Showing papers on "Equilibrium constant published in 2000"

A new approach to preparative enzymatic synthesis.

Abstract: A new approach to preparative organic synthesis in aqueous–organic systems is suggested. It is based on the idea that the enzymatic process is carried out in a biphasic system “water–water-immiscible organic solvent.” Thereby the enzyme is localized in the aqueous phase—this eliminates the traditional problem of stabilizing the enzymes against inactivation by a nonaqueous solvent. Hence, in contrast to the commonly used combinations “water–water-miscible organic solvent,” in the suggested system the content of water may be infinitely low. This allows one to dramatically shift the equilibrium of the reactions forming water as a reaction product (synthesis of esters and amides, polymerization of amino acids, sugars and nucleotides, dehydration reactions, etc.) toward the products. The fact that the system consists of two phases provides another very important sources for an equilibrium shift, i.e., free energies of the transfer of a reagent from one phase to the other. Equations are derived describing the dependence of the equilibrium constant in a biphasic system on the ratio of the volumes of the aqueous and nonaqueous phases and the partition coefficients of the reagents between the phases. The approach has been experimentally verified with the systhesis of N-acetyl-L-tryptophan ethyl ester from the respective alcohol and acid. Porous glass was impregnated with aqueous buffer solution of chymotrypsin and suspended in chloroform containing N-acetyl-L-tryptophan and ethanol. In water (no organic phase) the yield of the ester is about 0.01%, whereas in this biphasic system it is practically 100%. The idea is applicable to a great number of preparative enzymatic reactions.

First 17O NMR Observation of Coordinated Water on Both Isomers of [Eu(DOTAM)(H2O)]3+: A Direct Access to Water Exchange and its Role in the Isomerization

Abstract: A complete mechanistic study of the solution dynamics of [Eu(DOTAM)(H2O)]3+ is performed through 1H and 17O NMR variable pressure and temperature studies. An unambiguous understanding of the water exchange was possible thanks to the first 17O NMR observation of the bound water signal on both the M- and m-isomers (M: = 0.113 ± 0.013 × 103 s-1, = 8.3 ± 0.3 × 103 s-1, ΔH⧧ = 53.1 ± 2 kJ mol-1, ΔS⧧ = +8.4 ± 5 J K-1 mol-1, ΔV⧧ = +4.9 ± 1 cm3 mol-1; m: = 8.9 ± 0.6 s-1, = 327 ± 60 × 103 s-1, ΔH⧧ = 44.2 ± 2 kJ mol-1, ΔS⧧ = +8.8 ± 7 J K-1mol-1). The water exchange on m is about 50 times faster than on M, and even though the equilibrium constant K = [M]/[m] equals 4.5, the contribution of m to the overall exchange rate is 90%. These results can be transferred to an aqueous solution since they agree with the overall exchange rate obtained by 17O NMR for an aqueous solution of [Gd(DOTAM)(H2O)]3+. 2D-EXSY and variable temperature and pressure 1H NMR experiments reveal that the interconversion between the M and m isom...

Modeling Oxygen Solubility in Water and Electrolyte Solutions

Abstract: A model is presented for estimating oxygen solubility in water and solutions of inorganic electrolytes (I) as a function of oxygen pressure PO2 (atm), temperature T (K), and I. It is based on a thermodynamic analysis for water, where the molal concentration caq of oxygen follows an equation of the form caq = PO2k and k is a T-dependent function (equilibrium constant) related to the chemical potential, entropy, and partial molar heat capacity of the gaseous oxygen (O2)g and dissolved oxygen (O2)aq species. In the presence of I, the oxygen solubility becomes (caq)I = φcaq, where φ is a modifying factor < 1 that is dependent on I and its molal concentration CI. The decreasing molar heat capacity of (O2)aq with rising T, which affects k, is discussed. The decrease in φ with increasing CI is related in a general way to the decrease in partial molar volume of the water.

Hydration of Carbonyl Compounds, an Analysis in Terms of Multidimensional Marcus Theory

Abstract: Rate constants for base-catalyzed, uncatalyzed, and acid-catalyzed additions of water to 50 compounds (aldehydes, ketones, esters, thioesters, and amides) have been calculated using Multidimensional Marcus Theory. For all of these reactions except hydroxide addition to reactive aldehydes a single average value of the intrinsic barrier (8.51 kcal/mol) can be used to give calculated values within 1 kcal/mol (root-mean-square error) of the observed values. For the addition of hydroxide to reactive aldehydes it is necessary to use an intrinsic barrier linearly related to the equilibrium constant for hydroxide addition. The work term in Marcus Theory is approximated by a detailed model of the solvation and statistical cost of bringing reactants together.

