Enthalpy Of Complex Formation Research Articles

Experimental and theoretical approach to probe the aquatic speciation of transuranic (neptunyl) ion in presence of two omnipresent organic moieties

Pyrazines are omnipresent in nature and have their occurrence in plants, microbes, food supplies, marine arenas. The present studies aimed at aquatic speciation of the neptunyl ion (NpO2+) with two pyrazine compounds namely pyrazine monocarboxylic acid (PMC) and pyrazine dicarboxylic acid (PDC). Absorption spectrophotometry was used to probe the stability, speciation and spectral properties for the complexation process. NpO2+ forms a more stable complex with PMC than PDC for 1:1 (ML), while for 1:2 (ML2) the opposite trend is observed. The extent of shift in λmax, which is also an indicator for the strength of complexation, reflected similar trends for the complexation process. Isothermal titration calorimetry was employed to determine the enthalpies of complex formation, which is found to be endothermic. The complexation process is entropy driven. Linear free energy correlations were established to retrieve the coordination modes of the complexes. The variation in peak potentials (the cyclic voltammograms) with change in pH and metal to ligand ratio were explored to understand redox speciation, electron transfer kinetics and Eh-pH characteristics for the interaction of NpO2+ with pyrazine carboxylate ligands. Density functional theory calculations were employed to optimize the geometries and to calculate the bond distances and partial charges on key atoms of the optimized geometries. The theoretical calculations helped to reveal the contributions from two different configurations of the same geometry towards the optical absorption. The bond distances and partial charges estimated theoretically helped to understand the aqueous interactions at the molecular level.

Design Platform for Sustainable Catalysis with Radicals: Electrochemical Activation of Cp2 TiCl2 for Catalysis Unveiled.

The combination of synthesis, rotating ring‐disk electrode (RRDE) and cyclic voltammetry (CV) measurements, and computational investigations with the aid of DFT methods shows how a thiourea, a squaramide, and a bissulfonamide as additives affect the EqCr equilibrium of Cp2TiCl2. We have, for the first time, provided quantitative data for the EqCr equilibrium and have determined the stoichiometry of adduct formation of [Cp2Ti(III)Cl2]−, [Cp2Ti(III)Cl] and [Cp2Ti(IV)Cl2] and the additives. By studying the structures of the complexes formed by DFT methods, we have established the Gibbs energies and enthalpies of complex formation as well as the adduct structures. The results not only demonstrate the correctness of our use of the EqCr equilibrium as predictor for sustainable catalysis. They are also a design platform for the development of novel additives in particular for enantioselective catalysis.

Thermodynamics of antibiotics: natural cyclodextrin inclusion complex formation and covering of nano-metric calcite with these substances

Isothermal titration calorimetry (ITC) has been used to characterize inclusion complex formation of natural (α-, β-, γ-)cyclodextrins with antibiotics (ampicillin—sodium, penicillin G—sodium, streptomycin sulfate) in aqueous solutions. ITC measurements were taken at 298.15 K on Isothermal Titration Calorimeter Nano ITC. The experimental data were analyzed on the basis of the independent site model. Based on the experimental values of equilibrium constant (K) and enthalpy of complex formation (ΔH), the Gibbs energy of complex formation (ΔG) and the entropy of complex formation (ΔS) have been calculated. The analysis of the obtained inclusion complexes show that independently of the kind of cyclodextrin and antibiotics the obtained equilibrium constants are almost the same, but it is a difference between the changes in enthalpies of complex formation for the investigated systems. The stoichiometry of complexes formed both by ampicillin—sodium and by penicillin G—sodium with all the natural cyclodextrins is the same and equal to 1:1 and the complex formation is entropy driven. Three antibiotics (ampicillin—sodium, penicillin G—sodium and streptomycin sulfate) have been used further for covering of the monodisperse calcium carbonate nanoparticles obtained in the reactor (three-phase reaction) with rotating disks. Three series of experiments have been performed. First was connected with adsorption of the antibiotics on the surface on nano-calcite and two others with intercalation of the drugs into nano-metric CaCO3 structure (aggregates). An intercalation has been performed in two ways: one where the antibiotic was added to the reactor chamber at the beginning of the precipitation reaction and second where it was added just after the end of the reaction. Both pure CaCO3 nanoparticle and antibiotic-coated CaCO3 powders were deeply analyzed by the use of the thermogravimetric and the differential scanning calorimetry method. The performed investigations showed that all the antibiotics used can be both adsorbed and intercalated into the nano-metric CaCO3 obtained in the reactor with rotating disks. The different adsorption obtained by different antibiotics was caused by the different interaction between them and nano-calcite, caused by their different structure. The experimental data have allowed also for distinction between the antibiotics molecules present on calcite surface (adsorption) or antibiotics molecules intercalated into the nano-calcite structure. The conducted research shows that both approaches, i.e., formation of inclusion complexes with natural cyclodextrins and covering (adsorption and intercalation) of nano-metric CaCO3, can be successfully implemented for their pharmaceutical applications.

