Paradigms and paradoxes: Patterns and estimation of the entropy of formation of aqueous polynuclear oxyanions

A respectable number of energetic data and models of polynuclear oxyanions are known from the literature. However, we lacked information on “simple” energetics patterns of different aqueous polynuclear oxyanions; where “simple” means the use of parameters derived only from the stoichiometry of these species, and no additional extrathermodynamic quantities. A relationship between the entropy of formation and different parameters as negative value of the charge of the species, the number of oxygen atoms, the natural logarithm of the molecular weight, the total number of atoms and the number of central atoms that are gases is established and commented thereon.     

Paradoxes and paradigms: influence of the power of z on the estimation of entropies of formation of aqueous anions using simple parameters

We continue our use of “simple” energetic patterns, where simple means the use of parameters derived only from the stoichiometry of these species in our studies of the entropy of formation (TΔ_f S ^o) of aqueous anions. Relationships between the entropy of formation and different parameters such as the number of oxygen atoms, the natural logarithm of the molecular weight and the total number of atoms are explored. The charge of the species, z− continues to be explicitly considered where we now explore various choices of p and use of z ^p as a parameter.     

Thermodynamic study on inclusion complex formation of riboflavin with hydroxypropyl-β-cyclodextrin in water

The inclusion complex formation of riboflavin (RF) with hydroxypropyl-β-cyclodextrin (HP-β-CD) in water was investigated by ¹H NMR, UV-vis spectroscopy, and solubility methods. A 1:1 stoichiometry and thermodynamic parameters of complex formation (K, Δ_c G ⁰, Δ_c H ⁰, and Δ_c S ⁰) were determined. Complexation was characterized by negative enthalpy and entropy changes due to prevalence of van der Waals interactions and hydrogen bonding between polar groups of the solutes. A partial insertion of RF into macrocyclic cavity was revealed on the basis of ¹H NMR data and molecular mechanics calculation. Location of benzene ring of RF molecule inside the hydrophobic cavity of HP-β-CD results in an increase of aqueous solubility of the former.     

Inclusion Complex Formation of Thiacalix[4]arene and Xe in Aqueous Solution Studied by Hyperpolarized 129Xe NMR

The inclusion complex formation of 4-sulfothiacalix[4]arene sodium salt (STCAS) and Xe has been investigated by using hyperpolarized ¹²⁹Xe NMR spectroscopy. Our new continuous-flow type hyperpolarizing system has advantageous capabilities that can supply hyperpolarized gases continuously and directly to a sample solution in a NMR tube. Consequently saturated Xe concentration in the aqueous solution of STCAS is maintained during the NMR experiment, and ¹²⁹Xe NMR spectra can be obtained in remarkably short time. STCAS concentration dependence of ¹²⁹Xe chemical shift has been analyzed in an elaborated way by a computer method as well as a simple graphic method that we have proposed. The association constant K:13.6±0.8 M⁻¹ at 25 °C was obtained, and further analysis of the temperature dependence has successfully given thermodynamic parameters of enthalpy (ΔH) and entropy (ΔS) for the inclusion complex formation: ΔH = −11.9±1.9 kJ mol⁻¹ and ΔS = −17.4±5.8 JK⁻¹ mol⁻¹. The energetic aspects of complex formation are discussed from the size effect and from the molecular theory of standard entropy, and a release of definite number of water molecules from STCAS cavity is suggested in the inclusion complex formation with Xe.     

Polynuclear thallium(I) N,N-cyclo-hexamethylenedithiocarbamate [Tl2{S2CN(CH2)6}2]n: Chemisorption properties and copper(II) fixation modes

The Cu²⁺ sorption properties of the polynuclear thallium(I) N,N-cyclo-hexamethylenedithiocarbamate complex [Tl₂{S₂CN(CH₂)₆}₂]n (I) are reported. The sorption capacity of the complex is ∼67 mg/g. According to EPR data, freshly precipitated I binds copper from the aqueous phase by chemisorption, which results in the formation of the β-form of the paramagnetic trinuclear complex [CuTl₂{S₂CN(CH₂)₆}₄] (containing a small amount of its α-form). Scanning electron microscopic data indicate cardinal reformation of the chemisorbent particles upon copper(II) sorption, including changes in the particle shape and size.     

