Electrostatic Interaction Parameters Research Articles

Machine‐Learning‐Based Exploration of Bending Flexoelectricity in Novel 2D Van der Waals Bilayers

AbstractAccurate examination of electricity generation stemming from higher‐order deformation (flexoelectricity) in 2D layered materials is a highly challenging task to be investigated with either conventional computational or experimental tools. To address this challenge herein an innovative and computationally efficient approach on the basis of density functional theory (DFT) and machine‐learning interatomic potentials (MLIPs) with incorporated long‐range interactions to accurately investigate the flexoelectric energy conversion in 2D van der Waals (vdW) bilayers is proposed. In this approach, short‐range interactions are accurately defined using the moment tensor potentials trained over computationally inexpensive DFT‐based datasets. The long‐range electrostatic (charge and dipole) and vdW interaction parameters are calibrated from DFT simulations. Elaborated comparison of mechanical and piezoelectric properties extracted from the herein proposed approach with available data confirms the accuracy of the devised computational strategy. It is shown that the bilayer transition metal dichalcogenides can show a flexoelectric coefficient 2–7 times larger than their monolayer counterparts. Noticeably, this enhancement reaches up to 20 times for Janus diamane and fluorinated boron‐nitrogen derivatives of diamane bilayers. The presented results improve the understanding of the flexoelectric effect in vdW heterostructures and moreover the proposed MLIP‐based methodology offers a robust tool to improve the design of novel energy harvesting devices.

Phase Behavior of Melts of Diblock-Copolymers with One Charged Block.

In this work, we investigated the phase behavior of melts of block-copolymers with one charged block by means of dissipative particle dynamics with explicit electrostatic interactions. We assumed that all the Flory–Huggins χ parameters were equal to 0. We showed that the charge- correlation attraction solely can cause microphase separation with a long-range order; a phase diagram was constructed by varying the volume fraction of the uncharged block and the electrostatic interaction parameter λ (dimensionless Bjerrum length). The obtained phase diagram was compared to the phase diagram of “equivalent” neutral diblock-copolymers with the non-zero χ-parameter between the beads of different blocks. The neutral copolymers were constructed by grafting the counterions to the corresponding co-ions of the charged block with further switching off the electrostatic interactions. Surprisingly, the differences between these phase diagrams are rather subtle; the same phases in the same order are observed, and the positions of the order-disorder transition ODT points are similar if the λ-parameter is considered as an “effective” χ-parameter. Next, we studied the position of the ODT for lamellar structure depending on the chain length N. It turned out that while for the uncharged diblock copolymer the product χcrN was almost independent of N, for the diblock copolymers with one charged block we observed a significant increase in λcrN upon increasing N. This can be attributed to the fact that the counterion entropy prevents the formation of ordered structures, and its influence is more pronounced for longer chains since they undergo the transition to ordered structures at smaller values of λ, when the electrostatic energy becomes comparable to kbT. This was supported by studying the ODT in diblock-copolymers with charged blocks and counterions cross-linked to the charged monomer units. The ODT for such systems was observed at significantly lower values of λ, with the difference being more pronounced at longer chain lengths N. The fact that the microphase separation is observed even at zero Flory–Huggins parameter can be used for the creation of “high-χ” copolymers: The incorporation of charged groups (for example, ionic liquids) can significantly increase the segregation strength. The diffusion of counterions in the obtained ordered structures was studied and compared to the case of a system with the same number of charged groups but a homogeneous structure; the diffusion coefficient along the lamellar plane was found to be higher than in any direction in the homogeneous structure.