Complexation of Eu(III) with furan monocarboxylates in aqueous medium at variable temperatures: Luminescence and computational studies

Luminescence of Eu(III) with furan monocarboxylates (FCA) ligands namely, 2 furoate (2FCA) and 3 furoate (3FCA) have been studied in aqueous medium. Both the ligands sensitized the luminescence of Eu(III) to a different extent. The magnitude of luminescence intensity was higher for the Eu(III)-2FCA than Eu(III)-3FCA complex due to the higher absorbance of 2FCA ligand as corroborated by UV–Vis spectroscopy. The formation of 1:1 Eu(III) complexes with both the ligands was confirmed through luminescence lifetime measurements. Stability constants of these complexes, calculated using HypSpec computation program were found to increase with temperature. The coordination mode of both the ligands was found to be monodentate. Van't Hoff equation was used to calculate enthalpy of formation of Eu(III) complexes using stability constants at different temperatures (283, 298, 313, 328, 343 K). The enthalpy of complex formation of 2FCA and 3FCA with Eu(III) was endothermic. Density functional theory (DFT) and time dependent (TD)-DFT calculations were performed to corroborate experimental observations.

Structural Parameters and Thermodynamics of the Formation of Molecular Water Clusters

The formation of molecular water clusters is simulated using the theoretical density functional theory/ B3LYP/6-311+G(d,p) method. The spatial configurations of 29 clusters with 2 to 28 water molecules are calculated. The dipole moments, the complete complex-formation enthalpy, and the enthalpy of the successive joining of water molecules are determined with the basis-set superposition error taken into account. The features of the geometric structure and the hydrogen-bond strength of water clusters are analyzed on the basis of the obtained theoretical data. The complex-formation enthalpy is revealed to depend periodically on the number of water molecules in a cluster. It is found that clusters with molecules whose number is a multiple of four are energetically most advantageous. When a molecular cluster is built starting with 17 molecules, the cluster structure is changed, resulting in that one end of the complex rolls up into a prismatic configuration.

Influence of reagents solvation on [Ag18C6]+ complex formation in methanol‒acetonitrile mixed solvents

The influence of the composition of the methanol—acetonitrile (MeOH—AN) solvent on the solvation of 18-crown-6 ether (18C6) has been studied. Increasing fraction of AN in the binary solvent stabilizes the 18C6 solvation complex by increasing the exothermicity of 18C6 solvation. Analysis of the solvation contributions of the reagents to the enthalpies of complex formation of silver(I) ion with 18C6 in MeOH—AN mixtures has been performed to establish that the decrease in the exothermicity of complex formation in going from MeOH to AN is mostly associated with the solvation contribution of Ag(I).

IR spectra of halothane–acetone complex in liquefied noble gases (Kr and Xe)

IR absorption spectra of solutions of halothane (C2HBrClF3) and acetone ((CD3)2CO) mixtures in liquefied noble gases (krypton and xenon) have been recorded and analyzed. Bands due to weak hydrogenbonded complexes are identified. The complex-formation enthalpy is estimated in a series of temperature experiments on the change in the total intensity of the bands due to monomers and complexes. Second-order bands are found, which are assigned to the first overtone of stretching vibration CH of halothane and the Raman band related to simultaneous excitation of stretching vibration CH of halothane and stretching vibration CO of acetone. The results of ab initio calculation performed within the МР2/6-311++G(d, p) approximation are used to analyze the spectroscopic data.

