Showing papers on "Equilibrium constant published in 2004"

Toward a new type of anhydrous organic proton conductor based on immobilized imidazole

Abstract: Extensive studies on a model system (imidazole-terminated ethylene oxide oligomers doped with small amounts of strong acids) for a proton-conducting polymer functioning without a liquid phase, but instead using imidazole tethered to the backbone via flexible spacers as a proton solvent, are presented and the parameters governing conductivity and its mechanism are discussed. Temperature-dependent conductivities are well described by free-volume considerations (VTF-behavior). Thus, besides a high density of imidazole moieties, a low Tg is in favor of high proton conductivity, which experimentally is shown to be predominantly due to structure diffusion. The available free volume is suggested to correlate with the rate of hydrogen bond breaking and forming processes within the dynamical hydrogen bond networks, which generally limit the rate of long-range diffusion of protonic defects via structure diffusion. The equilibrium constants for the protonation of imidazole by two different dopants in an oligo(ethyle...

Salt effects on the apparent stability of the cucurbit[7]uril-methyl viologen inclusion complex.

Abstract: The effects of the medium ionic composition on the apparent equilibrium association constant (K) for the formation of a 1:1 inclusion complex between the guest methyl viologen (MV2+) and the host cucurbit[7]uril (CB7) were studied in aqueous solutions. The K values were found to decrease with increasing ionic strength, with more pronounced effects for solutions containing divalent Ca2+ ions than for solutions containing monovalent Na+ ions. The competing ion−dipole interactions between Ca2+ or Na+ and MV2+ ions appear to be responsible for the remarkable modulation of the K values observed in this work.

Consistent experimental and theoretical evidence for long-lived intermediate radicals in living free radical polymerization

Abstract: The cumyl dithiobenzoate (CDB)-mediated reversible addition fragmentation chain transfer (RAFT) polymerization of styrene at 30 degrees C is studied via both kinetic experiments and high-level ab initio molecular orbital calculations. The kinetic data clearly indicate the delayed onset of steady-state behavior. Such an observation is consistent with the slow fragmentation model for the RAFT process, but cannot be reconciled with the cross-termination model. The comprehensive failure of the cross-termination model is quantitatively demonstrated in a detailed kinetic analysis, in which the independent influences of the pre-equilibria and main equilibria and the possible chain length dependence of cross-termination are fully taken into account. In contrast, the slow fragmentation model can describe the data, provided the main equilibrium has a large fragmentation constant of at least 8.9 x 10(6) L mol(-1). Such a high equilibrium constant (for both equilibria) is consistent with high-level ab initio quantum chemical calculations (K = 7.3 x 10(6) L mol(-1)) and thus appears to be physically realistic. Given that the addition rate coefficient for macroradicals to (polymeric) RAFT agent is 4 x 10(6) L mol(-1) s(-1), this implies that the lifetime of the RAFT adduct radicals is close to 2.5 s. Since the radical is also kinetically stable to termination, it can thus function as a radical sink in its own right.

Chemical speciation of environmentally significant heavy metals with inorganic ligands

Abstract: This document presents a critical evaluation of the equilibrium constants and reaction enthalpies for the complex formation reactions between aqueous Hg(II) and the common environmental inorganic ligands Cl–, OH–, CO3 2–, SO4 2–, and PO4 3–. The analysis used data from the IUPAC Stability Constants database, SC-Database, focusing particularly on values for 25 °C and perchlorate media. Specific ion interaction theory (SIT) was applied to reliable data available for the ionic strength range Ic ≤ 3.0 mol dm –3. Recommended values of log10 βp,q,r° and the associated reaction enthalpies, ∆rHm°, valid at Im = 0 mol kg –1 and 25 °C, were obtained by weighted linear regression using the SIT equations. Also reported are the equations and specific ion interaction coefficients required to calculate log10 βp,q,r values at higher ionic strengths and other temperatures. A similar analysis is reported for the reactions of H+ with CO3 2– and PO4 3–. Diagrams are presented to show the calculated distribution of Hg(II) amongst these inorganic ligands in model natural waters. Under typical environmental conditions, Hg(II) speciation is dominated by the formation of HgCl2(aq), Hg(OH)Cl(aq), and Hg(OH)2(aq).

