Thermochemical behaviour of crown ethers in the mixtures of water with organic solvents. Part III. Hydrophobic hydration and preferential solvation of crown ethers in water-amide mixtures at 298.15 K

Enthalpies of solution of 15-crown-5 (15C5) and benzo-15-crown-5 ether (B15C5) in the mixture of water with formamide (F) and N-methylformamide (NMF) have been measured at 298.15 K. The values of standard enthalpies of solution of 15C5 are negative and those of B15C5 are positive in the mixtures. The results of calorimetric measurements of dissolution enthalpies of 15C5 and B15C5 in the mixture of water and organic solvents at 298.15 K are discussed with regard to the intermolecular interactions that occur in the solution. Considering the properties and molecular structure of the examined crown ethers, particular attention is given to the hydrophobic hydration and preferential solvation of solute in the mixed solvents. The preferential solvation in the examined systems was analysed using the Covington model developed by Balk and Somsen. The relative importance of the solvation mechanism appeared to depend on co-solvent properties.     

紫外光谱法研究环糊精／精噁唑禾草灵超分子复合体

采用紫外光谱法研究了β-环糊精（β-CD）、羟丙基-β-环糊精（HP-β-CD）和部分甲基化β-环糊精（RAMEB）与精噁唑禾草灵（FE-p）间的超分子作用。在体积比为1：4的乙醇水溶剂中，在298.15～318.15K的温度范围内，3种环糊精均能与FE-p形成稳定的1：1型的超分子复合体，其结合常数值大小依次是KRAMEB〉KHP-βCD〉KβCD，且随着温度的升高而降低。经过计算，热力学参数AG，△H和△S均为负值，说明结合反应是放热反应且能自发进行，焓变是形成超分子复合体的主要驱动力，符合熵焓补偿效应。     

Experimental evidence of chiral crown ether complexation with aromatic amino acids

The complexation of L ‐ and D ‐enantiomers of phenylglycine, phenylalanine, and tryptophan with D ‐mannonaphto‐crown‐6‐ether in methanol solution was studied by NMR and isothermal titration calorimetry (ITC) at 298.15 K. The total heat effects attributed to the binding phenomena were measured in the range of 1.8 to 7.7 mJ, and the complexation was found stereo‐specific. The binding topologies were estimated basing on ¹H 2D‐ROESY experiments. The analysis of Job plots obtained from ¹H NMR‐monitored titrations proved the coexistence of 1:1 and 1:2 (crown ether:amino acids) complexes, which thermodynamic parameters, K_s, ΔG, ΔH°, and TΔS were determined with the aid of ITC. The 1:1 complexes were found enthalpically stabilized, generally by electrostatic interactions between the charged NH group of amino acid and crown ether macrocyclic moiety, while the binding of the second amino acid molecule was driven entropically due to solvatophobic effect. Strong enthalpy–entropy compensation points towards the uniform binding mode of all complexes studied. The mode of complex formation was found solvent dependent. For phenylalanine guest studied in various solvent systems, in contrast to the aqueous media, the noticeable chiral recognition is observed in methanol solution, and the complex stoichiometry (1:2 ether:Phe) differs from the 2:1 one, determined previously for the same host‐guest system in water (J. Thermal. Anal. Cal. 2006; 83: 575–578). Copyright © 2007 John Wiley & Sons, Ltd.     

Solvation thermodynamics of xylitol in water and in aqueous amino acids at 298.15 K

The scaled particle theory has been applied to calculate the free energy, ΔG_solv, enthalpy, ΔH_solv, and entropy, ΔS_solv of solvation for xylitol in water and in aqueous amino acids (glycine, alanine and valine) at 298.15 K. The solvation energy, enthalpy and entropy of xylitol are expressed in terms of their various ingredients. The results show that the interaction terms contribute favorably to the process of solvation. The results suggest that the cavity formation for accommodation of xylitol molecules in aqueous amino acids is an enthalpy‐dominated process. Furthermore, the investigated parameters indicate that xylitol–amino acid interactions follow the sequence: glycine alanine valine water. The findings of the present work may help to throw light on the role that xylitol can play to stabilize macromolecules like proteins in aqueous solutions. Copyright © 2012 John Wiley & Sons, Ltd.     