Thermodynamics of reactions of complex formation for Ce3+ and Er3+ ions with glycine in an aqueous solution

Enthalpies of complex formation for glycine (HL±) with Ce3+ and Er3+ ions at 298.15 K and the value of the ionic strength of 0.5 (KNO3) are determined by calorimetric means using two independent procedures. Thermodynamic characteristics of the reactions of formation for complexes of glycine with Ce3+ and Er3+ ions at various [metal]: [ligand] molar ratios are calculated.

Standard thermodynamic functions and constants of complex formation of Nd3+ and La3+ with L-asparagine in aqueous solutions at 298 K

The enthalpies of complex formation of L-asparagine (HAsn±) with Nd3+ and La3+ ions are determined calorimetrically at 298.15 K and an ionic strength of 0.5 (KNO3). The thermodynamic characteristics of the formation of the NdAsn2+, NdAsn2+, LaAsn2+, and LaAsn2+ complexes are calculated.

Study of chemical bonding in the interhalogen complexes based on density functional theory

The density functional theory analysis was used for a number XYL complexes (XY is a dihalogen molecule and L is a Lewis base), formed between molecules I2, ICl, IBr and pyridine. The calculated geometrical parameters, IR spectra and nuclear quadrupole interaction constants of iodine are consistent with the data of microwave spectroscopy and nuclear quadrupole resonance. The good correlation between the experimental and calculated binding energies of the inner electrons of iodine, chlorine and nitrogen atoms were found with the calculation using both Gaussian and Slater functions. The comparison of experimental and calculated changes in the electron density on the atoms upon complex formation suggested the choice of scheme for calculating the effective charge on the atoms, which allow us to interpret the experimental spectra. It is shown that the use of both calculated schemes allows us to predict the enthalpy of complex formation in close agreement with the experimental values. The energy analysis shows that in the complexes the electrostatic binding energy dominates that of covalent binding.

Thermodynamics of β-cyclodextrin–ephedrine inclusion complex formation and covering of nanometric calcite with these substances

Isothermal titration calorimetry (ITC) has been used to characterize inclusion complex formation of β-cyclodextrin (β-CD) with ephedrine in aqueous solutions. ITC measurements were taken at 298.15 K on a MicroCal OMEGA ultrasensitive titration calorimeter (MicroCal Inc.). The experimental data were analyzed on the basis of the model of a single set of identical sites (ITC Tutorial Guide). Based on the experimental values of equilibrium constant (K) and enthalpy of complex formation (ΔH), the Gibbs energy of complex formation (ΔG), and the entropy of complex formation (ΔS), has been calculated. Obtained results showed that β-CD forms inclusion complex of stoichiometry 1:1 with ephedrine and the complex formation is entropy driven. Ephedrine and its complex with β-CD have been further used for covering of the obtained in a controlled way nanometric CaCO3 (calcite), which served as a solid supports for drug depositing. The calcite coating has been analyzed by the use of thermogravimetric method. The size of aggregates of pure calcite particles as well as CaCO3 particles covered by ephedrine and its complex with β-CD have been measured by DLS method. It has been found that pure CaCO3 aggregates are almost monodispersed with the mean diameter equal to 329 nm (±5 nm). Ephedrine and its complex with β-CD layers formed in situ on precipitated calcite surface prevented from crystallites aggregation and decreases particles mean diameter up to 274 nm (±5 nm) for ephedrine and 211 nm (±5 nm) for β-CD complex with ephedrine.

Isothermal titration calorimetry (ITC) study of natural cyclodextrins inclusion complexes with tropane alkaloids

Isothermal titration calorimetry (ITC) has been used to characterize inclusion complex formation of natural cyclodextrins (α- and β-cyclodextrin) with three tropane alkaloids (scopolamine, homatropine hydrobromide and atropine sulfate) in aqueous solutions. ITC measurements were taken at 298.15 K on a MicroCal OMEGA ultrasensitive titration calorimeter (MicroCal Inc.). The experimental data were analyzed on the basis of the model of a single set of identical sites (ITC Tutorial Guide). β-CD forms inclusion complexes of stoichiometry 1:2 with homatropine hydrobromide and 1:1 with scopolamine and atropine sulfate. The smaller molecule of α-CD forms very weak inclusion complexes with the tropane alkaloids. So, only one complex of the same stoichiometry 1:2 with homatropine hydrobromide has been detected by the ITC experiment. Based on the experimental values of equilibrium constant (K) and enthalpy of complex formation (ΔH), the Gibbs energy of complex formation (ΔG) and the entropy of complex formation (ΔS) have been calculated, for all the investigated systems. Obtained results showed that complex formation of both α- and β-CD with all the investigated tropane alkaloids is entropy driven. This indicated that the difference in the cavity dimensions is not reflected in different driving forces of complex formation and binding modes which resulted in the same stoichiometry of the obtained inclusion complexes.

