Dissociation Reaction Rate Research Articles

Effect of total aluminum concentration on the formation and transformation of nanosized Al13 and Al30 in hydrolytic polymeric aluminum aqueous solutions

Influence of total aluminum concentration (CAlT) on the generation and transformation of nanosized Al13 and Al30 in hydrolytic polyaluminum aqueous solutions was investigated using high field 27Al NMR and time-developed Al-Ferron complex colorimetry. When prepared at the optimal basicity (B) of Al13 generation and 80°C, the Al13 species in polyaluminum solution tends to further polymerize and convert to Al30 and higher polymers when CAlT>0.2 mol · L−1, but Al13 does not convert to Al30 quantificationally, as the formation of Alu from Al13 and Al30 is accelerated in the same way. The conversion rate of Al13 is accelerated by the increase in CAlT. When CAlT>0.75 mol·L−1, Al13 content decreases rapidly, and Al30 content increases continuously and becomes the dominant nanometer polynuclear aluminum species. Alm is one of prerequisites of Al13 conversion to Al30. When CAlT increases and B reduces, the polymerization rate between Al13 and Al13 increases, and at the same time, the dissociation reaction rate of Al13 and Al30 by H+ also increases. The latter becomes the dominant reaction in polyaluminum solution with low B value, so Al30 decreases with the increasing CAlT. The hydrolytic polyaluminum solution with Al13 content beyond 80% can only be prepared under the condition of CAlT<0.5 mol · L−1 and optimal B value.

Theoretical Study of Oxygen Isotope Exchange and Quenching in the O(1D) + CO2 Reaction

Ab initio multireference configuration interaction calculations have been carried out for the CO3 system in singlet and triplet electronic states to investigate the mechanism of the O(1D) + CO2 reaction. The reaction has been shown to occur through the formation of an O−CO2 complex s0, which then isomerizes to a cyclic O(CO2) structure s1 over a barrier at s-TS0 located ∼0.3 kcal/mol below the reactants. The cyclic isomer s1, 48.8 kcal/mol lower in energy than O(1D) + CO2, can in turn rearrange to a D3h structure s2, only slightly higher in energy. The isomers s1 and s2 formed in the reaction possess high internal energy and can decompose into the initial reactants or undergo singlet−triplet intersystem crossing to form the triplet isomer t1, which can dissociate to O(3P) + CO2. If the attacking oxygen atom is isotope-labeled, isotope exchange can occur due to symmetry properties of s1 and s2. The ab initio energies, molecular parameters, and spin−orbit coupling constants were employed in statistical calculations of various isomerization and dissociation reaction rates. For intersystem crossing rates, we used the theory of radiationless transitions, in which the rates are determined as a product of the overlap of electronic wave functions (spin−orbit coupling) and the overlap of vibrational wave functions (Franck−Condon factor). The calculated rate constants were then used to compute the product branching ratios both for the case of the 16O(1D) + 44CO2 reaction and for the isotope-labeled 18O(1D) + 44CO2 reaction. For the latter, the calculated relative branching ratios of the 16O(1D) + 46CO2 and 16O(3P) + 46CO2 products at collision energies of 4.2 and 7.7 kcal/mol, 17/83 and 42/58, agree reasonably well with the experimental values of 16/84 and 33/67 (ref 4), respectively, and reproduce the qualitative trend with Ecol. The overall relative yields computed for 44CO2 and 46CO2 show that the attacking O(1D) atom can be incorporated into the product CO2 molecule with a near-statistical probability of 2/3.

Dissociation and recombination rate constants for N 2 on Cu and Ni group transition metal surfaces

We report dissociation and recombination reaction rate constants for N 2 on the fcc(1 1 1) surfaces of Ni, Pd, Pt, Cu, Ag and Au from molecular dynamics simulations employing our normalized bond index-reactive potential functions. The Arrhenius pre-exponentials for recombination of N 2 on these surfaces are three to four orders of magnitude greater than the dissociation pre-exponentials, which is similar to results we obtained for CN. On the series of metals considered herein, the reaction energetics favor dissociation on the more active metals and favor recombination on the least active metals. The recombination of N 2 on gold and silver is diffusion controlled. The large differences in the dissociation/recombination pre-exponentials express the tendency of the reaction entropy to favor the recombination on the surfaces investigated. We discuss the low sticking probability of N 2 in terms of our previous modeling of adsorption/desorption processes.