Thermodynamic Characterization of the Association of Cyanine Dyes with DNA

Abstract: The association of three monomeric cyanine dyes with double-stranded DNA was investigated. The oxazole yellow dye (YO-PRO-1) and the two thiazole orange dyes (TO-PRO-1 and TO) are structurally similar, differing only in their charge and in the heteroatom in their benzothiazole and benzoxazole rings. On the basis of changes in the UV−visible absorption spectra of the dyes when bound to DNA, equilibrium constants were measured to investigate the factors that influence their intercalative association with DNA. When these positively charged dyes associate with DNA, the number of condensed Na+ ions that are released from the DNA backbone is correlated with the structural charge of the dye. A comparison of the equilibrium constants demonstrates that the charge and structure of the dye influences the strength of its interaction with DNA. As evident from larger equilibrium constants in buffers containing tetramethylammonium cations as opposed to sodium cations, the identity of the cations also influences the stab...

A practical kinetic model that considers endproduct inhibition in anaerobic digestion processes by including the equilibrium constant.

Abstract: The classical Michaelis-Menten model is widely used as the basis for modeling of a number of biological systems. As the model does not consider the inhibitory effect of endproducts that accumulate in virtually all bioprocesses, it is often modified to prevent the overestimation of reaction rates when products have accumulated. Traditional approaches of model modification use the inclusion of irreversible, competitive, and noncompetitive inhibition factors. This article demonstrates that these inhibition factors are insufficient to predict product inhibition of reactions that are close the dynamic equilibrium. All models investigated were found to violate thermodynamic laws as they predicted positive reaction rates for reactions that were endergonic due to high endproduct concentrations. For modeling of biological processes that operate close to the dynamic equilibrium (e.g., anaerobic processes), it is critical to prevent the prediction of positive reaction rates when the reaction has already reached the dynamic equilibrium. This can be achieved by using a reversible kinetic model. However, the major drawback of the reversible kinetic model is the large number of empirical parameters it requires. These parameters are difficult to determine and prone to experimental error. For this reason, the reversible model is not practical in the modeling of biological processes.This article uses the fundamentals of steady-state kinetics and thermodynamics to establish an equation for the reversible kinetic model that is of practical use in bio-process modeling. The behavior of this equilibrium-based model is compared with Michaelis-Menten-based models that use traditional inhibition factors. The equilibrium-based model did not require any empirical inhibition factor to correctly predict when reaction rates must be zero due to the free energy change being zero. For highly exergonic reactions, the equilibrium-based model did not deviate significantly from the Michaelis-Menten model, whereas, for reactions close to equilibrium, the reaction rate was mainly controlled by the quotient of mass action ratio (concentration of all products over concentration of all substrates) over the equilibrium constant K. This quotient is a measure of the displacement of the reaction from its equilibrium. As the new equation takes into account all of the substrates and products, it was able to predict the inhibitor effect of multiple endproducts. The model described is designed to be a useful basis for a number of different model applications where reaction conditions are close to equilibrium.

Experimental Evidence for the Existence of the HO2−H2O Complex

Abstract: Complex formation in the interaction of HO2 with H2O was studied using Fourier transform infrared absorption spectroscopy. Studies were carried out between 230 and 298 K in the presence of excess H2O. On the basis of measurements of HO2 disappearance, equilibrium constants (Kc) for the formation of the complex were determined to be 1.7 × 10-16, 4.1 × 10-17, and ≤4 × 10-18 cm3 molecule-1 at 230, 250, and 298 K, respectively. The enthalpy for the reaction forming the complex was determined to be −36 ± 16 kJ mol-1. This is in good agreement with previous estimates made from kinetic measurements and theoretical calculations.

Highly stable C(60)/poly(vinylpyrrolidone) charge-transfer complexes afford new predictions for biological applications of underivatized fullerenes.