Ethidium Bromide Interaction with Poly(G)

In current work we investigate the interaction between polyguanylic acid and Ethidium Bromide ( EtBr ) and the changes of thermodynamic parameters due to those interactions by spectrophotometric methods. From the binding isotherms, binding constants were calculated for three different temperatures and the changes of Gibbs free energy, entropy and enthalpy for complex-formation were determined. Complex-formation with four-stranded poly(G) structure needs less conformational changes in contrast with poly(G)poly(C), and due to that, EtBr binds with 5 and not 4 nucleotides. Obtained results confirm the concept that polyguanylic acids may form four-stranded complex structures with intercalating ligand EtBr in the aqueous solution.

Effect of temperature on the process of complex formation crown ether 15C5 with Na+ in the (water + ethanol) mixture at temperatures from (293.15 to 308.15) K

• The system 15C5/Na + complex + water + ethanol has been investigated. • The thermodynamic functions of 15C5/Na + complex formation were calculated. • The enthalpy of complex formation has been calculated using calorimetric method. • The enthalpy of complex formation has been calculated from van’t Hoff's equation. • The obtained results have been compared. Thermodynamic functions of complex formation of 15-crown-5 ether (15C5) with sodium cation (Na + ) in the mixtures of water (W) with ethanol (EtOH) at different temperatures have been calculated. To obtain this function two experimental methods have been used. The equilibrium constants of complex formation have been determined by conductivity measurements and the enthalpic effect of complex formation has been measured by the calorimetric method and has been also calculated using van’t Hoff's equation. The obtained results were compared and discussed. The complexes are enthalpy stabilized but entropy destabilized.

Interactions of DNA bases with individual water molecules. Molecular mechanics and quantum mechanics computation results vs. experimental data

To elucidate details of the DNA-water interactions we performed the calculations and systemaitic search for minima of interaction energy of the systems consisting of one of DNA bases and one or two water molecules. The results of calculations using two force fields of molecular mechanics (MM) and correlated ab initio method MP2/6-31G(d, p) of quantum mechanics (QM) have been compared with one another and with experimental data. The calculations demonstrated a qualitative agreement between geometry characteristics of the most of local energy minima obtained via different methods. The deepest minima revealed by MM and QM methods correspond to water molecule position between two neighbor hydrophilic centers of the base and to the formation by water molecule of hydrogen bonds with them. Nevertheless, the relative depth of some minima and peculiarities of mutual water-base positions in' these minima depend on the method used. The analysis revealed insignificance of some differences in the results of calculations performed via different methods and the importance of other ones for the description of DNA hydration. The calculations via MM methods enable us to reproduce quantitatively all the experimental data on the enthalpies of complex formation of single water molecule with the set of mono-, di-, and trimethylated bases, as well as on water molecule locations near base hydrophilic atoms in the crystals of DNA duplex fragments, while some of these data cannot be rationalized by QM calculations.

Thermochemical study of 4-(dimethylamino)pyridine complexes of some bivalent metal bromides

Interaction of some bivalent metal bromides with 4-(dimethylamino)pyridine in ethanol solution leads to the formation of complexes of general formula: [MBr2(dmapy)n] (where M=Mn(II), Fe(II), Co(II), Ni(II), Cu(II) or Zn(II); dmapy=4-(dimethylamino)pyridine; n=0.75, 1, 1.25, or 2). Melting point measurements, elemental analysis, TG/DTG and DSC curves, and IR and electronic spectroscopy characterized the complexes. The measure of the enthalpies of solution of complexes, salts and ligand leads to the determination of several thermochemical parameters by the use of thermochemical cycles. The mean standard enthalpies of the metal-nitrogen coordinated bonds and the enthalpies of complex formation in the gaseous phase also were determined.