Determination of the mode and efficacy of the cross-linking of guar by borate using MAS 11B NMR of borate cross-linked guar in combination with solution 11B NMR of model systems

Abstract: The reaction product of boric acid and the polysaccharide guaran (the major component of guar gum) has been investigated by 11B NMR spectroscopy. By comparison with the 11B NMR of boric acid and phenylboronic acid complexes of 1,2-diols (HOCMe2CMe2OH, cis-C6H10(OH)2, trans-C6H10(OH)2, o-C6H4(OH)2), 1,3-diols (neol-H2), monosaccharides (L-fucose, mannose and galactose) and disaccharides (celloboise and sucrose) it is found that the guaran polymer is cross-linked via a borate complex of two 1,2-diols both forming chelate 5-membered ring cycles ([B52]), this contrasts with previous proposals. Based upon steric constraints we propose that preferential cross-linking the guaran polymer occurs via the 3,4-diols of the galactose side chain. The ΔH and ΔS for complexation of boric acid to cis- and trans-1,2-cyclohexanediol have been determined, from the temperature dependence of the appropriate equilibrium constants, and used in conjunction with ab initio calculations on model compounds, to understand prior conflicting proposals for guaran–boric acid interactions. 11B NMR derived pH dependent equilibrium constants and ab initio calculations have been used to understand the reasons for the inefficiency of boric acid to cross-link guaran (almost 2 borate ions per 3 monosaccharide repeat units are required for a viscous gel suitable as a fracturing fluid): the most reactive sites on the component saccharides (mannose and galactose) are precluded from reaction by the nature of the guar structure; the comparable acidity (pKa) of the remaining guaran alcohol substituents and the water solvent, results in a competition between cross-linking and borate formation; a significant fraction of the boric acid is ineffective in cross-linking guar due to the modest equilibrium (Keq). In contrast to prior work, we present evidence for the reaction of alcohols with boric acid, rather than the borate anion. Based upon the results obtained for phenylboronic acid, alternative cross-linking agents are proposed.

Ab Initio Study of the Addition-Fragmentation Equilibrium in RAFT Polymerization: When Is Polymerization Retarded?

Abstract: High level ab initio calculations of the addition−fragmentation equilibrium constants at 333 K were performed for model RAFT reactions: R• + SC(Z)SCH3 → R−SC•(Z)SCH3, for all combinations of Z = CH3, phenyl, or benzyl with R = CH3, benzyl, CH2COOCH3, or C(CH3)2CN. The results indicate that slow fragmentation of the polymeric RAFT-adduct radical is responsible for rate retardation in cumyl dithiobenzoate mediated polymerization of styrene and methyl acrylate. They also confirm that rate retardation should be relieved by changing the phenyl Z substituent to a benzyl group and that inhibition should be relieved by using a RAFT agent with a C(CH3)2CN leaving group. The equilibrium constants are extremely sensitive to the nature of the R and Z substituents, with a 13 orders of magnitude variation over the 12 reactions considered. The effects of these substituents on the reaction enthalpy are generally consistent with the trends expected on the basis of radical stabilization arguments. However, there are addit...

Aggregation of Rhodamine B in Water

Abstract: Protolytic and associative equilibria in aqueous solutions of rhodamine B were studied. The curves of potentiometric titration with NaOH solution of the dye hydrochloride within the 5 ×10-4-3 ×10-2 M range and at an ionic strength of 0.1 M (NaCl or KCl) were treated by chemometric methods of successive modification of the equilibrium system models with regard to spectrophotometric data. The most probable value of the equilibrium constants of the reaction iH+ + jR ⇄ H i R j i+ were determined.

Critical Evaluation of Rate Constants and Equilibrium Constants of Hydrogen Peroxide Photolysis in Acidic Aqueous Solutions Containing Chloride Ions

Abstract: Equilibrium constants and rate constants involving Cl⋅(aq), Cl−, Cl2−⋅(aq), HO⋅, H2O, and H2O2(aq) determined at 297±2 K in the aqueous phase are updated and evaluated. Most of the rate constants and equilibrium constants are obtained by either pulse radiolysis or laser flash photolysis. The recommended values of rate constants and equilibrium constants are achieved by un-weighted averaging of the reliable experimental measurements.