Optical spectroscopic investigation of Ba3Tb(PO4)3 single crystals for visible laser applications

Optical spectroscopic characterization of Ba3Tb(PO4)3 single crystals grown by the Czochralski technique was performed in order to evaluate the potential for visible laser application. Powder X-ray diffraction patterns at different temperatures were recorded to calculate the thermal expansion for temperatures between 298 and 1073 K. Refractive indices in the visible spectral region were determined and fitted into a Sellmeier equation. Absorption spectra were measured in the UV and visible range to obtain the ground-state absorption cross-sections, which were used for a Judd-Ofelt analysis. Electrostatic and spin-orbit interaction parameters were fitted, allowing for more precise Judd-Ofelt calculations. The Judd-Ofelt intensity parameters were calculated and employed to study the radiative transition probabilities, branching ratios and radiative lifetime associated to the potential upper laser level 5D4. The fluorescence spectrum shows several lines in the visible range, which can be assigned to 5D4 → 7FJ (J=6,5,4,3,2,1,0) transitions of Tb3+. These data enabled the determination of the branching ratio of the transitions from the 5D4 excited state. The stimulated emission cross-section of the 5D4 → 7F5 transition was found to be 5.9 × 10−22 cm2 at a wavelength of 549.6 nm by using the Füchtbauer-Ladenburg equation. Fluorescence decay curves were recorded to obtain the fluorescence lifetime. The resulting value of 3.17 ms is in reasonable agreement with the theoretical value of 3.54 ms pointing towards a high radiative quantum efficiency of 90% despite a high doping ion density of 3.5 × 1021 cm−3 and revealing the low effect of detrimental inter-ionic processes in Tb3+.

On Lewis acidity/basicity and hydrogen bonding in the equation-of-state approach

The central objective of this work is to revisit the development of the widely used Lewis acidity/basicity scales and seek possibilities for their eventual unification. The primary focus is on hydrogen bonding interactions which are dealt with the estimation of hydrogen-bond-donor (HBD) and hydrogen-bond-acceptor (HBA) capacity of the compounds. The developments are made by having in mind their implementation in a thermodynamic equation-of-state framework for the reliable estimation of all basic thermodynamic quantities of compounds over a broad range of external conditions. Gutmann’s donor and acceptor numbers, the Hard–Soft Acid and Base principle, Drago’s electrostatic and covalent interaction parameters E and C, the LFER/LSER acidity and basicity, α and β (A and B), parameters, the Hansen partial hydrogen – bonding solubility parameters and Klamt’s quantum-chemically derived hydrogen – bonding cosmoments are all combined with the recent partial solvation parameter (PSP) approach in an effort to develop a unified hydrogen bonding scale. The challenges and the perspectives of this unification are critically discussed.

Equilibrium fluid-crystal interfacial free energy of bcc-crystallizing aqueous suspensions of polydisperse charged spheres.

The interfacial free energy is a central quantity in crystallization from the metastable melt. In suspensions of charged colloidal spheres, nucleation and growth kinetics can be accurately measured from optical experiments. In previous work, from these data effective nonequilibrium values for the interfacial free energy between the emerging bcc nuclei and the adjacent melt in dependence on the chemical potential difference between melt phase and crystal phase were derived using classical nucleation theory (CNT). A strictly linear increase of the interfacial free energy was observed as a function of increased metastability. Here, we further analyze these data for five aqueous suspensions of charged spheres and one binary mixture. We utilize a simple extrapolation scheme and interpret our findings in view of Turnbull's empirical rule. This enables us to present the first systematic experimental estimates for a reduced interfacial free energy, σ(0,bcc), between the bcc-crystal phase and the coexisting equilibrium fluid. Values obtained for σ(0,bcc) are on the order of a few k(B)T. Their values are not correlated to any of the electrostatic interaction parameters but rather show a systematic decrease with increasing size polydispersity and a lower value for the mixture as compared to the pure components. At the same time, σ(0) also shows an approximately linear correlation to the entropy of freezing. The equilibrium interfacial free energy of strictly monodisperse charged spheres may therefore be still greater.

On the use of one-step perturbation to investigate the dependence of different properties of liquid water on a variation of model parameters from a single simulation†

One-step perturbation is an efficient technique to explore the relation between force-field or model parameters and properties of a molecular system in the condensed phase from a single simulation of the latter using a particular set of model parameters. The accuracy of the prediction of different properties as a function of the extent of perturbation of different model parameters is evaluated using liquid water at ambient temperature and pressure as the test system. The accuracy of predicted changes in free enthalpy, energy, radial distribution functions and dielectric permittivity due to variations in geometric, van der Waals or electrostatic interaction parameters is strongly dependent on the type of property analysed and the type of parameter varied. This is illustrated by the application of one-step perturbation to predict the properties of four models of liquid water from the four ensembles generated for these models.