Abstract: The equilibrium constants for complexation of C(60) with poly(vinylpyrrolidone) (PVP) in water have been determined by UV-visible spectroscopy. The magnitude of the equilibrium constants was found to describe the formation of a charge-transfer (CT) saturated 1:1 complex for [PVP] = 2.62-5.25 x 10(-2) M (K = 1331.3 M(-1)), and a contact-pairs complex for [PVP] = 7.0-12.25 x 10(-2) M (K = 20.64 x 10(-2) M(-1)). These results indicate that the binding affinity of C(60) for drug receptors, base pairs in double-stranded DNA, or AT-rich segments of its minor groove to form CT complexes is limited by the strong coordination in the C(60)/PVP-saturated CT complex or envelopment by the polymer ligand in the contact CT complex.

Photophysical studies of the pheophorbide a dimer.

Abstract: This work reports on a monomer–dimer equilibrium state of pheophorbide a in solution. A methodology for controlling the equilibrium constant by use of temperature and solvent variation is described. The absorption spectrum of the dimer is calculated, using different prepared equilibria of monomer and dimer in solution. We propose that these aggregates provide a good model for understanding the dimerization process in tetrapyrroles.

Coupled energetics of lambda cro repressor self-assembly and site-specific DNA operator binding I: analysis of cro dimerization from nanomolar to micromolar concentrations.

Abstract: The cro repressor from bacteriophage lambda is an important and classical transcription regulatory protein that binds DNA operator sites as a dimer. Therefore, a complete understanding of gene regulation by cro requires knowledge of the coupled energetics of its protein dimerization and site-specific DNA binding. A method is described by which cro repressor can be labeled in vivo with [(35)S]methionine to a specific activity of 2 x 10(15) cpm/mol. As a prelude to binding studies, the association equilibrium of cro was determined over the range 10(-)(9)-10(-)(3) M using large-zone analytical gel chromatography with radiolabeled repressor. The data are best described by a monomer-dimer stoichiometry with an equilibrium constant of 3.07 (+/-1.08) x 10(6) M(-)(1) total cro monomer. Stokes radii for monomers and dimers were evaluated from the resolved gel partition coefficients. Under the conditions employed in this study (10 mM Bis-Tris, 200 mM KCl, 2.5 mM MgCl(2), 1 mM CaCl(2), 100 microg/mL BSA, pH 7.0, 20 degrees C), self-association of cro to species with assembly states greater than dimers is not observed.

Complexes of Hydroxyl and Hydroperoxyl Radical with Formaldehyde, Acetaldehyde, and Acetone

Abstract: Likewise, for the organic molecules studied, the strength of binding energies is consistently acetone > acetaldehyde > formaldehyde. Vibrational frequencies are also calculated and some relationships between these and binding energies and geometries are discussed. These data may be useful when laboratory data is present for these complexes. Equilibrium constants for the formation of these complexes are also calculated. As a result of this, it is predicted that the strongest of these complexes, the hydroperoxyl radical -acetone complex, will exist in the atmosphere under certain conditions.

Comparison of sorbents and isotherm models for NH3-gas separation by adsorption

Abstract: Adsorption equilibrium isotherms of ammonia gas were measured at temperatures between 298 and 393 K on 13X zeolite, 4A zeolite, alumina, silica gel, and activated carbon. The applicability of these sorbents to ammonia gas separation was compared based on equilibrium data. In the pressure range of 1 to 100 kPa activated carbon has its highest working capacity (5.5 mmol·g−1) at 298 K, and the working capacity drops rapidly with temperature, reaching its lowest point at 393 K. The two zeolites provide almost the same working capacity, 3.0–3.5 mmol·g−1, over the entire temperature range. Silica gel and alumina showed low working capacities. The experimental equilibrium data were fitted to 16 different isotherm models, with accuracy and reliability statistically evaluated. The Langmuir–Freundlich model with the van't Hoff equation for the equilibrium constant and with a thermal expansion equation for the saturation sorbate concentration provided the most accurate fit for the 13X and 4A zeolites. This model was also very accurate for the alumina and silica gel data, even though the Dubinin–Astakhov model gave slightly higher predictions. The Henry and vacancy solution models provided the best fit for activated carbon.

