István Nagypál

NMR relaxation studies in solution of transition metal complexes. V. Proton exchange reactions in aqueous solutions of VO2+-oxalic acid, -malonic acid systems

Abstract The equilibria existing in the VO 2+ -oxalic acid and -malonic acid systems have been studied pH-metrically and the proton exchanges between the bulk water and the different paramagnetic species have been insvestigated by measuring the T 2 relaxation time of water protons at 298 K, I = 1.0 M NaClO 4 . Mainly VOL and VOL 2− 2 type complexes are formed in both systems, the relative amounts of the protonated and mixed hydroxo complexes being very small up to pH 5. The first order rate constants of the proton exchange between the bulk water and the paramagnetic species are given. The surprisingly high value for the VOL 2− 2 type complexes is interpreted by a continuous intramolecular rearrangement of the ligands from two equatorial positions to axial-equatorial position and vice versa. The unexpectedly high rate constant for the protonated complex VOmalH + is interpreted by its fast acid dissociation process.

Thermodynamic study of the parent and mixed complexes of aspartic acid, glutamic acid and glycine with copper(II)

Abstract The equilibrium constants and in part the ΔH and ΔS data for the parent and mixed complexes copper(II)-aspartic acid, copper(II)-glutamic acid, copper(II)-aspartic acid-glycine, copper(II)-glutamic acid-glycine and copper(II)-aspartic acid-glutamic acid were determined pH-metrically and calorimetrically. Protonated complexes are formed in significant concentration in the copper(II)-aspartic acid and copper(II)-glutamic acid systems. The thermodynamic data led to the assumption that in the CuA complex of aspartic acid the β-carboxyl group of the ligand also forms a bond with the metal. The glutamic acid is bound “glycine-like” to the copper(II). There is an appreciable stabilization in the copper(II)-aspartic acid-glycine mixed complex, while in the copper(II)-glutamic acid-glycine system the stability of the mixed complex agrees with the statistical value. The above statements are supported by the dependence of the stability constants on the ionic strength.

Analytical evaluation of the derivatives used in equilibrium calculations

The derivatives of the total concentrations and other measured quantities with respect to the formation constants are calculated in general by a numerical method using finite increments. It is shown in this paper that these values can be calculated analytically, whereby the disadvantages of numerical differentiation may be avoided. The use of this method gives the possibility of writing faster programs for the calculation of the stability constants, based on the SCOGS and LETAGROP principle, without as large a memory requirement as in the LEAST and MINIQUAD programs.

Thermodynamic relations of parent and mixed complexes of asparagine and glutamine with copper(II)

A study was made of the equilibrium and, in part, thermodynamic relations of the parent complexes copper(II)-asparagine and copper(II)-glutamine, and of the mixed complexes copper(II)-asparagine-glycine, copper(II)-asparagine-serine, copper(II)-glutamine-glycine and copper(II)-glutamine-serine. The higher stabilities of the parent complexes copper(II)-asparagine compared to the copper(II) complexes of α-amino-butyric acid, the larger enthalpy change accompanying the formation of the complex CuA, and the deprotonation of the complex CuA2 above pH ∼ 9·5, all indicate that the amide group in the asparagine is also coordinated to the copper(II) ion. The results for the complexes copper(II)-glutamine strongly suggest that the glutamine forms bonds to the copper(II) ion only via its amino and carboxyl groups. The results obtained with the mixed ligand complexes support the above findings.

NMR relaxation studies in solutions of transition metal complexes. VI. Equilibria and proton exchange processes in aqueous solutions of VO2+-glycine system

The equilibria in aqueous solution of the VO2+-glycine system has been studied by pH-metry in very high ligand excess, to avoid the hydrolysis of vanadyl ions. The complexes VOG+, VOG2, VOGH2+, VOG2− H+ and VOG2H−−1 = VOG2(OH)− are dominating in the system; their formation constants are given. The formation of complexes of VOGH−1 = VOG(OH), VOGH−−2 = VOG(OH)−2 and (VO)2G2H−2 = (VO)2G2(OH)2 have also been detected. It is stated that the VO2G2(OH)2 is not the only binuclear (polynuclear) complex in the system; the composition of the other polynuclear complexes, however, cannot be stated unambiguously because of their very low concentrations. The rate constants of the proton exchange between the bulk water and the different complexes have been determined by measuring the T2 relaxation time of the water protons. In contrast to the oxalate, and some other vanadyl complexes, the rate constant decreases by the decrease of the number of water molecules remaining in the first coordination sphere of the vanadyl ion. The exceedingly high proton exchange rate constant for the VOG2(OH)− mixed hydroxo complex is interpreted as being due to the direct proton exchange between the bulk water and the coordinated OH group.