Thermodynamic Study of the Inclusion Interaction between Gemini Surfactants and Cyclodextrins by Isothermal Titration Microcalorimetry

The inclusion complexes of a series of bis-quarternary ammonium surfactants, (C _n N)₂Cl₂ (where n = 12, 14, 16) and sodium bis(2-ethylhexyl) sulfosuccinae (AOT), with α-cyclodextrin (α-CD), β-cyclodextrin (β-CD) and γ-cyclodextrin (γ-CD) in aqueous solutions were investigated by using isothermal titration calorimetry (ITC) at 298.15 K. The stability constants, stoichiometry, and formation enthalpies, entropies and Gibbs energies for the complexes in aqueous solutions have been derived from the calorimetric data. The values of the binding constant, K ^∘ _i , are very large, which indicates that these complexes are quite stable in their aqueous solutions. The enthalpy changes (ΔH ^∘) for all of the inclusion processes are negative, showing that the complex process is enthalpy driven. The entropy effect (TΔS ^∘) is negative, so the inclusion process is entropically unfavorable. The large negative Gibbs energy changes indicate that formation of host-guest inclusion complexes is generally a spontaneous process. The thermodynamic parameters are discussed in the light of the different structures of the host and guest molecules.     

Calorimetry to Evaluate Inclusion Mechanism in the Complexation Between 2-hydroxypropyl-β-cyclodextrin and Barbiturates in Aqueous Solution

Two different types (structures) of inclusion complexes with a 1:1 stoichiometry between barbiturates and 2-hydroxypropyl-β-cyclodextrin (HPCyD) were realized in aqueous solution using isothermal titration calorimetry and molecular dynamics simulation. The first type of complex with a higher association constant was entropy driven and the substituent R ₂ was inserted into the HPCyD cavity by hydrophobic interaction. The barbituric acid ring contributed to the second type of complex, which was characterized by large negative values of ΔH and small positive ΔS reflecting van der Waals interaction and/or hydrogen bonding formation between the hetero atoms in the barbituric acid ring and the secondary hydroxyl groups of HPCyD. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermodynamic properties of molybdenum-transition metal-hydrogen (nitrogen) solid solutions

Using the experimental values ΔH _H^exc and ΔS _H^exc (ΔH _N^exc and ΔS _N^exc) for solid solutions of hydrogen and nitrogen in molybdenum, we calculate the parameters ɛ_H^Mand ɛ_N^M of interaction between hydrogen (nitrogen) and molybdenum lattice doped with small additives of transition metals M; the values of the activity coefficients on hydrogen and nitrogen γ_H^M and γ_N^M, ΔH _H, ΔH _N, ΔS _H, ΔS _N; enthalpy (η_H^M and η_N^M) and entropy (σ_H^M and σ_N^M) parameters of interaction over the 1100–1300 K range for alloys of Mo_{1 − y} M _y H _x and Mo_{1 − y} M _y N _x types (y = 0.01 and 0.02; x = 0.01 and 0.02).     

Host–Guest Complexation of <Emphasis Type="Italic">β</Emphasis>-, <Emphasis Type="Italic">γ</Emphasis>-Cyclodextrin with Alkyl Trimethyl Ammonium Bromides in Aqueous Solution

As a continuation of our previous investigation, interactions between cyclodextrin (β-CD), γ-cyclodextrin (γ-CD) and alkyl trimethylammonium bromides in aqueous solutions have been studied with titration calorimetry and ¹H NMR at 298.15 K. The results are discussed in terms of the amphiphilic interaction of CD with surfactants and the iceberg structure formed by water molecules existing around the hydrophobic tail of surfactant molecules. The stoichiometry of the β-CD–surfactant system is 1:1 whereas that of the γ-CD–surfactant system is 1:2. The corresponding formation enthalpy (negative) of the complexes of the two systems decreases with an increase in the number of carbon atoms (n) in hydrophobic chain of surfactant molecule, C _n H_2n+1, whereas the entropy increases with the enlargement of n.     