Infrared Spectral Study of Crown Ether Molecular Complexes with 4-Nitroaniline

Abstract

The IR spectra of the crystalline complexes of 4-nitroaniline with crown ethers were studied, viz., 18-crown-6, benzo-18-crown-6, dibenzo-18-crown-6, dicyclohexano-18-crown-6 and dibenzo-24-crown-8. The hydrogen bonding interaction was found to be predominant in these complexes. The strongest among them was the one formed with the most flexible crown ether dicyclohexano-18-crown-6, in which both the N-H bonds of 4-nitroaniline were shown to be equivalently hydrogen bonded to the crown ether oxygens. The shift of the NO₂ absorption bands indicated that the resonance form (^?C₂N-C₆H₄=N⁺H₂) is favored upon complexation with the larger ring size dibenzo-24-crown-8, while for the other complexes, the resonance form (C₂N-C₆H₄-NH₂) is favored; a fact which was attributed to geometrical factors.     

Sensing and fluorescence behaviors of complex formation between new lariat crown ethers bearing fluorescence sidearm and metal ions in methanol solution

Fluorescence spectroscopic properties of the complex formation between six new C-pivot16-crown-5 ethers bearing fluorescence side arms and metal ions are studied. Six new C-pivot 16-crown-5 ethers that carry fluorescence side arms were synthesized. The research also included the examination of different fluorescent behavior of the new lariat ethers in the presence of silver and sodium ions (Ag⁺ and Na⁺) in methanol. Because of the good fit of the cavity size of these 16-crown-5 ethers with the Ag⁺ and Na⁺ ions, remarkable change in fluorescent spectra were observed. The results showed that the new lariat crown ethers exhibited enhanced selectivity towards sodium ion, but silver ion show quenching abilities towards the ligands. Cooperative participations of the C-pivot side arm oxygen with the ring oxygen molecules in complexation was also confirmed by the single crystal X-ray crystallography of the complex, (C₂₇H₃₀O₇)·NaClO₄. The freezing of crown ring conformation and the diminishment of the chance of sidearm oscillation that occurred upon complexation with sodium ion are the main causes of fluorescent enhancement. The quenching caused by the addition of silver ion was found to explainable by the photo-induced electron transfer that results in the reduction of silver ions.     

Transformational Homologies in Amino Acid Sequences Suggest Memberships in Protein Families

The 20 amino acid monomers composing polymeric proteins are encoded using their individual properties relatable to thermodynamic potentials such as aqueous partial specific volumes, aqueous molar volumes, and free energies of transfer from hydrocarbon to water solvents. These principally hydrophobic solvation, “hydrophobicity”-derived free energies are minimized in protein folding as well as protein–protein and peptide–receptor interactions. Sequential patterns in the one-dimensional distribution of these energies, reflected in dominant wavelengths of amino acid hydrophobicity, and the locations of singular hierarchical, secondary and supersecondary structures are elucidated by orthogonal decomposition and eigenfunction construction followed by continuous wavelet and all poles, maximum entropy power spectral transformations. The resulting graphs discriminate among examples of structural families of proteins.     

Fluorescence enhancement of the aflatoxin B1 by forming inclusion complexes with some cyclodextrins and molecular modeling study

The interaction between the aflatoxin B₁ (AFB₁) and three cyclodextrins, α-cyclodextrin (α-CD), β-cyclodextrin (β-CD) and heptakis-2,6-dimethyl-o-β-cyclodextrin (ome-CD), was studied by spectrofluorescence technique. It was found that the inclusion association behavior occurs for the complexes of cyclodextrins with AFB₁. The fluorescence of AFB₁ is generally enhanced in the complexes with cyclodextrins in aqueous solutions. The inclusion complex constants of the three types of cyclodextrins at different temperatures were evaluated from Benesi-Hildebrand plot and also by non-linear regression analysis. These cyclodextrins can only form the 1:1 (host:guest) inclusion complex in the studied temperature range of 20-50 °C. The enthalpy (ΔH°) and entropy (ΔS°) changes of complexation were extracted from the temperature dependency of complex formation constants (K). Temperature-dependent measurements showed that the association step is controlled by enthalpy-entropy compensation effect. The use of ome-CD generally resulted in the greatest fluorescence intensity. On the other hand, the discrepancy between the exhibited enhanced fluorescence and thermodynamic parameters (ΔG°) is proposed to be different only by the orientation of the AFB₁ within the cyclodextrin cavity. To find the most favorable structure, the geometry of complex was investigated by molecular modeling approach employing the semiemperical HF-SCF calculations.     