Substituent and temperature controlled tautomerism: multinuclear magnetic resonance, X-ray, and theoretical studies on 2-phenacylquinolines

Abstract: Proton-transfer equilibria in chloroform solution of twelve 2-phenacylquinolines were studied by 1H, 13C and 15N NMR spectroscopies. The (Z)-enaminone form stabilized by an intramolecular hydrogen bond was found to prevail in all cases. Electron-donating substituents in the phenacyl part of the molecule lead to an increase of the ketimine form (to 33% for p-NMe2). Variable temperature 1H NMR measurements show that higher temperatures have the same effect. The negative logarithm values of the equilibrium constant, pKT, were found to be linearly dependent on Hammett σ substituent constants. The pKTvs. temperature correlation also has a linear character. In general, strong electron-withdrawing substituents cause transformation of the ketimine to the enaminone form to become more exothermic but values of the heat of reaction for 2-phenacylquinolines studied are not linearly dependent on σ. X-Ray data show that the strength of the internal hydrogen bond in the enaminone form increases for strong electron-withdrawing substituents. Rough estimation shows this bond to be stronger in chloroform solution than in the crystalline state. π-Electron delocalization in the six-membered quasi-ring involving the H⋯O bond is very strong. This effect is responsible for the predominance of the tautomeric enaminone form in 2-phenacylquinolines. On the other hand, semiempirical AM1 and PM3 calculations show that in the gas phase the ketimine tautomer is energetically favored in most cases.

Kinetics and Equilibrium Constants for Reactions of α-Phenyl-Substituted Cyclopropylcarbinyl Radicals

Abstract: Laser-flash photolysis methods were used to determine Arrhenius functions for cyclizations of the 4,4-diphenyl-3-butenyl (2) and trans-4-phenyl-3-butenyl (5) radicals to the 1,1-diphenylcyclopropylcarbinyl (1) and 1-phenylcyclopropylcarbinyl (4) radicals, respectively. At 20 °C, the cyclization rate constants are 1.7 × 107 and 5.4 × 106 s-1. Equilibrium constants for the two processes were estimated and evaluated with thermochemical data and via computational methods, and Arrhenius functions for the ring-opening reactions of the cyclopropylcarbinyl radicals were calculated. The cyclization reactions of 2 and 5 are strongly enthalpy controlled. Production of radicals 1 and 2 from the corresponding tert-butylperoxy esters in the presence of Et3SnH gave diphenylcyclopropylmethane and 1,1-diphenyl-1-butene from H-atom trapping of radicals 1 and 2 and 4-phenyl-1,2-dihydronaphthalene which derives from the product radical formed by addition of the radical moiety in 2 to the cis-phenyl group. Rate constants for ...

A theoretical study of tautomerism of cytosine, thymine, uracil and their 1-methyl analogues in the gas and aqueous phases using AM1 and PM3

Abstract: Heats of formation, entropies, Gibbs free energies, relative tautomerisation energies, relative proton affinities, tautomeric equilibrium constants, dipole moments, and ionisation potentials for the tautomers of cytosine, uracil, thymine and their 1-methyl analogues have been studied using semiempirical AM1 and PM3 quantum-chemical calculations at the SCF level in the gas and aqueous phases, with full geometry optimisation. The COSMO solvation model was employed for aqueous solution calculations. The calculations show that the oxo-amino or dioxo tautomers are the most stable both in the gas and aqueous phase. The results are in good agreement with the available experimental and theoretical data. The entropy effect on the Gibbs free energy of the pyrimidine bases is very small and there is little significance for the tautomeric equilibria of pyrimidine bases. The enthalpic term is dominant also in the determination of the equilibrium constant.

The competing effects of π–π packing and hydrogen bonding in a hexabenzocoronene carboxylic acid derivative: A 1H solid-state MAS NMR investigation