Thermodynamic enthalpy–entropy compensation effects observed in the complexation of basic drug substrates with β-cyclodextrin

Measurement of the variation of inherent drug solubility (S _o) and 1:1 drug/cyclodextrin complex formation constants (K ₁₁) with temperature were used to estimate the thermodynamic parameters (ΔH ^o, ΔS ^o and ΔG^o). A plot of TΔS ^o against ΔH ^o indicates the extent of enthalpy–entropy compensation; that is, how much of the enthalpic gain is cancelled by entropy loss or vice versa (the slope indicates the fraction of conformational change contribution to enthalpy gain that is cancelled by an accompanying entropy loss). The remaining fraction of enthalpy gain contributes to complex formation. The intercept is the inherent contribution to complex stability, which is due to desovation. Extensive phase solubility studies combined with rigorous analysis were conducted in the temperature range 20–45°C for the following basic drugs complexing with β-cyclodextrin (β-CD): astemizole (Astm), cisapride (Cisp), dipyridamole (Dipy), ketotifen (Keto), pizotifen (Pizo), terfenadine (Terf), fexofenadine (Fexo), sildenafil (Sild), and celecoxib (Celox). The results indicate that inherent drug solubility is accompanied by unfavorable conformational changes to the extent of 86%, which are counterbalanced by opposite favorable entropy changes. Only 14% of the favorable enthalpy change contributes to drug solubility. The extent of solvation (hydration) accompanying solubility amounts to −30 kJ/mol, which retards solubility as an unfavorable entropy change. In contrast, 1:1 drug/β-CD complex formation is accompanied by favorable conformational changes to the extent of 94%, which are counterbalanced by unfavorable entropy changes. Only about 6% of enthalpy changes contribute to complex stability. However, the extent of favorable entropy change (desolvation) accompanying complex formation amounts to 26 kJ/mol.     

Aqueous Chemistry of Zirconium(IV) in Carbonate Media

The interactions between carbonate ions and zirconium oxychloride are studied by potentiometry, dialysis, and ¹³C‐ and ¹⁷O‐NMR spectroscopy in aqueous media. The nature of the soluble carbonatohydroxo complexes depends on the proportions of hydrogencarbonate and carbonate ions in solution before the addition of zirconium oxychloride. Carbonate media lead to polynuclear entities containing no more than two complexed carbonate ions per Zr⁴⁺. The presence of hydrogencarbonate favors the formation of less condensed and more carbonated complexes such as [Zr(CO₃)₄]⁴⁻. The polycondensation degree of the species decreases when the number of carbonates linked per Zr⁴⁺ increases. In all complexes, the carbonate is bidentate, and the metal atoms are linked via hydroxo bridges. The complexation of carbonate with Zr⁴⁺ occurs for a total carbonate concentration higher than 0.1M . Consequently, in natural medium, the speciation of this metal is governed only by the formation of hydroxo complexes.     

Calorimetric investigation of interactions in the LaNi4.75Al0.25−H2 and LaNi4.8Sn0.2−H2 systems

The interaction of hydrogen with intermetalic compounds LaNi_4.75Al_0.25 and LaNi_4.8Sn_0.2 has been studied in the temperature range 308–353 K by the calorimetry titration method. The mechanism of hydrogenation was investigated. It was shown that, as the temperature increases, the initial concentration of hydrogen in the metal lattice needed for β-hydride formation decreases. It was assumed that this effect is related to the concentration of H^δ+ atoms, which “oxidize” the metallic matrix according to the scheme H^δ++M⁰→H^δ−+M⁺. The enthalpy and entropy of hydrogenation for the LaNi_4.75Al_0.25−H₂ system were calculated from thep-C-T curves and the calorimetry results. The thermodynamic parameters of the LaNi_4.8Sn_0.2−H₂ system were obtained for the first time. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1841–1844, October, 1999.     

