solution-phase spectra. An accelerator mass spectrometer was used for absorption spectroscopy where the absorption is represented by the yield of photofragment ions versus excitation wavelength (action spectroscopy). The luminescence experiments were done with a newly built ion trap setup equipped with an electrospray ion source, and some details on the mass selection technique will be given which have not been described before. In vacuo, the absorption and emission maxima are at 580 ± 10 nm and 628 ± 1 nm. These values are somewhat blue-shifted relative to those obtained in most solvents; however, they are much further to the red than those in some of the most non-polar solvents. Furthermore, the Stokes shift in the gas phase (1300 cm(-1)) is much smaller than that in these non-polar solvents but similar to that in polar ones. An explanation based on charge localization by solvent dipoles, or by counterions in some non-polar solvents, can fully account for these findings. Hence in the case of ions, it is nontrivial to establish intrinsic electronic transition energies from solvatochromic shifts alone.

Unified Electrostatic Understanding on the Solvation-Induced Changes in the CN Stretching Frequency and the NMR Chemical Shifts of a Nitrile

Understanding on the spectroscopic properties of a functional group is essential to use it to detect changes in the structural and/or dynamical properties through the situations of intermolecular interactions. The present study is devoted to elucidating the factors that control the solvation-induced changes in the C≡N stretching frequency and the (13)C and (15)N NMR chemical shifts of the nitrile group. It is shown that the nonelectrostatic contribution of the hydration-induced changes in the C≡N stretching frequency as previously thought, as well as the specific effect of hydrogen bonding on the (13)C and (15)N chemical shifts, actually originate from the spatially inhomogeneous nature of the electrostatic situation generated by the hydrogen-bond donating water molecule, especially by the OH bond dipole. On this basis, a unified electrostatic interaction model that encompasses the cases of both hydration and dipolar solvation is constructed. The responses of electrons in these two cases are also discussed.

Optimized Synthesis According to One-Step Process of a Biobased Thermoplastic Polyacetal Derived from Isosorbide.

This paper describes both the synthesis and characterization of a biobased and non-aromatic polyacetal produced from the reaction between isosorbide and methylene chloride. The reaction was conducted in an aprotic dipolar and harmless solvent using a one-step, fast and economical procedure. The chemical composition of this polymer was investigated using Nuclear Magnetic Resonance and Fourier Transform Infra-Red spectroscopies. The molecular weights were examined by size exclusion chromatography and MALDI-TOF spectrometry. The synthesis conditions (concentration, mixing speed, solvent nature, stoichiometry, addition mode of one reactan) were found to strongly influence both polymer architecture and reaction yield. Under moderated stirring conditions, the polyacetal was characterized by a larger amount of macro-cycles. Inversely, under higher intensity mixing and with an excess of methylene chloride, it was mainly composed of linear chains. In this latter case, the polymeric material presented an amorphous morphology with a glass transition temperature (Tg) close to 55 °C. Its degradation temperature was evaluated to be close to 215 °C using thermogravimetry according to multi-ramp methodology. The chemical approach and the physicochemical properties are valuable in comparison with that characteristic of other isosorbide-based polyacetals.

Solvatochromism and linear solvation energy relationship of the kinase inhibitor SKF86002

We studied the spectroscopic characteristics of SKF86002, an anti-inflammatory and tyrosine kinase inhibitor drug candidate. Two conformers SKF86002A and SKF86002B are separated by energy barriers of 19.68kJ·mol−1 and 6.65kJ·mol−1 due to H-bonds, and produce the three major UV–Vis absorption bands at 325nm, 260nm and 210nm in cyclohexane solutions. This environment-sensitive fluorophore exhibited emission in the 400–500nm range with a marked response to changes in environment polarity. By using twenty-two solvents for the solvatochromism study, it was noticed that solvent polarity, represented by dielectric constant, was well correlated with the emission wavelength maxima of SKF86002. Thus, the SKF86002 fluorescence peak red shifted in aprotic solvents from 397.5nm in cyclohexane to 436nm in DMSO. While the emission maximum in hydrogen donating solvents ranged from 420nm in t-butanol to 446nm in N-methylformamide. Employing Lippert-Mataga, Bakhshiev and Kawski models, we found that one linear correlation provided a satisfactory description of polarity effect of 18 solvents on the spectral changes of SKF86002 with R2 values 0.78, 0.80 and 0.80, respectively. Additionally, the multicomponent linear regression analysis of Kamlet-Taft (R2=0.94) revealed that solvent acidity, basicity and polarity accounted for 31%, 24% and 45% of solvent effects on SKF86002 emission, respectively. While Catalán correlation (R2=0.92) revealed that solvatochromic change of SKF86002 emission was attributed to changes in solvent dipolarity (71%), solvent polarity (12%), solvent acidity (11%) and solvent basicity (6%). Plot of Reichardt transition energies and emission energies of SKF86002 in 18 solvents showed also a linear correlation with R2=0.90. The dipole moment difference between excited and ground state was calculated to be 3.4–3.5debye.