Thermodynamic and NMR studies of some copper(II)-diaminomonocarboxylate equilibrium systems in aqueous solution

Abstract pH-metric calorimetric and NMR studies were carried out on the copper(II) complexes of 2,3-diaminopropionic acid (dapa), 2,4-diaminobutyric acid (daba), 2,5-diaminopentanoic acid (ornithine) and 2,6-diaminohexanoic acid (lysine). It was concluded that lysine is coordinated to the copper(II) ion in a “glycine-like” manner. With dapa, daba and ornithine, however, the ω-amino groups also coordinate in complexes containing the ligand in fully deprotonated form. NMR results show that with the ornithine complex a fast dynamic equilibrium could be assumed between those forms which contain the ω-amino group in coordinated and non-coordinated forms. A similar equilibrium, shifted toward diamine-like coordination, is probable in the daba complex. In practice, dapa behaves like a substituted ethylenediamine in its copper(II) complex.

Stochastic behavior and stirring rate effects in the chlorite–iodide reaction

The autocatalytic reaction between chlorite and iodide ions in a closed system is a clock reaction, showing a sudden appearance of brown I2 followed by a rapid disappearance of the color. Under certain conditions, the reaction time displays a striking irreproducibility. This stochastic behavior is studied potentiometrically and spectrophotometrically as a function of initial [I− ], stirring rate and solution volume. The results imply that the irreproducibility is an inherent feature of the reaction generated by fluctuations in the solution after it is ‘‘well mixed.’’ The key contributors to the stochasticity are local concentration inhomogeneities resulting from imperfect stirring and the ‘‘supercatalytic’’ reaction kinetics. A qualitative explanation is given that incorporates these aspects.

Studies on transition-metal–peptide complexes. Part 1. Equilibrium and thermochemical study of the copper(II) complexes of glycylglycine, glycyl-DL-α-alanine, DL-α-alanylglycine, and DL-α-alanyl-DL-α-alanine

pH Titrimetry and calorimetry have been used to determine the stoicheiometries, stability constants, and enthalpies and entropies of formation of the complexes formed in the systems of copper(II) with glycylglycine, glycyl-DL-α-alanine, DL-α-alanylglycine, and DL-α-alanyl-DL-α-alanine, at ionic strength I= 0.2 mol dm–3 KCl and at 25 °C. There is no possibility of the titrimetric indication of the formation of the complexes [CuL2] and [CuL2H–2]2–(L = dipeptide anion H2N–CHR–CO–NH–CHR′–CO2–) in these equilibrium systems, even when their maximum concentration is 10–15%. The systematic changes within the series have been interpreted by simultaneous consideration of the electron-donating and steric-hindrance effects of the methyl groups. From the values of the formation constants and enthalpy changes, it is concluded that equatorial–axial co-ordination of the second peptide ligand occurs in the complex [CuL2H–1]–, while in the binuclear complex [Cu2L2H–3]– an [OH]– ligand bridges the two [CuLH–1] units equatorially.

Effect of Chloride Ion on the Kinetics and Mechanism of the Reaction between Chlorite Ion and Hypochlorous Acid

The effect of chloride ion on the chlorine dioxide formation in the ClO 2 (-)-HOCl reaction was studied by following .ClO 2 concentration spectrophotometrically at pH 5-6 in 0.5 M sodium acetate. On the basis of the earlier experimental data collected without initially added chloride and on new experiments, the earlier kinetic model was modified and extended to interpret the two series of experiments together. It was found that the chloride ion significantly increases the initial rate of .ClO 2 formation. At the same time, the .ClO 2 yield is increased in HOCl but decreased in ClO 2 (-) excess by the increase of the chloride ion concentration. The two-step hydrolysis of dissolved chlorine through Cl 2 + H 2O left harpoon over right harpoon Cl 2OH (-) + H (+) and Cl 2OH (-) left harpoon over right harpoon HOCl + Cl (-) and the increased reactivity of Cl 2OH (-) compared to HOCl are proposed to explain these phenomena. It is reinforced that the hydrolysis of the transient Cl 2O 2 takes place through a HOCl-catalyzed step instead of the spontaneous hydrolysis. A seven-step kinetic model with six rate parameters (constants and/or ratio of constants) is proposed on the basis of the rigorous least-squares fitting of the parameters simultaneously to 129 absorbance versus time curves measured up to approximately 90% conversion. The advantage of this method of evaluation is briefly outlined.

Response reactions in chemical thermodynamics

The fundamental differential equations of chemical thermodynamics within the De Donder (stoichiometric) approach are reformulated in terms of a special class of chemical reactions, which we call response reactions, RERs. The RERs have the remarkable property of being independent of the actual choice of the stoichiometrically independent reactions, by means of which the chemical processes in the system considered are usually described. The concept of RERs is shown to be applicable in both equilibrium and non-equilibrium chemical thermodynamics. This approach enables one to overcome some difficulties and ambiguities of the standard thermodynamic description of complex chemical systems. In particular, the concept of thermodynamic coupling of chemical reactions may be given a new qualitative and quantitative meaning.