Mechanism of Oxidation of <Emphasis Type="SmallCaps">L</Emphasis>-Cysteine by Hexachloroiridate(IV) — A Kinetic Study

The kinetics of oxidation of L-Cysteine in aqueous HClO₄ medium were studied using a one-equivalent oxidant, hexachloroiridate(IV). The reaction exhibits second-order dependence with respect to hexachloroiridate(IV) and first-order in cysteine. The rate decreases with increase in hydrogen ion concentration indicating that the zwitterionic form of cysteine is more reactive. Cysteic acid is identified as the product of oxidation. A suitable mechanism involving the formation of [IrCl₆]²⁻ – sulphur bonded intermediate is proposed. The activation parameters of the reaction are computed using the linear least squares method and the values of E_a and ΔS^# are found to be 27.97±1.82 kJ mol⁻¹ and −51.30±6.0 J K⁻¹mol⁻¹, respectively.     

Thermodynamic Properties of Hydrofullerene C60H36 from 5 to 340 K

The temperature dependence of the molar heat capacity (C⁰ _p) of hydrofullerene C₆₀H₃₆ between 5 and 340 K was determined by adiabatic vacuum calorimetry with an error of about 0.2%. The experimental data were used for the calculation of the thermodynamic functions of the compound in the range 0 to340 K. It was found that at T=298.15 K and p=101.325 kPa C⁰ _p (298.15)=690.0 J K⁻¹ mol⁻¹,H^o(298.15)−H^o(0)= 84.94 kJ mol⁻¹,S^o(298.15)=506.8 J K⁻¹ mol⁻¹, G^o(298.15)−H^o(0)= −66.17 kJ mol⁻¹. The standard entropy of formation of hydrofullerene C₆₀H₃₆ and the entropy of reaction of its formation by hydrogenation of fullerene C₆₀ with hydrogen were estimated and at T=298.15 K they were Δ_fS^o= −2188.4 J K⁻¹ mol⁻¹ and Δ_rS^o= −2270.5 J K⁻¹mol⁻¹, respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Conductometric studies of hexadecyltrimethylammonium bromide in aqueous solutions of ethanol and ethylene glycol

The effect of cosolvent on micellization of hexadecyltrimethyl ammonium bromide (CTAB) in aqueous solutions was studied. The conductivity of a mixture (cosolvent + water) as function of CTAB concentration was measured at different temperatures. Ethylene glycol and ethanol were used as a cosolvent. The conductivity data were used to determine the critical micelle concentration (CMC) and the effective degree of counterion dissociation of micelle in the temperature range 303.2 to 313.2 K. In all the cases studied, a linear relationship between log([CMC]_mix/mol dm⁻³) and the mass fraction of cosolvent in solvent mixture has been observed. The free energy (ΔG _mic ⁰ ), enthalpy (ΔH _mic ⁰ ), and entropy (ΔS _mic ⁰ ) of micellization were determined using the temperature dependence of CMC. The dependence of these thermodynamic parameters on solvent composition was determined. The standard free energy of micellization was found to be negative in all cases and becomes less negative as the cosolvent content increases. The enthalpy and entropy of micellization are independent of temperature in pure water, while ΔH _mic ⁰ and ΔS _mic ⁰ decrease dramatically with temperature in mixed cosolvents. Furthermore, the entropic contribution is larger than the enthalpic one in pure water, while in the mixed solvents, the enthalpic contribution predominates. The text was submitted by the authors in English.     

Structure of polynuclear palladium(ii) hydroxocomplexes formed upon alkaline hydrolysis of palladium(ii) chloride complexes

The structure of polynuclear Pd^II hydroxocomplexes (PHC) formed as a result of alkaline hydrolysis of Pd^II chloride complexes was studied by EXAFS, SAXS, and TEM methods. It is established that in aqueous solutions a hydroxocomplex particle is a filament curled into a ball containing about 100 Pd atoms. The filament consists of planar coordination squares of PdO₄ units linkedvia one or two oxygen bridges of different geometry. Aging of samples results in an increase in the number of single bridging ligands and a decrease in the diameter of particles. Interatomic distances around palladium atoms were determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimlcheskaya, No. 10, pp. 1901–1905, October, 1995.     