Dependence of the dielectric constant of electrolyte solutions on ionic concentration: A microfield approach.

We present a microfield approach for studying the dependence of the orientational polarization of the water in aqueous electrolyte solutions upon the salt concentration and temperature. The model takes into account the orientation of the solvent dipoles due to the electric field created by ions, and the effect of thermal fluctuations. The model predicts a dielectric functional dependence of the form ɛ(c)=ɛ_{w}-βL(3αc/β),β=ɛ_{w}-ɛ_{ms}, where L is the Langevin function, c is the salt concentration, ɛ_{w} is the dielectric of pure water, ɛ_{ms} is the dielectric of the electrolyte solution at the molten salt limit, and α is the total excess polarization of the ions. The functional form gives a remarkably accurate description of the dielectric constant for a variety of salts and a wide range of concentrations.

Frequency-dependence of electric double layer capacitance without Faradaic reactions

This report reviews the time-dependence of the double layer (DL) capacitors in the polarized potentials domains and the related subjects, which have been observed as the behavior of the constant phase element (CPE). The DL capacitance decreases with an increase in the ac-frequency of the applied voltage, obeying a power law of the frequency. This variation necessarily yields an electric resistance component at the interfaces. The DL resistance does not really exist but is involved inevitably in the measured current as a result of the time-dependent capacitance. It cannot be discriminated against the heterogeneous kinetics with the Butler-Volmer type. The frequency-dependence agrees with the behavior of the CPE. The DL capacitance is brought about by orientation of solvent molecules, independent of concentrations of salts, kinds of salts, and applied dc-potential. The orientation is modeled by combination of the external electric field, the image force of polarized charge of solvent on the electrode, and the interaction of nearest neighboring solvent molecules. The combination belongs to cooperative phenomena because the orientation is competitive with the molecular interactions energetically. The competition generates two-dimensional local phases, prolonging the orientation much larger than flip rates of solvent dipoles.

Identifying the Electrochemical Conditions of Underpotential Deposition and Its Role in Shape-Controlled Synthesis from First Principles

Recently, much attention has been devoted to the development of synthetic approaches that can exert facile control over the size, shape, architecture and composition of transition metal nanoparticles (TMNPs). The advent of such approaches could enable the rational design of nanoelectrocatalysts, as the catalytic properties of TMNPs are highly sensitive to their morphologies and elemental compositions. One strategy that is actively being investigated involves the underpotential deposition (UPD) of transition metals onto the surfaces of seed TMNPs. UPD-based approaches for shape control are attractive since the deposition process is surface-sensitive and is easily controlled by varying the synthesis conditions, such as the transition metal salt concentration and the pH of the electrolyte. Nevertheless, the ability to predict the equilibrium shape of UPD-modified TMNPs is challenging because of difficulties in modeling the UPD process in electrolytic media. Therefore, in order to develop a strong connection between the synthesis conditions and the equilibrium shapes of UPD-modified TMNPs, better theoretical descriptions of UPD phenomena are required. In this work, we report progress on the first-principles modeling of UPD phenomena under realistic electrochemical conditions. Key to this model is the ability to include the influence of solvent dipoles and the electrical double layer along the solid–liquid interface. We capture these effects through a recently released solid–solution interface model. Employing this approach, we are able to accurately calculate the measurable shift in redox potential associated with the UPD process on planar surfaces, as well as the adsorption isotherms including entropic effects. The development of this predictive first-principles UPD model and extensions to modeling equilibrium TMNP shapes are discussed.

Dielectric relaxation in ionic liquid/dipolar solvent binary mixtures: A semi-molecular theory.