Equilibrium‐range constant in wind‐generated wave spectra

Abstract: [1] The equilibrium range in wind-driven wave spectra is characterized to have an f−4 form in deep water (k−5/2 shallow-water form). Absolute energy levels for six physically diverse sites are shown to be well fit by an expression involving the wind speed measured at a fraction of a wavelength above the sea surface in combination with the phase speed of the spectral peak. Observed energy levels suggest that nonlinear wave-wave interactions are capable of maintaining the observed energy-flux balance in the equilibrium range.

Calorimetric measurement of phospholipid interaction with methyl-beta-cyclodextrin.

Abstract: Cyclodextrins are able to bind hydrophobic molecules in their interior cavity and as such have received a great deal of attention as carriers of cholesterol, lipophilic drugs, and other sparingly soluble compounds. Despite the importance of these biochemical applications, relatively little is known about the interactions of cyclodextrins with phospholipid membranes. Here we characterize the binding of randomly methylated beta-cyclodextrin (m beta CD) to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using right-angle light scattering and isothermal titration calorimetry. Existing models of lipophile-membrane interactions are inadequate to describe the observed binding; we introduce a modified chemical reaction model in which the chemical activity of the phospholipid is independent of its concentration. We find that an average of four m beta CD molecules bind to each POPC molecule with an enthalpy of reaction of 46 kJ mol(-1) and an equilibrium constant of 90 M(-3). These results are consistent with earlier qualitative observations and suggest that disruption of phospholipid membranes may be minimized if the concentration of m beta CD is kept below about 15 mM.

A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the O ? H Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants

Abstract: New experimental results on the determination of the bond dissociation enthalpy (BDE) value of 3,5-di-tert-butylcatechol, a model compound for flavonoid antioxidants, by the EPR radical equilibration technique are reported. By measurement of the equilibrium constant for the reaction between 3,5-di-tert-butylcatechol and the 2,6-di-tert-butyl-4-methylphenoxyl radical, in UV irradiated isooctane solutions at different temperatures, it has been shown that the thermodynamic parameters for this reaction are DeltaH degrees = -2.8+/-0.1 kcal mol(-1) and DeltaS degrees = +1.3+/-0.2 cal mol(-1) K(-1). This demonstrates that the entropic variations in the hydrogen exchange reaction between phenols and the corresponding phenoxyl radicals are also negligible when one of the reacting species is a polyphenol and that the EPR radical equilibration technique also allows the determination of the Obond;H BDEs in intramolecularly hydrogen-bonded polyphenols. The BDE of 3,5-di-tert-butylcatechol (78.2 kcal mol(-1)) was determined to be identical to that of alpha-tocopherol. Through use of the group additivity rule, this piece of data was also used to calculate the strength of the intramolecular hydrogen bond between the hydroxyl proton and the oxygen radical centre in the corresponding semiquinone radical (5.6 kcal mol(-1)), which is responsible both for the excellent antioxidant properties of catechols and for the BDE of catechol (81.8 kcal mol(-1)). These values are in poor agreement with those predicted by DFT calculations reported in the literature (9.5 kcal mol(-1) and 77.6 kcal mol(-1), respectively). Extensive theoretical calculations indicate that the BDE of catechol is reproduced well (81.6 kcal mol(-1)) by use of diffuse functions on oxygen and the CCSD method.