Dissociation Kinetics of Cerium(lll) Complexes of Macrocyclic Polyaza Polycarboxylate Ligands TETA and DOTA

AbstractThe acid‐catalyzed dissociation rate constants of the cerium(III) complexes of 1,4,8,11‐tetraazacyclo‐tetradecane‐1,4,8,11‐tetraacetic acid (TETA) and 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA) have been determined at four different temperatures (i.e., 25.0°C,32.0°C,39.0°C,45.0°C) in aqueous media (μ = 0.10 M, HCl/KCl) to obtain additional kinetic data and to evaluate possible effects of ligand pre‐organization for metal ion complexation. The rates are much faster for Ce(TETA)− than for Ce(DOTA)−, indicating the lower thermodynamic stability of the former. In the presence of excess strong acid, 0.1‐1.0 MHCl, the dissociation reactions follow the rate law: ‐d[ML]T/dt = (kd + kH[H+])[ML]T and ‐d[ML]T/dt = (kH[H+] + kH2[H+]2)[ML]T, respectively, where kd is acid‐independent dissociation reaction rate constant and kH and kH2 are the respective dissociation rate constants for the pathways involving monoprotonated and diprotonated species. The rate activation parameters, ΔH≠, ΔS≠ and ΔG≠, for each dissociation pathway have been obtained and their values are consistent with the proposed mechanisms. In particular, the rate difference between Ce(TETA)−and Ce(DOTA)− for the monoprotonated complex dissociation pathway is mainly due to difference in ΔHH≠. It has been concluded that ligand pre‐organization results in more stable complexes and slower complex dissociation rates.

Dissociation and recombination rate constants for CN on Cu and Ni group transition metal surfaces

We report dissociation and recombination reaction rate constants for CN on the fcc(111) surfaces of Ni, Pd, Pt, Cu, Ag and Au from molecular dynamics simulations employing our normalized bond index-reactive potential functions (NBI-RPF). The Arrhenius pre-exponentials for recombination of CN on these surfaces are about three orders of magnitude greater than the dissociation pre-exponentials. On the series of metals considered herein, the reaction energetics favor dissociation on the more active metals and favor recombination on the least active metals. However, the differences in the pre-exponentials of nearly a factor of 10 3 express the tendency of the reaction entropy to favor the recombination on the surfaces investigated. We also discuss the implications of these results in terms of the thermodynamics of the surface reactions.

A plasma kinetics model: analysis of wall loss reactions in dry etching of silicon dioxide

Computational models for gas-phase chemical reactions in plasmas and for sticking reactions on metallic Al walls have been developed and applied to the plasma chemistry of dry etching of silicon dioxide. Dissociation paths and threshold energies of gases are determined by using an ab initio density functional molecular orbital method, and dissociation cross sections are approximated. The electron energy distribution function is determined by using a particle-in-cell model with the Monte Carlo collision method, and dissociation reaction rates are determined. Plasma densities, electron temperatures. and radical densities are calculated by a kinetic model which consists of the fluid equations for plasmas and rate equations for radicals. The model effectiveness was confirmed by results comparison for the Ar discharge in an radio-frequency device. The chemical compositions of the dry etching plasmas have been investigated for C 4F 8. Calculated electron temperatures and densities agree with experimental results within factors of three. Correlations could be found between the composition of radicals in the plasma and the etch selectivities in C 4F 8. Adsorption potentials of fluorocarbon radicals on Al (III) surface clusters have been calculated by using molecular orbital method, and sticking coefficients are estimated. Sticking coefficient of fluorine atom is the largest and decrease in order of F, C, CF, H, CF 2, and CF 3. Effects of hypervalence bonding at Al surface are discussed. Phenomena [1]of no film depositions in CF 4 RF plasma and film depositions in RF plasma of CF 4 mixed with H 2 were explained by hypervalence reactions of Al.