A surface site interaction model for the properties of liquids at equilibrium

The electrostatic solvent competition model developed to estimate solvent effects on solution phase association constants for formation of 1 : 1 complexes between two solutes has been extended to provide a general treatment of intermolecular interactions in the liquid state. The interactions of a molecule with its solvation shell are described by a set of surface site interaction points (SSIPs). An SSIP represents a molecular surface area of 9.5 A2, a volume of 5 A3, and is characterised by an electrostatic interaction parameter, ei, obtained from the molecular electrostatic potential surface calculated in the gas phase or from functional group H-bond parameters experimentally determined in solution. A liquid is treated as an ensemble of SSIPs that interact with a probability governed by the sum of the electrostatic interaction energy, given by eiej, and a constant van der Waals term of −5.6 kJ mol−1. The speciation of SSIP contacts is determined as a Boltzmann-weighted population of states, and this allows calculation of a variety of thermodynamic properties of solutions. The model assumes that unbound states are possible for SSIPs due to the large amount of void space present in a liquid. This provides a straightforward thermodynamic connection between different phases, because unbound states have the same chemical potential in different environments. The free energy of transfer of a molecule between two different liquid phases is calculated as the sum of a binding energy, which is a measure of the total interaction of all SSIPs in the molecule with the liquid, and a confinement energy, which is a measure of the entropic cost of confining the molecular SSIPs to that phase. Calculated liquid–liquid transfer free energies agree with experiment (±1 to 3 kJ mol−1) for a collection of alkanes, ethers, alcohols and water. The calculations provide insight into the molecular basis of the hydrophobic effect, the origin of the difference in H-bond populations in water and alcohols, and the differences between 1-octanol and n-hexadecane partition coefficients as measures of hydrophobicity. Solvent effects on association constants for formation of 1 : 1 H-bonded complexes are also reproduced by the calculations (±0.5 log units). In addition to comparison with experimental thermodynamic data, this model can also be validated using spectroscopic data on the speciation of different H-bonded states in solution.

Physically Motivated, Robust, ab Initio Force Fields for CO2 and N2

We present a novel methodology for developing physically motivated, first-principles polarizable force fields and apply these techniques to the specific cases of CO(2) and N(2). Exchange, electrostatic, induction, and dispersion interaction parameters were fit to symmetry adapted perturbation theory (SAPT) dimer energy calculations, with explicit terms to account for each of the dominant fundamental interactions between molecules. Each term is represented by a physically appropriate functional form and fitted individually based on the results of the SAPT decomposition. The resulting CO(2) and N(2) force field was benchmarked against a diverse set of experimental data, including the second virial coefficient, density, scattering structure factor, heat capacity, enthalpy of vaporization, and vapor-liquid coexistence curves. In general, excellent agreement with experimental data is obtained with our model. Due to the physical nature of their construction, these force fields are robust and transferable to environments for which they were not specifically parametrized, including gas mixtures, and we anticipate applications in modeling CO(2)/N(2) adsorption in polar and/or heterogeneous media.

Improvements of COSMO-SAC for vapor–liquid and liquid–liquid equilibrium predictions

The COSMO-SAC model has been revised for a better description of the nonideality of liquid mixtures, and thus more accurate predictions of both vapor–liquid equilibrium (VLE) and liquid–liquid equilibrium (LLE). Two major modifications are introduced. First, the electrostatic interaction parameter is made temperature-dependent, which is important in LLE predictions for non-hydrogen-bonding systems. Second, the variation in the strength of hydrogen-bonding interactions involving different types of functional groups is treated using separate sigma-profiles. The overall RMS error in LLE predictions of 278 binary mixtures (4281 data points) from the new model is 0.1047, which is 30% lower than that from the original COSMO-SAC (0.1446), 10% lower from that of UNIFAC-LLE (0.1161), and comparable to that from the modified UNIFAC model (0.1048). The accuracy in VLE predictions is also improved. The average deviation in total pressure at a fixed liquid composition is 6.54% and the overall average deviation in vapor phase composition is 2.57% for 1338 binary mixtures, compared to 7.25% and 2.83% from the original model.