A Raman study on preferential interactions in the formamide/pyridine/pyridazine system and complementary thermodynamic information on the formamide/pyridazine mixture

The formamide (FA)/pyridine (py)/pyridazine (prd) system was investigated by FT-Raman spectroscopy and the results in situ show that FA is preferentially bound to py, as experimentally pointed out by the proton affinity (PA) values of these azabenzenes. Temperature-dependent further investigations for the FA/prd mixture clearly show that the 2:1 FA:prd complex is more stable than the 1:1 FA:py adduct, even though its formation is influenced by the extremely negative entropy of the system, which becomes the spontaneously unfavorable global process. Linear relationships of the complex formation enthalpies (ΔH°) with band shifts (Δν) and dipole moments (μ) of these azabenzenes are observed and are in excellent agreement with our recent studies.

Isothermal titration calorimetry (ITC) study of natural cyclodextrins inclusion complexes with drugs

Isothermal titration calorimetry (ITC) was used to characterize inclusion complex formation of natural cyclodextrins (α- and β-cyclodextrin) with three drugs ((+)brompheniramine, (±)brompheniramine, cyclopentolate) in aqueous solutions. ITC measurements were carried out at 298.15 K on a Microcal OMEGA ultrasensitive titration calorimeter (MicroCal Inc.). The experimental data were analyzed on the basis of the model of a single set of identical sites (ITC tutorial guide). β-CD forms inclusion complexes of stoichiometry 1:1 with the all investigated drugs. In turn, smaller molecule of α-CD forms inclusion complexes of two different stoichiometry: with bigger molecules ((+)brompheniramine and (±)brompheniramine) of a stoichiometry 2:1 and with smaller molecules (cyclopentolate) of a stoichiometry 1:2. Based on the experimental values of equilibrium constant (K) and enthalpy of complex formation (ΔH), the Gibbs energy of complex formation (ΔG), and the entropy of complex formation (ΔS), have been calculated, for all the investigated systems. Obtained results showed that complex formation of β-CD (bigger molecule with wider cavity compared to β-CD) with both (+)brompheniramine, (±)brompheniramine, and cyclopentolate is enthalpy driven while complexes of α-CD with the all investigated drugs are enthalpy-entropy stabilized. This indicated that the difference in the cavity dimensions is reflecting in different driving forces of complex formation and binding modes what resulted in different stoichiometry of the obtained inclusion complexes.

Synthesis and Calorimetric, Spectroscopic, and Structural Characterization of Isocyanide Complexes of Trialkylaluminum and Tri-tert-butylgallium

Addition of tert-butylisocyanide or 2,6-dimethylphenylisocyanide to a solution of trialkylaluminum or trialkylgallium results in formation of complexes R(3)M·C≡N(t)Bu (M = Al, R = Me (1), Et (2), (i)Bu (3), (t)Bu (4); M = Ga, R = (t)Bu (9)) or R(3)M·C≡N(2,6-Me(2)C(6)H(3)) (M = Al, R = Me (5), Et (6), (i)Bu (7), (t)Bu (8); M = Ga, R = (t)Bu (10)), respectively. Complexes 1, 4, 5, and 8-10 are isolated as solids, whereas the triethylaluminum and triisobutylaluminum adducts 2, 3, 6, and 7 are viscous oils. Complexes 1-10 were characterized by NMR ((1)H, (13)C) and IR spectroscopies, and the molecular structures of 4, 5, and 8-10 were also determined by X-ray crystallography. The frequency of the C≡N stretch of the isocyanide increased by 58-91 cm(-1) upon complexation, consistent with coordination of the isocyanide as a σ donor. Enthalpies of complex formation for 1-10 were determined by isothermal titration calorimetry. Enthalpy data suggest the following order of decreasing Lewis acidity: (t)Bu(3)Al ≫ (i)Bu(3)Al ≥ Me(3)Al ≈ Et(3)Al ≫ (t)Bu(3)Ga. In the absence of oxygen and protic reagents, the reported complexes do not undergo insertion or elimination reactions upon heating their benzene-d(6) solutions to 80 °C.

Enthalpies of formation of the complexes [Co2+-N,N-bis(carboxymethyl)aspartic acid] in aqueous solutions at 298.15 K

Enthalpies of complex formation between N,N-bis(carboxymethyl)aspartic acid (H4Y) with Co2+ ion at 298.15 K are determined by calorimetry over a wide range of ionic strengths (KNO3). Thermodynamic characteristics of the formation reactions of the complexes CoY2−, CoY26−, and Co(OH)Y3− are calculated at zero and other values of ionic strength. The obtained values are interpreted.