Abstract: 1 H solid-state NMR methods employing fast (νR=30 kHz) magic-angle spinning (MAS) are applied to the investigation of both the solid and columnar liquid-crystalline (LC) phases of HBC-C10COOH, a discotic molecule consisting of a hexa-peri-hexabenzocoronene core with six symmetrically attached alkyl chains, each chain being capped by a COOH group. In the solid phase at T=320 K, the presence of hydrogen-bonded COOH dimers is demonstrated by the observation of an auto COOH peak in the 1H double-quantum (DQ) MAS NMR two-dimensional spectrum, with the interproton distance being determined to be 0.279±0.09 nm. Considering the COOH peak in the one-dimensional 1H MAS spectra, both a shift to high field of the peak position as well as an initial increase followed by a subsequent decrease in the linewidth are observed upon heating. These observations are interpreted in terms of a chemical exchange process involving the making and breaking of hydrogen bonds, with the coalescence point corresponding to T=362 K. The equilibrium constant at a given temperature is calculated from the observed chemical shift, and a thermodynamic analysis yields for the opening of the hydrogen-bonded dimers: ΔH=45±4 kJ mol−1 and ΔS=113±11 J K−1 mol−1. In 1H DQ-filtered (DQF) MAS spectra, the intensity of the COOH peak is observed, upon heating, to reduce faster than expected from thermodynamic factors alone, and above T=380 K, no signal is detected. Using the fact that the observation of a DQ signal relies on the existence of a DQ coherence for the duration of the experiment, the kinetics of dimer opening are determined: the extracted activation energy and Arrhenius parameter equal 89±10 kJ mol−1 and 4.2×1016 s−1, respectively. On the transition to the LC phase, a marked narrowing of all resonances is observed, with the COOH chemical shift initially having a very high-field value, 9.0 ppm, which is indicative of free COOH groups. An analysis of DQ MAS spinning-sideband patterns shows that the dipolar coupling in the LC phase is reduced by a factor of 0.43±0.04.

Equilibrium and kinetic studies of the aquation of the dinuclear platinum complex [[trans-PtCl(NH3)2]2(mu-NH2(CH2)6NH2)]2+: pKa determinations of aqua ligands via [1H,15N] NMR spectroscopy.

Abstract: By the use of [1H,15N] heteronuclear single quantum coherence (HSQC) 2D NMR spectroscopy and electrochemical methods we have determined the hydrolysis profile of the bifunctional dinuclear platinum complex [[trans-PtCl(15NH3)2]2(mu-15NH2(CH2)(6)15NH2)]2+ (1,1/t,t (n = 6), 15N-1), the prototype of a novel class of potential antitumor complexes Reported are estimates for the rate and equilibrium constants for the first and second aquation steps, together with the acid dissociation constant (pKa1 approximately pKa2 approximately pKa3) The equilibrium constants determined by NMR at 25 and 37 degrees C (I = 01 M) were similar, pK1 approximately pK2 = 39 +/- 02, and from a chloride release experiment at 37 degrees C the values were found to be pK1 = 411 +/- 005 and pK2 = 42 +/- 05 The forward and reverse rate constants for aquation determined from this chloride release experiment were k1 = (85 +/- 03) x 10(-5) s-1 and k-1 = 091 +/- 006 M-1 s-1, where the model assumed that all the liberated chloride came from 1 When the second aquation step was also taken into account, the rate constants were k1 = (79 +/- 02) x 10(-5) s-1, k-1 = 118 +/- 006 M-1 s-1, k2 = (106 +/- 30) x 10(-4) s-1, k-2 = 15 +/- 06 M-1 s-1 The rate constants compare favorably with other complexes with the [PtCl(am(m)ine)3]+ moiety and indicate that the equilibrium of all these species favors the chloro form A pKa value of 562 was determined for the diaquated species [[trans-Pt(15NH3)2(H2O)]2(mu-15NH2(CH2)(6)15NH2)]4+ (3) using [1H,15N] HSQC NMR spectroscopy The speciation profile of 1 and its hydrolysis products under physiological conditions is explored

Structure-Reactivity Relationships and Intrinsic Reaction Barriers for Nucleophile Additions to a Quinone Methide: A Strongly Resonance-Stabilized Carbocation

Abstract: Second-order rate constants kNu (M-1 s-1) were determined for addition of a wide range of nucleophiles to the simple quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) to give the nucleophile adduct 1-Nu in water. Equilibrium constants were determined for the overall addition of HBr and HI to 1 to give H-1-Nu, and the data were used to calculate equilibrium constants for the addition of Br- and I- to 1, and to estimate equilibrium constants for the addition of Cl- and AcO-. The values of log kNu show a linear correlation with the Ritchie nucleophilicity parameter N+ with a slope s = 0.92 ± 0.10 that is essentially the same as the electrophile-independent value of 1.0 for highly resonance-stabilized carbocations. Marcus intrinsic barriers Λ of 12.4, 13.9, 15.4, and 19.8 kcal/mol are reported for the addition of I-, Br-, Cl-, and AcO- to 1, respectively. The thermodynamic barriers ΔG° and intrinsic barriers Λ for addition of Br-, Cl-, and AcO- to 1 are 8.4 ± 1.0 and 5.2 ± 0.2 kcal/mo...