Characterization of the surface chemistry of carbon materials by potentiometric titrations and temperature-programmed desorption

Two methods currently used to characterize oxygen-containing functional groups on the surface of carbon materials are compared, namely potentiometric titration (PT) and temperature-programmed desorption (TPD). Two materials were used, activated carbon and multi-walled carbon nanotubes, which were subsequently modified by oxidative treatments in order to produce samples with different amounts of surface groups. The concentrations of carboxylic acid groups determined by both techniques are in good agreement, but the quantitative result obtained by TPD is closer to the analytical concentration of the species obtained by elemental analysis. Discrepancies between the quantitative results are more pronounced at higher p K a values (weak acids), where the concentrations determined by PT are lower than those obtained by TPD. This directly reflects the effects of neglecting the electrostatic interaction parameter. The TPD method was particularly suited for the characterization of samples modified with ethylenediamine, which is anchored to specific oxygenated groups. PT results are useful to describe the material behaviour in aqueous solutions, where the activity of the surface groups depends not only on their concentrations, but also on their environment.

Study on physical and electrostatic interactions of counterions in poly(perfluorosulfonic) acid matrix: Characterization of diffusion properties of membrane using radiotracers

The self-diffusion coefficients of water and ions were used to study the physical (tortuosity) and electrostatic interactions of counterions in poly(perfluorosulfonic) acid membrane (Nafion-117) matrix. The self-diffusion coefficients of water ( D H 2 O m ) were measured in the water swollen Nafion-117 membrane with Zn 2+, Ca 2+, Sr 2+, and Fe 2+ counterions by analyzing the experimental exchange rates between tritium tagged water (HTO) in membrane and equilibrating water. In order to study the effects of equilibrating solution, the HTO-desorption rate profiles between the membrane samples in H + or Cs + forms and equilibrating solution containing CsCl or HCl (0.25 mol/L) were measured. It was observed that the HTO-exchange rate profile was slower in case of membrane sample in Cs +-from equilibrated with salt/acid solution than that equilibrated with deionized water in same ionic form. However, HTO-exchange rate profile did not alter in case of H +-form of membrane on equilibration with salt or acid solution. The variation of ln D H 2 O m with polymer volume function V p/(1 − V p), where V p is polymer volume fraction, indicated that: (i) D H 2 O m in the membrane with multivalent counterions was lower than that reported for membrane with monovalent counterions at same V p, and (ii) the linear trends observed in variation of ln D H 2 O m with V p/(1 − V p) for multivalent and monovalent counterions were significantly different. The values of D H 2 O m in membrane normalized with D H 2 O m at V p = 0 were taken as an estimate of the tortuosity factor for self-diffusion of ions in the membrane matrix. The self-diffusion coefficients of ions reported in the literature along with tortuosity factor obtained from D H 2 O m in the corresponding ionic forms of the membrane were analyzed to obtain the charge ( Z i ) independent electrostatic interaction parameter g( φ) of monovalent and divalent ions in the membrane. This analysis indicated that g( φ) also vary exponentially as a function of V p/(1 − V p) irrespective of charge on counterions. In order to study the influence of V p on diffusional transport rates of Na + and Cs + ions in membrane, a permeation experiment was carried out using H +-form of membrane having high water volume fraction. The diffusional transport rates of Cs + and Na + in H +-form of membrane were found to be similar indicating that the water volume fraction in membrane has strong influence on the parameters that govern the diffusion across the Nafion-117 membrane.