Fluorescence, Induced Circular Dichroism and Molecular Mechanics of 1-Methyl Naphthalenecarboxylate Complexes with 2-Hydroxypropyl Cyclodextrins

Steady-state, time-resolved fluorescence, Circular Dichroism and Molecular Mechanics techniques were used to study the complexation of 1-methyl naphthalenecarboxylate (1MN) with the 2-hydroxylpropyl-α-, -β- and -γcyclodextrins (HPCDs). The emission spectrum of 1MN shows two bands whose intensity ratios (R) are sensitive to complexation. The stoichiometry, binding constants and thermodynamics parameters upon complexation were obtained from the variation of fluorescence intensity, R, and lifetime averages, , with [HPCD] and temperature. They were then compared with the ones obtained for the complexation of 1MN with the non-substituted α-, β- and γCDs. Like the 1MN:CD complexes, the 1MN:HPCD ones showed 1:1 stoichiometries, but they resulted relatively more stable. Molecular Mechanics calculations in the presence of water allowed us to understand the structure of the complexes and the possible driving forces responsible for the complexation. Geometry agrees with the experimental stoichiometry and the signs of enthalpy and entropy changes. R for the complexes, quenching, fluorescence depolarization measurements and induced circular dichroism spectra also supported the proposed structures.     

Crown ether enhancement of ionic conductivity in a polymer-salt system

Crown ethers are a class of organic compounds that form complexes with inorganic cations. When crown ethers are added to poly (vinylene carbonate) containing dissolved lithium salt, ionic charge transport in the solid electrolyte is assisted. The ionic conductivity of the polymer containing 12-crown-4 is three orders of magnitude greater than the conductivity in the polymer without crown ether. The conductivity of this system at room temperature is approximately 2.5×10⁻⁴ S cm⁻¹, higher than any polymer-lithium salt system yet reported. The effects of various crown compounds as well as their concentration effects are examined.     

Density functional theory study of the bis‐3‐benzo‐crown ethers and their complexes with alkali metal cations Na+, K+, Rb+ and Cs+

The binding interactions of bis‐3‐benzo‐15‐crown‐5 ethers and bis‐3‐benzo‐18‐crown‐6 ethers (neutral hosts) with a series of alkali metal cations Na⁺, K⁺, Rb⁺ and Cs⁺ (charged guests) were investigated using quantum chemical density functional theory. Different optimized structures, binding energies and various thermodynamic parameters of free crown ethers and their metal cation complexes were obtained based on the Becke, three‐parameter, Lee–Yang–Parr functional using mixed basis set (C, H, O, Na⁺ and K⁺ using 6‐31 g, and the heavier cation Rb⁺ and Cs⁺ using effective core potentials). Natural bond orbital analysis is conducted on the optimized geometric structures. The main types of driving force host–guest interactions are investigated. The electron donating O offers a lone pair of electrons to the contacting LP* (1‐center valence antibond lone pair) orbitals of metal cations. The bis‐3‐benzocrown ethers are assumed to have sandwich‐like conformations, considering the binding energies to gauge the exact interactions with alkali cations. It is found that there are two different types of complexes: one is a tight ion pair and the other is a separated ion pair. Copyright © 2011 John Wiley & Sons, Ltd.     

Spectroscopic Study of Some Alkali and Alkaline Earth Complexes with Benzo Crown Ethers in Ethanol Solution

The interaction between Na⁺, K⁺, Mg²⁺ and Ca²⁺ ions and benzo crown ethers B15C5, DB18C6, DB21C7, DB24C8 and DB30C10 in ethanol solution has been studied spectrophotometrically at 25°C. The formation constants for 1:1 complexes were determined by computer fitting of the resulting UV-absorbance-mole ratio data. In the case of all crown ethers used, K⁺ ion was found to form the most stable complexes. The observed selectivities of different benzo crowns for the cations used are discussed in terms of the cavity-cation size ratio, number of the donating oxygens participating in the cation binding, conformations of the free and complexed ligands and ionic solvation.     