Breakthrough analysis for adsorption of sulfur-dioxide over zeolites

Abstract: The adsorption experiments were carried out under dynamic conditions for the removal of trace sulfur-dioxide (SO 2 ) in nitrogen by 5A zeolites. The experiments were conducted to characterize the breakthrough characteristics of SO 2 in a fixed bed under different operating conditions including temperature, pellet size, concentration levels, and gas flowrate. At a reaction temperature of 70 °C, the breakthrough time was found to be maximum. The adsorption isotherm was found to be linear over the gas concentration range from 1000 to 10 000 ppm. The exothermic heat of adsorption assuming Arrehenius type of temperature dependence of the equilibrium constant was determined to be 9.8 kc/mol. The mathematical model was developed to predict the breakthrough profiles of SO 2 during adsorption over the biporous zeolites (containing both macro and micro-pores). The model incorporates all resistances to mass transfer, namely: diffusion in the gas film around pellets in the bed, diffusion in the binder-phase of zeolites and within the crystals, and adsorption/desorption at the interface of binder-phase and crystals. The model was successfully validated with the observed experimental breakthrough data. The study showed potential application of 5A zeolites in controlling SO 2 emissions at trace levels.

Thermodynamic and Dynamic Interfacial Properties of Binary Carbon Dioxide−Water Systems

Abstract: We have measured the interfacial tension (γ) between H2O and CO2 with a drop tensiometer that permitted us to regulate the drop area and to perform elasticity measurements. We observed that γ decreased with time and with the CO2 pressure. We first analyzed the adsorption of CO2 onto surface water by plotting the initial interfacial tension (γ0) versus CO2 fugacity and applied Gibbs and then Langmuir-Freundlich equations. We have calculated the interfacial area occupied by CO2 molecules onto H2O as well as the equilibrium constant and the thermodynamics parameters of adsorption. This permitted us to understand that the CO2 was adsorbed onto the surface water by formation of a one-to-one H-type complex with H2O. Second, we attributed the kinetics phenomena to the formation of interfacial clusters between water and CO2, which gave interfacial quasi-crystalline structure as shown by compression of the drop area at the end of the adsorption kinetics. We studied this interfacial organization by measuring the ap...

The adsorption of amino acids on the surface of highly dispersed silica

Abstract: The adsorption of arginine, histidine, lysine, and ornithine on the surface of highly dispersed silica from aqueous solutions was studied as a function of pH. The equilibrium constants of the formation of surface complexes were calculated using the Stern model for the electrical double layer. It was shown that the possibility of adsorption of amino acids on the silica surface is determined by the presence of additional basic groups in their molecules.

Water Dimers in the Atmosphere II: Results from the VRT(ASP-W)III Potential Surface

Abstract: We report refined results for the equilibrium constant for water dimerization (KP), computed as a function of temperature via fully coupled 6D calculation of the canonical (H2O)2 partition function on VRT(ASP-W)III, the most accurate water dimer potential energy surface currently available. Partial pressure isotherms calculated for a range of temperatures and relative humidities indicate that water dimers can exist in sufficient concentrations (e.g., 1018 m-3 at 30 °C and 100% relative humidity) to affect physical and chemical processes in the atmosphere. The determinations of additional thermodynamic properties (ΔG, ΔH, ΔS, CP, and CV) for (H2O)2 are presented, and the role of quasi-bound states in the calculation of KP is discussed at length.

Mechanism of stereo- and regioselectivity in the Paternò-Büchi reaction of furan derivatives with aromatic carbonyl compounds: importance of the conformational distribution in the intermediary triplet 1,4-diradicals.

Abstract: Temperature and substituent effects on the stereo- and regioselectivity have been investigated in the photochemical [2 + 2] cycloaddition reaction, the so-called Paterno-Buchi (PB) reaction, of unsymmetrically substituted furans 2a,b (2-methyl- and 3-methylfuran) with aromatic carbonyl compounds 1a,b (benzaldehyde and benzophenone). The regio-random but stereoselective (exo/endo > 97/3) formation of lower substituted oxetane 3a and higher substituted oxetane 4a is found in the reaction with benzaldehyde (1a). The exclusive stereoselectivity is not dependent on the position of methyl substituent on the furan ring and the reaction temperature. The double-bond selection (3a versus 4a) is slightly dependent on the reaction temperature (3a/4a = 55/45 to 40/60). The Eyring plots of the regioselectivity are linear. Contrastively, in the reaction with benzophenone (1b), the double-bond selection (3b versus 4b) largely depends on the reaction temperature. The Eyring plots are not linear, but the inflection points are observed. The transient absorption spectroscopic analyses (picosecond time scale) clarify the intervention of triplet 2-oxabutane-1,4-diyls in the photochemical processes. Computational studies reveal the equilibrium structures of the triplet diradicals, energy barriers between the conformers, and the equilibrium constants. A rational mechanism is herein proposed by the support of both experimental and computational investigations to account for not only the exclusive formation of the exo-configured oxetanes 3a and 4a but also the nonlinear Eyring plots observed in the reaction with 1b.