Substituent and temperature controlled tautomerism of 2-phenacylpyridine: the hydrogen bond as a configurational lock of (Z)-2-(2-hydroxy-2-phenylvinyl)pyridine

Abstract: 2-Phenacylpyridines substituted in the benzene ring are in equilibrium with (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines when dissolved in chloroform. The substituent affects significantly the tautomeric equilibrium [the amount of the enolimine form stabilized by the intramolecular hydrogen bond is 1 and 92% for R = p-N(CH2)4 and p-NO2, respectively]. The negative logarithm of the tautomeric equilibrium constant, KT, is linearly dependent on the Hammett σ substituent constants. The dependence of KTvs. temperature is exponential in character: the more electron-withdrawing is the substituent, the more distinct is the influence of temperature. Unexpectedly, the tautomer present in the crystalline state is not the same for all compounds studied (it is the ketimine one for those carrying strong electron-donor groups). Among the different ab initio methods used to calculate the enthalpy of the proton transfer in chloroform solution, MP2/6-31G** gives the best results.

Langmuir and Dubinin–Radushkevich analyses on equilibrium adsorption of activated carbon fabrics in aqueous solutions

Abstract: The present study investigated the adsorption of phenol, iodine and tannic acid onto activated carbon fabrics in aqueous solutions. The carbon fabrics were made of polyacrylonitrile with different extents of activation in steam. The increasing extent of activation is accompanied by increasing carbon porosity as well as liquid-phase adsorption capacity. Langmuir and Dubinin–Radushkevich (D–R) models were employed to analyze the equilibrium adsorption data. The fractional filling of carbon micropores and the mean free energy for adsorption were estimated according to these models. It is suggested on the basis of the present work that the mesopore fraction of the fabrics, the extent of carbon activation, and the adsorbate size are important as well as competitive factors determining the adsorption capacity. Both the equilibrium constant and free energy for adsorption increase with the mesopore fraction, indicating that the mesopores promote the adsorption onto high energy sites, whereas the fraction filling micropores decreases with the extent of carbon activation and the size of the adsorbates. © 2000 Society of Chemical Industry

Adsorption and Desorption of n-Hexane, Methyl Ethyl Ketone, and Toluene on an Activated Carbon Fiber from Supercritical Carbon Dioxide

Abstract: The adsorption equilibrium and regeneration efficiency of n-hexane, methyl ethyl ketone (MEK), and toluene under supercritical CO2 on an activated carbon fiber were experimentally determined. When the densities of the supercritical fluids were fixed at 0.32, 0.45, and 0.62 g/cm3, the Langmuir model agreed with the experimental isotherms at the temperatures 308, 318, and 328 K. As the concentration of adsorptive in supercritical CO2 was kept constant, the crossover of the equilibrium loading at different temperatures was observed at relatively high pressure and the crossover pressure of MEK was lower than that of n-hexane. In both cases, the crossover pressure increased with an increase in concentration. Also, the activated carbon fiber loaded with toluene and MEK was regenerated by supercritical CO2 in the range of 0.69−0.90 g/cm3. Desorption became favorable with an increase in pressure, and there was an optimal temperature at every pressure condition. However, the regeneration efficiency increased with ...

Structures of the linear silicon carbides SiC4 and SiC6: Isotopic substitution and Ab Initio theory

Abstract: The structures of two linear silicon carbides, SiC4 and SiC6, have been determined by a combination of isotopic substitution and large-scale coupled-cluster ab initio calculations, following detection of all of the singly substituted isotopic species in a supersonic molecular beam with a Fourier transform microwave spectrometer. Rotational constants obtained by least-squares fitting transition frequencies were used to derive experimental structures; except for those nearest the center of mass, individual bond lengths for both chains have an error of less than 0.008 A. Accurate equilibrium structures were derived by converting the experimental rotational constants to equilibrium constants using the vibration–rotation coupling constants from coupled-cluster calculations, including connected triple substitutions. Equilibrium dipole moments and harmonic vibrational frequencies were also calculated for both chains. On the basis of the calculated vibration–rotation and l-type doubling constants, weak rotational...