Hydrophobic/hydrophilic solvation: inferences from Monte Carlo simulations and experiments

In this study Monte Carlo simulations are used to determine the solvation properties of model hydrophobic (xenon and hard sphere) and hydrophilic (dimethyl ether) solutes in SPC/E water. Various contributions to the experimental solvation entropy, including the solvent reorganization entropy, have been determined. The main conclusion drawn, which is in accord with solubility data, is that poor solubility correlates with poor solute-water interaction. At room temperature, energy dominates the aqueous solubility of both hydrophobic and hydrophilic solutes, rather than entropy. However, at higher temperatures the solubility can pass through a minimum, and then entropy becomes dominant. Another interesting finding is the presence of larger than expected cavities in water. Two different simulation results support this finding. This unexpected hollow structure in water explains why a hard sphere solute is more soluble in water than in a comparable hard sphere or Lennard-Jones solvent. Hydrogen bonding causes water to aggregate into clusters that produce a few large cavities rather than many smaller cavities. The propensity for clustering also explains why water gives the illusion of being a low density liquid. Sufficient theoretical apparatus is developed to connect theoretical solvation properties to those measured by simulation and experiment. Finally, based on gas solubility, an intuitive hydrophobic/hydrophilic scale is developed.     

DFT study of solvation of Li~+/Na~+ in fluoroethylene carbonate/vinylene carbonate/ethylene sulfite solvents for lithium/sodium-based battery

Choosing suitable solvent is the key technology for the electrochemical performance of energy storage device.Among them,vinylene carbonate(VC),fluoroethylene carbonate(FEC),and ethylene sulfite(ES)are the potential organic electrolyte solvents for lithium/sodium battery.However,the quantitative relation and the specific mechanism of these solvents are currently unclear.In this work,density functional theory(DFT)method is employed to study the lithium/sodium ion solvation in solvents of VC,ES,and FEC.We first find that 4VC-Li+,4VC-Na+,4ES-Li+,4ES-Na+,4FEC-Li+,and 4FEC-Na+are the maximum thermodynamic stable solvation complexes.Besides,it is indicated that the innermost solvation shells are consisted of 5VC-Li+/Na+,5ES-Li+/Na+,and 5FEC-Li+/Na+.It is also indicated that the Li+solvation complexes are more stable than Na+complexes.Moreover,infrared and Raman spectrum analysis indicates that the stretching vibration of O=C peak evidently shifts to high frequency with the Li+/Na+concentration reducing in nVC-Li+/Na+and nFEC-Li+/Na+solvation complexes,and the O=C vibration peak frequency in Na+solvation complexes is higher than that of Li+complexes.The S=O stretching vibration in nES-Li+/Na+solvation complexes moves to high frequency with the decrease of the Li+/Na+concentration,the S=O vibration in nES-Na+is higher than that in nES-Li+.The study is meaningful for the design of new-type Li/Na battery electrolytes.     

Calculation of the thermodynamic potentials of changes in translational, rotational, and vibrational degrees of freedom in the dimerization of aromatic molecules

The work presents a method for calculating the thermodynamic potentials (free Gibbs energy, enthalpy, and entropy) and an analysis of the contributions of changes in translational, rotational, and vibrational degrees of freedom to the energy characteristics of the dimerization of aromatic compounds. On the whole, changes in the Gibbs energy caused by these contributions were shown to exceed the experimental energies of dimerization in solution in magnitude, which was evidence of the necessity of taking them into account for the aggregation of molecules. We determined changes in the Gibbs energy and entropy of translational, rotational, and vibrational (mechanical) degrees of freedom and the enthalpy of vibrations, which can be used to analyze the energy characteristics of dimerization reactions. These dimerization energy components should be calculated separately for each compound.     