The Thermal Dissociation of Phenol-Blocked Toluene Diisocyanate Crosslinkers

Abstract The thermal dissociation reaction of different phenol-blocked toluene diisocyanate crosslinkers has been studied by the use of differential scanning calorimetry. It was found that dissociation occurs after melting of the adduct. The dissociation reaction rate constants, activation energies, and the heat of reaction values are reported. Adducts with orthosubstituted phenols dissociate at a faster rate than those with para isomers. The regeneration of isocyanate functionality was identified by an infrared spectrophotometer.

A detergent-solubilized nicotinic acetylcholine receptor of caenorhabditis elegans

We have used a spin column assay to study the detergent-solubilized levamisole receptor, a nicotinic acetylcholine receptor of the nematode Caenorhabditis elegans. The receptor can be successfully solubilized in detergent solutions of Triton X-100, Lubrol PX, or sodium cholate. Centrifugal gel fitration assay using the tritiated ligand [ 3H] metaaminolevamisole ([ 3H]MAL) provides a greater signal and a better signal-to-noise ratio for soluble levamisole receptor binding than either polyethylene glycol precipitation or DEAE filter assay with the same ligand. As for membrane-bound receptor, the detergent-solubilized levamisole receptor consists of more than one affinity state. Detergent solubilization appears to increase the affinity of all states for [ 3H]MAL (K d for the highest affinity solubilized [ 3H]MAL binding state, 41 ± 5 pM). Data is presented on the equilibrium binding and the association and dissociation reaction rates of the receptor. The similar relative efficacy with which various compounds inhibit specific [ 3H]MAL binding and deficiencies in solubilizable high affinity specific [ 3H]MAL binding in two receptor mutants show that the solubilized receptor is the same nicotinic acetylcholine receptor that is detected by assaying membrane-bound specific [ 3H]MAL binding. The detergent-solubilized levamisole receptor is stable at 0° to 4°C, making receptor purification feasible.

Kinetics and thermochemistry of ClCO formation from the Cl+CO association reaction

Laser flash photolysis of Cl2/CO/M mixtures (M=N2, CO, Ar, CO2) has been employed in conjunction with Cl(2PJ) detection by time-resolved resonance fluorescence spectroscopy to investigate equilibration kinetics in the reactions Cl(2PJ) +CO↔ClCO as a function of temperature (185–260 K) and pressure (14–200 Torr). The association and dissociation reactions are found to be in the low-pressure limit over the range of experimental conditions investigated. In N2 and/or CO buffer gases, the temperature dependences of the ClCO formation and dissociation reaction rate constants are described by the Arrhenius expressions k1=(1.05±0.36)×10−34 exp[(810±70)/T] cm6 molecule−2 s−1 and k−1=(4.1±3.1)×10−10 exp[(−2960±160)/T] cm3 molecule−1 s−1 (errors are 2σ). Second- and third-law analyses of the temperature dependence of the equilibrium constant (k1/k−1) lead to the following thermodynamic parameters for the association reaction: ΔH0298 =−7.7±0.6 kcal mol−1, ΔH00 =−6.9±0.7 kcal mol−1, ΔS0298 =−23.8±2.0 cal mole−1 K−1, ΔH0f,298 (ClCO)=5.2±0.6 kcal mol−1 (errors are 2σ). The results reported in this study significantly reduce the uncertainties in all reported kinetic and thermodynamic parameters.

Concentration polarization and water dissociation in ion-exchange membrane electrodialysis. Mechanism of water dissociation

A relationship between the current density i and the current efficiency η of H+ and OH– ions generated by dissociation of water above the limiting current density in ion-exchange membrane electrodialysis has been proposed. Generation rates of H+ and OH– ions in the dissociation of water, H2O [graphic omitted] H++ OH– are given as the difference between the forward and the reverse reaction rate. The transport of H+ and OH– ions generated is described according to the Nernst–Planck equation. The relationship between i and η is derived by using the conservation law for the generation and the transport of H+ and OH– ions. Comparing the calculated values with experimental ones, it is clear that the water dissociation reaction rate constant increases with increasing i. Such an increase is usually larger for an anion-exchange membrane than for a cation-exchange membrane. An especially remarkable increase is seen for a cation-exchange membrane placed in aqueous MgCl2, NiCl2 or CoCl2 solution. This is due to violent water dissociation generation by cascade (snow slide) splitting of water molecules in the water dissociation layer.