Dioxygen-Binding Kinetics and Thermodynamics of a Series of Dicopper(I) Complexes with Bis[2-(2-pyridyl)ethyl]amine Tridendate Chelators Forming Side-On Peroxo-Bridged Dicopper(II) Adducts

Abstract: Copper-dioxygen interactions are of interest due to their importance in biological systems as reversible O2- carriers, oxygenases, or oxidases and also because of their role in industrial and laboratory oxidation processes. Here we report on the kinetics (stopped-flow, -90 to 10 degrees C) of O2-binding to a series of dicopper(I) complexes, [Cu2(Nn)(MeCN)2]2+ (1Nn) (-(CH2)n- (n = 3-5) linked bis[(2-(2-pyridyl)ethyl]amine, PY2) and their close mononuclear analogue, [(MePY2)Cu(MeCN)]+ (3), which form mu-eta 2:eta 2-peroxodicopper(II) complexes [Cu2(Nn)-(O2)]2+ (2Nn) and [(MePY2)Cu]2(O2)]2+ (4), respectively. The overall kinetic mechanism involves initial reversible (k+,open/k-,open) formation of a nondetectable intermediate O2-adduct [Cu2(Nn)(O2)]2+ (open), suggested to be a CuI...CuII-O2- species, followed by its reversible closure (k+,closed/k-,closed) to form 2Nn. At higher temperatures (253 to 283 K), the first equilibrium lies far to the left and the observed rate law involves a simple reversible binding equilibrium process (kon,high = (k+,open/k-,open)(k+,closed)). From 213 to 233 K, the slow step in the oxygenation is the first reaction (kon,low = k+,open), and first-order behavior (in 1Nn and O2) is observed. For either temperature regime, the delta H++ for formation of 2Nn are low (delta H++ = -11 to 10 kJ/mol; kon,low = 1.1 x 10(3) to 4.1 x 10(3) M-1 s-1, kon,high = 2.2 x 10(3) to 2.8 x 10(4) M-1 s-1), reflecting the likely occurrence of preequilibria. The delta H degree ranges between -81 and -84 kJ mol-1 for the formation of 2Nn, and the corresponding equilibrium constant (K1) increases (3 x 10(8) to 5 x 10(10) M-1; 183 K) going from n = 3 to 5. Below 213 K, the half-life for formation of 2Nn increases with, rather than being independent of, the concentration of 1Nn, probably due to the oligomerization of 1Nn at these temperatures. The O2 reaction chemistry of 3 in CH2Cl2 is complicated, including the presence of induction periods, and could not be fully analyzed. However, qualitative comparisons show the expected slower intermolecular reaction of 3 with O2 compared to the intramolecular first-order reactions of 1Nn. Due to the likelihood of the partial dimerization of 3 in solution, the t1/2 for the formation of 4 remains constant with increasing complex concentration rather than decreasing. Acetonitrile significantly influences the kinetics of the O2 reactions with 1Nn and 3. For 1N4, the presence of MeCN inhibits the formation of a previously (Jung et al, J. Am. Chem. Soc. 1996, 118, 3763-3764) observed intermediate. Small amounts of added MeCN considerably slow the oxygenation rates of 3, inhibit its full formation to 4, and increase the length of the induction period. The results for 1Nn and their mononuclear analogue 3 are presented, and they are compared with each other as well as with other dinucleating dicopper(I) systems.

Concerning the self-association of N-vinyl pyrrolidone and its effect on the determination of equilibrium constants and the thermodynamics of mixing

Abstract: Hydroxylated (co)polymers form miscible blends with a wide variety of (co)polymers that contain proton acceptor groups, such as esters, ethers, pyridines, etc. The former strongly self-associate through the formation of OH/OH hydrogen bonded interactions, while the latter do not self-associate to any measurable exent. With appropiate self- and interassociation equilibrium constants and anthalpies of hydrogen bond formation, we have been able in the past to successfully predict the phase behavior of such hydrogen bonded polymer blends. At first glance, the vinyul pyrrolidone group appears to be just another proton acceptor and if the appropiate interassociation equilibrium constant value is determined we should be able to predict the phase behavior of hydroxylated (co)polymer blends with (co)polymers containing the vinyl pyrrolidone group. However, it is not that simple. Using infrared spectroscopy we demnstrate in this work that the pyrrlidone group strongly self-associates through transition dipole coupling. Ramifications to the determination of equilibrium constants, the free energy of mixing and the phase behavior of blends involving the vinyl pyrrolidone group are discussed.