Fe2+/Fe3+charge ordering in contact layers of lamellar magnetism: Bond valence arguments

Fe 2+ /Fe 3+ charge ordering in the contact layers of lamellar magnetism in the hematite-ilmenite series is not a postulate of the lamellar magnetism hypothesis. Such ordering is possible, however, and a model was suggested earlier in which contact layer Fe 2+ octahedra share faces with ilmenite layer Ti 4+ octahedra, and contact layer Fe 3+ octahedra share faces with hematite layer Fe 3+ octahedra, thus copying the shared-face configurations of ilmenite and hematite, respectively. This model and related charge-balance matters could be explored using bond-valence theory, a simplified picture of complex bonding which takes into account the relationship between bond strength and bond distance, but ignores magnetism. This has now been done, and shows that local oxygen charge satisfaction is strongly favored by a different charge-ordering scheme, in which contact layer Fe 3+ octahedra share faces with ilmenite layer Ti 4+ octahedra, and contact layer Fe 2+ octahedra share faces with hematite layer Fe 3+ octahedra. A new, more sophisticated, Monte Carlo simulation of the interface, including electrostatic and magnetic interaction parameters of cations, reported in detail elsewhere, independently shows the same charge-ordering scheme. For the cation ordered metastable ilmenite 50 composition, bond-valence theory indicates that, unlike the contact layers, the favored charge ordering scheme in Fe layers would have the same shared-face configurations as ilmenite and hematite.

A General Approach for the Prediction of the Intestinal Absorption of Drugs: Regression Analysis Using the Physicochemical Properties and Drug–Membrane Electrostatic Interaction

A general method for predicting the intestinal absorption of a wide range of drugs using multiple regression analysis of their physicochemical properties and the drug-membrane electrostatic interaction was developed. The absorption rates of tested drugs from rat jejunum were measured by the in situ single-pass perfusion technique. The drugs used in this study were divided into three groups for regression analysis, and a smaller “test” set of compounds was used to assess the predictive capacity of the regression equation. When the analysis was applied to each respective group of drugs (i.e., anionic, cationic, and nonionized compounds), obtained regression coefficients were 0.569, 0.821, 0.728 by using the organic solvent (n-octanol)/buffer partition coefficient, 0.730, 0.734, 0.914 using the permeation rate across a silicon membrane, and 0.790, 0.915, 0.941 using an EVA membrane, respectively. However, smaller regression coefficients of 0.377, 0.468, and 0.718 were obtained when these three groups of drugs were put together for prediction. Meanwhile, correlation was improved remarkably when drug-membrane electrostatic interactions, namely, hydrogen-bonding donor (Hα) and acceptor (Hβ) activity or index of electricity (Ec), were added to the other parameters of lipophilicity and permeation rate across the EVA membrane (r=0.880 and 0.883, respectively). Moreover, the equation obtained from these regression analyses was applicable even to the prediction of the absorption of the zwitterionic drugs. These results suggest that including the electrostatic interaction parameters in addition to lipophilicity and permeability across artificial membranes would afford a better prediction for the intestinal absorption of the vast majority of drugs.

Influence of monovalent cations on magnesium binding to poly-RNA by solution titration calorimetry: an analysis of the salt dependence of binding enthalpies and entropies

The salt dependence of the binding constant (K) and enthalpic (ΔH) and entropic (ΔS) components for magnesium binding to poly-RNA was determined as a function of the concentration and identity of monovalent counter ions (M+). Both ΔH and ΔS were found to vary linearly with ln [M+]. A theoretical analysis of the experimental data revealed that the temperature dependence of the product of the density of bound counter ions and the electrostatic interaction parameter, δ(m′ψ)/δT, is non-negligible, although it has previously been ignored. The sign of δ(m′ψ)/δT was negative for poly(A) and positive for poly(U), indicating that the charge density of poly(A) decreased with temperature, while that of poly(U) increased. These results are related to the distinct solution structures of the RNA homopolymers. Considerable support was lent to this calorimetric approach by the excellent agreement obtained in a test comparison between experimental and calculated parameters. From the intercept of energy term versus ln [M+] plots, the non-electrostatic contributions, ΔH° and ΔS°, were determined. For each polynucleotide, the similarity in ΔG° over the series of monovalent ions used in each study suggests a compensatory relationship between ΔH° and ΔS°, each of which shows significant variation. The non-electrostatic contribution to binding of divalent magnesium is generally entropically favorable and enthalpically unfavorable for both poly(A) and poly(U).