Mechanism of intercalation: ion effects on the equilibrium and kinetic constants for the interaction of propidium and ethidium with DNA.

AbstractThe effects of sodium ion concentration on the binding isotherms and association and dissociation reaction rates for the interaction of the closely related intercalating dication, propidium, and the monocation, ethidium, with DNA have been determined by spectrophotometric binding and stopped‐flow kinetics methods. The binding of propidium to DNA is best described by a neighbor‐exclusion binding isotherm (two base pairs per binding site) with negative ligand cooperatively on binding. The cooperativity parameter is fairly independent of salt concentration, while the log of the observed equilibrium binding constant varies with −log [Na+], with a slope of two for the propidium–DNA interaction. These effects of the sodium ion on the equilibrium binding of propidium with DNA are similar to those previously described for the dication, quinacrine [Wilson, W.D. &amp; Lopp, I.G. (1979) Biopolymers, 18, 3025–3041]. Ethidium behaves, as a function of salt, as a monocation binding to DNA with neighbor exclusion and without ligand cooperativity. Equations are derived for two limiting kinetics models for intercalation involving binding of the intercalator to a preequilibrium, open state of DNA (model I) or binding form a preequilibrium externally associated state of the intercalator with DNA (model II). Model II gives the best fit to all of the kinetic results if it is assumed that the initial external interaction of the intercalator with DNA is similar to the exchange of free and condensed simple counterions. Intercalation then occurs from this state following an opening transition of the base pairs of the double helix. This model predicts a larger effect of salt concentration on the association than on the dissociation reaction, and that is what is experimentally observed. The intercalation conformational change makes a significant contribution to the ionic effects for both the equilibrium binding and the kinetic constants. The dissociation results and the association reaction results under pseudo‐first‐order conditions could be fit with single exponential curves under the conditions of our experiments.

Thermodynamic Values and Rate of Exchange Constants of the Adducts Surrounding Extracted Metal Chelates

Abstract Overall dissociation reaction rates and activation parameters for the removal of adducts from several metal chelates in organic solvents has been determined by ultrasonic absorption. The ability of the metal chelates to hold onto adducts appears to be dependent upon the coordination sphere of the metal and the polarity of the ligand. The type of solvent used also has an effect.

Normal isotope effect on the rate of dissociation of carbazole in the excited state

The kinetics of the dissociation reaction of carbazole in the excited state has been investigated by measuring the fluorescence lifetime in the dilute alkaline H 2O and D 2O solutions. A normal isotope effect has been observed on the dissociation reaction rate, but the difference of the activation energies is less than the difference of the zero-point vibrational energies between NH and ND bonds.

A pressure- and flow-insensitive reference electrode liquid junction.

The design and construction of a pressure- and flow-insensitive reference liquid junction for use in ion concentration electrode measuring systems is described. The junction is inexpensive, is very easily and rapidly constructed, is rugged, and is adaptable to various applications. When used in a pH-measuring system, drift, pressure artifacts, and flow artifacts are negligible. The response time of the system appears to be less than 10 ms. Using the pH electrode device as described, the dissociation reaction rate constant of H2CO3 at 24 degrees C was determined to be 22 s-1.

Inhibition of the soluble adenosine triphosphatase from mitochondria by adenylyl imidodiphosphate

1. Adenylyl imidodiphosphate is an inhibitor with high affinity for the soluble ATPase (adenosine triphosphatase) from mitochondria. 2. The reaction of the inhibitor with the ATPase is slow and estimates for the association and dissociation reaction rate constants are given. 3. The number of binding sites for the inhibitor appears to be doubled in the presence of 2,4-dinitrophenol. 4. Adenylyl imidodiphosphate is less effective as an inhibitor of the ATPase activity of this enzyme than of the inosine triphosphatase activity. It is also less effective on the ATPase of frozen-thawed or intact mitochondria and did not inhibit ADP-stimulated respiration by intact mitochondria.