Study of an Alcohol’s Influence on the CMC and Thermodynamic Functions of Anionic Surfactants in DMA/Long-chain Alcohol Solutions Using a Microcalorimetric Method

The power-time curves for the micelle formation process were determined for two anionic surfactants, sodium laurate (SLA) and sodium dodecyl sulfate (SDS), in mixed alcohol + N,N-dimethylacetamide (DMA) solvent using titration microcalorimetry. From the data of the lowest point and the area of the power-time curves, their critical micelle concentration (CMC) and ΔH _m^o were obtained. The other thermodynamic functions of the micellization process (ΔG _m^o and ΔS _m^o) were also calculated with thermodynamic equations. For both surfactants, the effects of the carbon number (chain length) of the alcohol, the concentration of alcohol, and the temperature on the CMC and thermodynamic functions are discussed. For systems containing identical concentrations of a different alcohol, values of the CMC, ΔH _m^o and ΔS _m^o increased whereas ΔG _m^o decreased with increasing temperature. For systems containing an identical alcohol concentration at the same temperature, values of the CMC, ΔH _m^o,ΔG _m^o and ΔS _m^o decrease with increasing carbon number of alcohol. For systems containing the same alcohol at the same temperature, the CMC and ΔG _m^o values increase whereas ΔH _m^o and ΔS _m^o decrease with increasing alcohol concentration.     

Fluxional rearrangement in congested ruthenium(II) complexes containing pyrimidine- and/or pyridine-based dithioether ligands

The solvento species obtained by the treatment of cis-RuCl₂(N,N-L)₂ [L = di-2-pyridyl sulfide (dps), di-2-pyrimidyl sulfide (dprs)] with AgPF₆, reacted with dithioethers L′ [L′ = 2,6-bis(2-pyridylthiomethyl)pyridine (pytmp), 2,6-bis(2-pyrimidylthiomethyl)pyridine (prtmp) and 2,6-bis{2-(4-methyl)pyrimidylthiomethyl} pyridine (mprtmp)] to afford the compounds [Ru(N,N-L)₂(N,S-L′)][PF₆]₂. The ¹H NMR spectra indicate that L′ is chelated through S and N atoms with the formation of a four-membered ring. As a consequence, the ruthenium and sulfur atoms are stereogenic centers with ∆ and Λ and (R) and (S) configurations, respectively. NMR spectra, at low temperatures, show that two invertomers, of similar abundance, as enantiomeric couples ∆S, ΛR and ∆R, ΛS are present. In the methylene region, four AB systems are observed that in both the species contain two non-equivalent methylene groups. Variable-temperature NMR spectra and EXSY experiments show that the sulfur inversion produces an exchange between the invertomers. The one-dimensional band-shape analysis of the exchanging methylene signals showed that the energy barriers for the process are in the 43–52 kJ mol⁻¹ range. The possible mechanisms of the sulfur inversion are discussed.     

Hydrogen bonds in unpolar matrix — Comparison of complexation in polymeric and low molecular-weight systems

By ene-reaction of 4-phenyl-1,2,4-triazoline-3,5-dione with double bonds, polar 4-phenyl-1,2,4-triazolidine-3,5-dione (phenyl urazole) groups are introduced into unpolar matrices. Hydrogen-bond complexes are formed between two amide-like units. The temperature dependence of complex formation in dilute hydrocarbon medium is obtained from remperature-dependent IR spectra in the region of the C=O stretching vibration. Results obtained for a low molecular-weight model system are compared with modified polybutadienes, where the groups are attached statistically along the polymer backbone. The enthalpy and the entropy of complex formation (ΔH _f =−28.6 kJ/mol; ΔS _f =−52 J/mol K⁻¹) obtained for the low molecular-weight model prove a dimeric chelate like complex involving two hydrogen bonds. The lower equilibrium constants observed for the groups attached to the polymer are related to additional topological constraints emerging from the polymer backbone.