A semi-molecular theory is developed here for studying dielectric relaxation (DR) in binary mixtures of ionic liquids (ILs) with common dipolar solvents. Effects of ion translation on DR time scale, and those of ion rotation on conductivity relaxation time scale are explored. Two different models for the theoretical calculations have been considered: (i) separate medium approach, where molecularities of both the IL and dipolar solvent molecules are retained, and (ii) effective medium approach, where the added dipolar solvent molecules are assumed to combine with the dipolar ions of the IL, producing a fictitious effective medium characterized via effective dipole moment, density, and diameter. Semi-molecular expressions for the diffusive DR times have been derived which incorporates the effects of wavenumber dependent orientational static correlations, ion dynamic structure factors, and ion translation. Subsequently, the theory has been applied to the binary mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) with water (H2O), and acetonitrile (CH3CN) for which experimental DR data are available. On comparison, predicted DR time scales show close agreement with the measured DR times at low IL mole fractions (x(IL)). At higher IL concentrations (x(IL) > 0.05), the theory over-estimates the relaxation times and increasingly deviates from the measurements with x(IL), deviation being the maximum for the neat IL by almost two orders of magnitude. The theory predicts negligible contributions to this deviation from the x(IL) dependent collective orientational static correlations. The drastic difference between DR time scales for IL/solvent mixtures from theory and experiments arises primarily due to the use of the actual molecular volume (V(mol)(dip)) for the rotating dipolar moiety in the present theory and suggests that only a fraction of V(mol)(dip) is involved at high x(IL). Expectedly, nice agreement between theory and experiments appears when experimental estimates for the effective rotational volume (V(eff)(dip)) are used as inputs. The fraction, V(eff)(dip)/V(mol)(dip), sharply decreases from ∼1 at pure dipolar solvent to ∼0.01 at neat IL, reflecting a dramatic crossover from viscosity-coupled hydrodynamic angular diffusion at low IL mole fractions to orientational relaxation predominantly via large angle jumps at high x(IL). Similar results are obtained on applying the present theory to the aqueous solution of an electrolyte guanidinium chloride (GdmCl) having a permanent dipole moment associated with the cation, Gdm(+).

The coalescence of polystyrene in correlated binary solvents

ABSTRACTThe solution behavior of solvophobic polymers is crucial to the development of polymer coatings and polymeric drug delivery vehicles. In this article, the role of dipolar interactions is investigated in the solvophobic coalescence of polystyrene in binary correlated polar solvent mixtures. A simple model for coalescence thermodynamics is derived from correlations between thermally rotating dipole moments in the solvent. The stabilizing correlations lost to the solvent due to a solute's presence give rise to a driving force for the coalescence of solutes. This stabilization is offset by the entropy of mixing that favors the dispersion of solutes. Predictions are compared to the measured point of coalescence of polystyrene in acetone when different alcohols are titrated. The model is shown to capture this point of coalescence and conformation for a variety of systems. Our results suggest the significant property determining the solubility of nonpolar polymers in a polar liquid is a free energy resulting from attractive dispersion interactions between thermally rotating solvent dipole moments. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 948–955

Spectrophotometric study of dehalogenation of benzyl halides with nickel in aprotic dipolar solvents in the presence of oxygen

Oxidative dissolution of nickel in the benzyl halogenide – dipolar aprotic solvent system in the presence of air oxygen proceeds with the formation of benzaldehyde, benzyl alcohol, 1,2-diphenylethane, and trace amounts of 4,4'-ditolyl. Spectrophotometry studies have revealed that the oxidation products are formed in the solution rather than at the nickel surface. It has been shown that the oxygen adsorbed at the nickel surface practically does not pass into the solution.

NMR-Based Molecular view of the Biology and Biophysics of WIP, An Intrinsically Disordered Protein