Gas phase oxidation of benzene: Kinetics, thermochemistry and mechanism of initial steps

Abstract: A new investigation of the primary steps of the benzene oxidation, involving complementary experimental and theoretical approaches, is presented. The reactions of the OH-adduct (hydroxy-cyclohexadienyl radical c-C6H6-OH) were investigated using laser flash photolysis and producing OH radicals by H2O2 photolysis at 248 nm. It is confirmed that the adduct is in equilibrium with the corresponding peroxy radical RO2, near atmospheric conditions, the measured equilibrium constant being: Kc,2b = (2.62 ± 0.24) × 10−19 cm3 molecule−1 at 295 K, with the temperature dependent expression: ln(Kc,2b/cm3 molecule−1) = −63.29 + 6049/T, obtained by using the calculated entropy of reaction. The rate constant of the association reaction yielding RO2 is: k2b = (1.31 ± 0.12) × 10−15 cm3 molecule−1 s−1. Calculated data are in agreement with those values. In addition, data analysis shows that the reaction c-C6H6-OH + O2 involves an irreversible loss of radical species, yielding phenol and other oxidation products, with the global rate constant: kloss = (2.52 ± 0.40) × 10−16 cm3 molecule−1 s−1. Quoted errors are statistical (2σ), the possible total errors on the above values being estimated at around 40%. By comparison with the kloss value, the rate constant for phenol formation, calculated using a combination of DFT and ab initio CCSD(T) methods, corresponds to a phenol yield of about 55%, in reasonable agreement with experimental observations. Thermochemical and kinetic parameters have been calculated for the formation and for the reactions of the two RO2 stereoisomers, cis and trans. They show that the observed equilibrium must involve the trans isomer, which is more stable and is formed more rapidly than the cis isomer. Calculations show that the only possible reactions of peroxy radicals, under atmospheric conditions, is cyclisation yielding a bicyclic radical. However, cyclisation of the RO2(trans) is calculated to be too slow, compared to the rate of the irreversible radical loss, whereas it is very fast in the case of the cis isomer and can lead readily to oxidation products. On the basis of those results, a reaction mechanism is proposed for the first steps of benzene oxidation, consistent with all experimental and theoretical data, and which accounts for the principal oxidation products observed.

Hydrogen atom transfer from iron(II)-tris[2,2'-bi(tetrahydropyrimidine)] to TEMPO: a negative enthalpy of activation predicted by the Marcus equation.

Abstract: The transfer of a hydrogen atom from iron(II)−tris[2,2‘-bi(tetrahydropyrimidine)], [FeII(H2bip)3]2+, to the stable nitroxide, TEMPO, was studied by stopped-flow UV−vis spectrophotometry. The products are the deprotonated iron(III) complex [FeIII(H2bip)2(Hbip)]2+ and the hydroxylamine, TEMPO−H. This reaction can also be referred to as proton-coupled electron transfer (PCET). The equilibrium constant for the reaction is close to 1; thus, the reaction can be driven in either direction. The rate constants for the forward and reverse reactions at 298 K are k1 = 260 ± 30 M-1 s-1 and k-1 = 150 ± 20 M-1 s-1. Interestingly, the rate constant for the forward reaction decreases as reaction temperature is increased, implying a negative activation enthalpy: ΔH1⧧ = −2.7 ± 0.4 kcal mol-1, ΔS1⧧ = −57 ± 8 cal mol-1 K-1. Marcus theory predicts this unusual temperature dependence on the basis of independently measured self-exchange rate constants and equilibrium constants: ΔHcalcd⧧ = −3.5 ± 0.5 kcal mol-1, ΔScalcd⧧ = −42 ...