Ion-solvation and ion-pair equilibria in electrolyte solutions

In a concentrated acid or salt solution the amount of solvent molecules bound in the ionic solvation layers is an appreciable fraction of the total amount of solvent available. Some of the bound solvent molecules are released upon contact-ion-pair formation. The participation of the solvent is taken into account in the calculation of ion-pair formation by explicitly balancing the mixing entropy change with the standard and excess energy changes. The latter are taken from the mean spherical approximation theory for ion-ion interaction, augmented with a term for electrostatic ion-ion-pair interaction. A least-squares global optimization applied to a large body of available literature data on conductivities of univalent electrolytes over the concentration range up to saturated solutions is used for obtaining consistent values for ionic hydration numbers, standard free energies of ion-pair formation and electrostatic interaction parameters. With these values the measured conductivity data can be reproduced over the whole concentration range with an accuracy approaching that of the measurements themselves. As an example, the results for HCl and CsCl are presented and compared.

Multipolar electrolyte solution models. IV. Thermodynamic perturbation theory

Expressions for the equilibrium thermodynamic properties of a model electrolyte solution are obtained from the Taylor series expansion of the Helmholtz free energy in electrostatic interaction parameters. Rational polynomials, which are explicit in temperature, density, and composition, for ion–ion, ion–solvent, and solvent–solvent components of the free energy are used to extrapolate these series to low temperatures and finite ion concentrations. Comparisons of predicted internal energies, Helmholtz free energies, and compressibility factors are made with the results of Monte Carlo computer simulation. Predictions for the solutions are comparable in accuracy to those for the pure solvent. An expression for the solute concentration dependent dielectric response of the solution is developed and tested by comparison with simulation and experiment. This function introduces observed saturation with increasing ionic concentration to the ion–solvent and solvent–solvent contributions to the energies.

Hydrogen ion equilibria of .alpha.-globulin from Sesamum indicum L

The hydrogen ion equilibrium of the high mol. wt. protein alpha-globulin of sesame seed was investigated in the pH range 2.0-12.0. Analysis of the data gave pKint values of 4.37, 7.47 and 9.60 for carboxyl, imidazole and epsilon-amino groups, resp. The number of residues containing these groups agreed well with values obtained from amino acid analysis. Tyrosyl phenolic groups had a pKint of 9.41 as determined from spectrophotometric titration. The value of the electrostatic interaction parameter, w, varied with pH, and this is discussed in relation to the hydrodynamic properties of the protein.

Interplay of hydrophobic and electrostatic interactions in biopolymer chromatography : Effect of salts on the retention of proteins

The effect of salt on the retention behavior of proteins in electrostatic and hydrophobic interaction chromatography is described by a three-parameter equation, log k′ = A - B log m s + Cm s, where k′ is the retention factor and m s is the molality of the salt in the eluent. Parameter B, termed the electrostatic interaction parameter, depends on the characteristic charge of the protein and the salt counterion and governs the change of retention with the salt concentration in ion-exchange chromatography. According to the model the magnitude of the hydrophobic interaction parameter C is determined by the hydrophobic contact area upon protein binding at the stationary phase surface and the properties of the salt as measured by its molal surface tension increment. Retention data measured at different salt concentrations in the eluent on a variety of ion exchangers can be fitted to the above equation which yields U-shaped plots of log k′ against log m s. The limiting slopes of the appropriate plots at sufficiently low and high salt concentrations can be used to evaluate the electrostatic and hydrophobic interaction parameters, respectively. The approach, which is based on a combination of established treatments of electrostatic and hydrophobic interactions offers a convenient framework for analyzing retention data in biopolymer high-performance liquid chromatography and for the characterization of stationary phases. Furthermore, it may facilitate some characterization of protein molecules on the basis of their retention behavior as a function of the concentration and nature of the salt in the eluent. In the treatment of electrostatic interactions use is made of the counterion condensation theory that is believed to make possible a more comprehensive analysis than the traditional stoichiometric ion-exchange model which assumes binding of the proteins by coulombic interactions at discrete sites. The treatment of hydrophobic interactions is based on an adaptation of the solvophobic theory which predicts that the hydrophobic portion of the free energy of binding is proportional to the hydrophobic contact area and the microthermodynamic surface tension of the aqueous salt solution. Despite its simplicity the theory was successful in explaining the observed effect of the nature and concentration of salt in the eluent, the pH and the effect of the density of fixed charges at the surface of the stationary phase in the absence of specific salt effects.