高温高压下湿空气的焓和熵计算

本文以湿空气透平和压缩空气储能系统中的工质-湿空气为研究对象,运用所提出的一个适合计算高温高压湿空气热力学性质的对应态维里方程,对湿空气的偏差焓和偏差熵进行了计算.在湿度0≤ W≤1 kg/kg(A),温度小于573.15 K,压力小于5 MPa时,与ASHRAE用的多项式维里方程计算得到的偏差焓和偏差熵进行了比较,误差很小.用这个对应态维里方程外推计算了温度和压力分别达到600 K和15 MPa的湿空气焓和熵.计算结果表明对湿空气而言压力和湿度越大,偏差焓和偏差熵越大;温度越高,偏差焓和偏差熵越小.     

Vibrational and orientational relaxation of thiocyanate ion in crown ether complexes

The IR absorption and Raman spectra of the following complex compounds based on thiocyanates and crown ethers have been studied: KSCN-18-crown-6, KSCN-dibenzo-18-crown-6, NaSCN-dibenzo-18-crown-6, and NaSCN-benzo-15-crown-5. The shape of the contour of the spectral line corresponding to the stretching vibration ₁(CN) of the thiocyanate ion in the indicated compounds in the temperature interval involving solid and liquid phases have been investigated for the first time. The reorientation parameters and molecular-relaxation characteristics of the thiocyanate ion in the crown compounds have been calculated. It is found that an increase in temperature leads to gradual freeing of cations and to the properties of crown compounds becoming identical to those of pure salt melts.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 23–27, January–February, 2005.     

Gibbs energy of cooperative hydrogen‐bonding interactions in aqueous solutions of amines and pyridines

A method to determine the Gibbs energy of specific (hydrogen bonding) interactions of a solute with water is proposed. The energies of hydrogen bonding in bulk water are very difficult to determine by any method. The Gibbs energy of hydration of substances forming hydrogen bonds with water is considered as the sum of contributions due to non‐specific interactions, the hydrophobic effect, and specific interactions. The first two terms were found to be rather accurately described by empirical equations. The Gibbs energies of hydrogen bonding of aliphatic amines and pyridines in bulk water are determined, and the results are compared with the energies of their complexes with one molecule of water. The cooperativity of hydrogen bonding is proved in aqueous solutions of amines and pyridines. To use our equations, experimental values of the Gibbs energies of solvation in 'standard' solvents need to be known. The Gibbs energies of solvation of ten amines and pyridines in dimethyl sulfoxide are determined experimentally using chromatographic head space analysis. The tendencies observed for the series of amines and pyridines are in agreement with other studies. Copyright © 2009 John Wiley & Sons, Ltd.     

Solution calorimetry of organic nonelectrolytes as a tool for investigation of intermolecular interactions

The paper is mainly the review and generalization of the previous publications of the authors. It demonstrates that solution calorimetry method gives the opportunities of more detailed understanding of various aspects of intermolecular interactions in solution. We are assured that prerequisite to such an understanding is the successive analysis of various solute–solvent systems from the simplest ones which include alkanes as a solute or as a solvent to the most complex systems with solvent self‐association via hydrogen bonding. Particular findings discussed in this paper are (i) an inconspicuous contribution of electrostatic solute–solvent interaction to the solvation enthalpy and, accordingly, the dominating contribution of dispersion interactions for nonspecifically solvated solutes; (ii) new, very general method for the extraction of specific interaction enthalpy from the enthalpy of solvation; (iii) new method of determination of self‐association enthalpies for the solvents associated via hydrogen bonding; (iv) new method for determination of cooperative hydrogen bonding enthalpies of proton acceptors with associated species of alcohols; (v) the unique method of experimental determination of the hydrophobic effect enthalpy. Copyright © 2007 John Wiley & Sons, Ltd.     

Relation between structure and enthalpy for stacking interactions of aromatic molecules

An approximately linear correlation has been found between the enthalpy of complexation and the area of overlap of the chromophores using published structural and thermodynamical data on the self- and hetero-association of aromatic molecules measured under similar solution conditions. This finding is consistent with the assumption that short-range van-der-Waals forces dominate over other contributions to the enthalpy of stacking of aromatic molecules. It provides a ‘model-independent’ approach for a priori estimation of the enthalpy of aromatic–aromatic stacking interactions from knowledge of the structural properties or vice versa.