Intrinsically disordered proteins (IDPs) are multi-conformational polypeptides that lack a stable three-dimensional structure, forcing structural biology to reconsider the structure-function paradigm. The versatile IDPs play key roles in a multitude of biological processes, and, given their inherently flexible nature, nuclear magnetic resonance (NMR) is a leading method for investigating IDP behavior on the molecular level. Our research has focused on the intrinsically disordered WASp Interacting Protein (WIP) from human T cells, whose C-terminal binds Wiskott-Aldrich syndrome protein (WASp), and N-terminal interacts with actin, both crucial factors in mediating the cytoskeletal changes accompanying cell activation.A range of NMR experiments tailored for IDP study was employed to discover transient structural motifs in the two terminal WIP domains. Chemical shifts, relaxation rates, residual dipolar couplings and solvent exposure all concurred in identifying WIP segments exhibiting a secondary structure bias in their conformational ensemble. Remarkably, these transient structural elements echo the conformation of WIP bound to actin or WASp. In addition, in each domain we identified a previously unrecognized binding epitope, and later confirmed these with binding-induced effects observed by NMR.IDPs are essentially an ensemble of multiple fast-interchanging conformations, and we have utilized WIP also as a model to monitor this ensemble behavior under various conditions. Temperature effects were consistent with thermal unfolding and exhibited a strong correlation with structural propensities. Similarly, the effects of denaturing or crowding conditions could be evaluated with this approach. Finally, we obtain insight into how proteins fold and interact with peptidic ligands by following the effects of sub-stochiometric actin concentrations on the WIP conformational ensemble. Our findings highlight the potential impact of high-resolution NMR upon the field of IDPs, which can be enhanced by complementary biophysical and computational approaches.

Multinuclear NMR spectroscopy for differentiation of molecular configurations and solvent properties between acetone and dimethyl sulfoxide

The differences in molecular configuration and solvent properties between acetone and dimethyl sulfoxide (DMSO) were investigated using the developed technique of 1H, 13C, 17O, and 1H self-diffusion liquid state nuclear magnetic resonance (NMR) spectroscopy. Acetone and DMSO samples in the forms of pure solution, ionic salt-added solution were used to deduce their active sites, relative dipole moments, dielectric constants, and charge separations. The NMR results suggest that acetone is a trigonal planar molecule with a polarized carbonyl double bond, whereas DMSO is a trigonal pyramidal-like molecule with a highly polarized S–O single bond. Both molecules use their oxygen atoms as the active sites to interact other molecules. These different molecular models explain the differences their physical and chemical properties between the two molecules and explain why DMSO is classified as an aprotic but highly dipolar solvent. The results are also in agreement with data obtained using X-ray diffraction, neutron diffraction, and theoretical calculations.

Controllable defluorination of fluorinated graphene and weakening of C-F bonding under the action of nucleophilic dipolar solvent.

The effect of solvent on the chemical structure and properties of fluorinated graphene (FG) was particularly investigated in this work. It is found that the reduction of FG and the weakening of strong covalent C-F bonding take place under the action of some dipolar solvents even at room temperature. The rate of the C-F bond rupture reaction is positively influenced by the dipole moment of solvent and fluorine coverage of FG sheets. Meanwhile, defluorination of FG is controllable through the time and temperature of solvent treatment. These solvents function as the nucleophilic catalysts, promoting chemical transformation, which leads to a series of changes in the structure and properties of FG, such as a decline of fluorine concentration of about 40% and the reduction of thermal stability and band gap from 3 to 2 eV. After the treatment with dipolar solvent N-methyl-2-pyrrolidinone, FG maintained a capacity of 255 mA h g(-1) and a power density of 2986 W kg(-1) at a high discharge rate, while the pristine FG could not be discharged at all. This is called the "solvent activation" effect on the electrochemical performance of FG. The finding may draw attention to the effect of various external factors on the chemical structure and properties of FG, which is of great importance for the realization of the FG's potential.

Influence of asymmetric depletion of solvents on the electric double layer of charged objects in binary polar solvent mixtures.

For binary solvent mixtures composed of ions and two kinds of polar solvents, the electric double layer near a charged object is strongly affected by not only the binary solvent composition but also the nature of the solvents, such as the volume and dipole moment of the solvent molecule. Accounting for the difference in sizes of solvents and the orientational ordering of solvent dipoles, we theoretically obtain general expressions for the spatial distribution functions of solvents and ions, in planar geometry and within the mean-field approach. While focusing on long-range electrostatic interaction and neglecting short-range interactions such as preferential solvation, our approach predicts an asymmetric depletion of the two solvents from the charged surface and a behavior of decreased permittivity of the binary solvent mixture. Furthermore, we suggest that the key factor for the depletion is the ratio of the solvent dipole moment to the solvent volume. The influence of the binary solvent composition, the volume of solvent and the dipole moment of the solvent on the number density of solvents, the permittivity and the differential capacitance is presented and discussed. We conclude that accounting for the difference in the volume and dipole moment between polar solvents is necessary for a new approach to represent more realistic situations such as preferential solvation.