Metal chelate-interaction chromatography of proteins with iminodiacetic acid-bonded stationary phases on silica support

An iminodiacetic acid (IDA)-bonded stationary phase on a wide-pore microparticulate silica support was used for the chromatography of amino acids and proteins at pH 5.0 and 6.0. Without chelated metal the retention behavior of the stationary phase parallelled that of other silica-bound cation exchangers used in high-performance liquid chromatography of proteins. In metal chelate-interaction chromatography (MCIC) with IDA, chelated by Cu(II), Zn(II), Ni(II), or Fe(III), amino acids were most strongly retained on Cu(II)-IDA, whereas all metal chelates separated the proteins under investigation but with different selectivity. The effect of salt concentration in the eluent on protein retention was investigated and the pertinent electrostatic and hydrophobic interaction parameters were evaluated. The proteins were separated by MCIC with increasing salt gradient and, using the same column, by hydrophobic-interaction chromatography with decreasing salt gradient. In MCIC the addition of methanol to the mobile phase had disparate effect on protein retention, whereas addition of histidine or glycine, which acted as competing ligands, reduced the retention.

Optical spectra and crystal field analysis of Nd3+ in the cubic Cs2NaYCl6 host

Optical absorption and emission measurements are reported for the Cs2NaYCl6:Nd3+ (∼5 mol %) system under variable-temperature (4.2–296 K) conditions. The absorption measurements span the 12 000–24 000 cm−1 spectral region, and emission spectra are presented throughout the 13 300–18 860 cm−1 region. All the spectra are consistent with the retention of octahedral (Oh) site symmetry for the Nd3+ ions down to 4.2 K. Thirty-four crystal-field levels spanning 14 multiplets of the Nd3+ 4f3 electronic configuration are located and assigned on the basis of the experimental data. The most intense lines observed in each of the multiplet-to-multiplet transition regions are assigned to one-phonon (electric-dipole) vibronic transitions associated with the three odd-parity moiety modes of the NdCl3−6 cluster, ν3(tlu), ν4(tlu), and ν6(t2u). The ν3 lines generally appear as doublets (split by ∼10–15 cm−1) and are displaced from magnetic-dipole origins by ∼245–260 cm−1; the ν4 lines are displaced from origins by ∼100 cm−1, and are generally sharp and symmetric; and the ν6 lines are generally asymmetric in shape (or appear as doublets split by ∼2–6 cm−1), and are displaced from origins by 70–75 cm−1. Additional vibronic lines are observed persistently at displacement energies of ∼40–45 and ∼55–60 cm−1, but these lines are considerably less intense than those assigned to the ν3, ν4, and ν6 moiety-mode vibrations. The energy level data obtained from experiment is rationalized in terms of a 9-parameter model which includes the radial spin-orbit interaction parameter (ζso), the Slater–Condon one-particle electrostatic interaction parameters (F2, F4, and F6), the two-particle electrostatic configuration-interaction parameters (α, β, and γ), and the octahedral (Oh) crystal-field interaction parameters (B(4)0 and B(6)0). The values deduced for the crystal-field parameters are B(4)0 =2400 cm−1 and B(6)0 =−330 cm−1 (expressed with unit-tensor normalization).