Number Of Active Centers Research Articles

Designation of the number of active centers of acetylcholinesterase in motor endplates

32P or14C-labelled diisopropyl-fluoro-phosphate (DFP) was bound to endplates of mouse diaphragms by incubation. The number of molecules was determined by densitometric measurement of autoradiographs. One endplate contains 2.4·107 active centres for acetylcholine, blocked by DFP. The turnover time for acetylcholine is very short: 15µsec.

Deposition of various radioactive elements on the surface of a platinum anode—III: The number of active centres as a function of the anode voltage and the temperature

The change of the equilibrium distribution coefficient—which characterizes the anodic deposition of 144Ce—with increasing Ce 2(SO 4) 3 concentration was investigated. The Langmuir isotherm applies to the deposition data. The experimental data show that the number of active centres, which is a characteristic of the anodic deposition, depends on the anode voltage and, at constant voltage, on the temperature. The activation energy for a given voltage was determined from the temperature dependence of the number of active centres. The voltage dependence of the activation energy for deposition was studied by the evaluation of isotherms obtained for several voltages at different temperatures.

Deposition of various radioactive elements on the surface of the platinum anode

The anodic deposition of 204Tl, 144Ce, 90Y, 175Yb, 143Pr, 147Pm and 166Ho were investigated as a function of time, with special consideration of 204Tl and 144Ce. These studies show that when these atoms are anodically deposited, a plot of the equilibrium distribution coefficient as a function of the applied voltage passes through the maximum. The number of active centres on the surface of the platinum anode was determined for the anodic deposition of 204Tl and 144Ce by the isotherm method. A plot of the number of active centres as a function of the applied voltage also gave a curve with a maximum. The effect of hydrogen ion concentration was also investigated and a linear relation between the pH and the logarithm of the number of active centres found. Investigation of the deposition of 204Tl on platinum from a solution containing hydrogen peroxide shows that the chemisorbed oxide layer, which develops on the platinum anode, plays an important role in the anodic deposition.

Termination in anionic polymerization of octamethylcyclotetrasiloxane. formation of stable complexes on active sites

AbstractThe ionic polymerization of octamethylcyclotetrasiloxane (the tetramer) proceeds through successive additions of tetramer on active centers. The active centers consist of the ion‐pair magnified image (where M stands for Na, K, Rb, Cs, Me4N−, Bu4P−, etc.) formed through the reaction of the hydroxide (oxide) of the respective alkali metal (or the respective ammonium or phosphonium base) with the siloxane bond. Measurement of the rate of polymerization of tetramer indicated changes in the efficiency of magnified image active centers caused by additions of small amounts of substances capable of changing the character of donor‐acceptor reactions taking place during polymerization. These acids such as H2SO4 to very strong bases, such as hydroxides of alkaline metals. The substances were added in the order of their decreasing acidity ranging from very strong influence of substances being added, as judged by their influence on the overall rate of reaction, can be seen to fall into several classes. The most acidic substances (ranging from very strong mineral acids to weak organic acids, e.g., phenol) neutralize the active centers; weak acids (H2O, ROH, etc.) act (as judged by changes of the overall rate of reaction) only through physicochemical effects in the polymerizing system (for instance through changes in the quantitative composition of the solvate shell of ions and ion‐pairs and through changes of the dielectric constant). Very strong bases, as strong as, or stronger than, KOH increase the number of active centers. The most interesting effects are displayed on addition of relatively strong bases of the type of LiOH and NaOH. They cause a strong reduction of the efficiency of active centers, a deceleration of the polymerization and even its complete inhibition, in the whole range of dielectric constante of the polymerizing medium investigated by the authors (viz., in the range of dielectric constants from 3.4 to 8.7). This phenomenon of deceleration and even stopping of polymerization on addition of relatively strong bases to a basic active center has been called the “isobasic termination.” Similar effects were observed also in the case of the cationic polymerization of the tetramer, viz. a slowing‐down or even stopping of the H2SO4−induced polymerization on addition of weaker acids. The mechanism of “isobasic termination” is discussed.

The formation of carbon dendrites in the decomposition of alcohols on nickel

1. The carbon deposits formed in the decomposition of ethyl-, in-hexyl alcohol and cyclohexanol on nickel sheets at 450–820° have a dendritic structure. Stratified, nonbranched fiber-like dendrites and branched dendrites with conical terminations have been detected. 2. The carbonaceous substance of the dendrites has lattice parameters characteristic for graphite and contains from 1 to 6% metallic nickel crystallized in a hexagonal lattice. The formation of hexagonal, instead of cubic, nickel results from the two-dimensional geometric correspondence between the parameters of hexagonal nickel and graphite and from their crystallochemical interaction. 3. The terminations of screw dislocations allowing carbon dendrites to grow on them just in the form of such dislocations are the active centers for the deposition of carbon on nickel. At the same time these active centers are the suppliers of nickel migrating into the carbonaceous substance. 4. The enhanced catalytic activity, which for the catalyst getting covered with carbon is found during the first 2–3 hours of its use is connected with the increased number of active centers, as results from the joint formation of nickel- and carbon dendrites.

Chain-thermal flame propagation

1. Relations were obtained for determining the rate of flame propagation taking into account the following effects: linear chain branching and termination rates, second order chain termination rates, chain initiation rates, thermal conductivity and the diffusion coefficient of the active center. 2. From these relations it follows that second order chain termination has a greater effect on the flame propagation rate, the lower the combustion temperature and the smaller the ratio of the product of the heat capacity and density and the diffusion coefficient to the thermal conductivity. With a decrease in this ratio or with a fall in the active center concentration at the combustion temperature, the effect of linear chain branching and termination on the flame propagation rate increases as in these cases there is a relative fall in the number of active centers transferred by diffusion. 3. The relations obtained were applied to two cases of hydrazine decomposition flames and the calculations illustrate the effect of second order chain termination and linear chain branching on the flame propagation rate.

Über zusammenhange zwischen dem vernetzungsgrad und der degradation von naturkautschuk

AbstractOxidation reactions, as followed by infrared spectrometry, may serve as a measure for judging the relative extent of oxidative degradation and crosslinking of two independent parallel reactions subsequent upon the primary destruction. Such information can be derived from a kinetic analysis of the processes taking place in the rubber as a result of the absorption of energy, using the expression w = (k/b) (1 − e−bη) (1 − η). Since in this equation k is proportional to the rate of chain branching (which leads predominantly to oxidation), and b is proportional to the rate of recombination of the radicals (leading to crosslinking), the total degree of crosslinking will increase with increasing values of b/k. This assumption has been verified experimentally for the thermal and photochemical ageing of rubber, both in the absence of additives and in the presence of sulfur, of vulcanizing mixtures, and of copper as a catalyst of oxidation. At increased temperatures and pressures (130°C., 3 atm.) in the absence of vulcanizing additives and in a nitrogen atmosphere, radicals of long life are formed; this leads to an increased rate of the subsequent oxidative degradation. The number of active centers formed, and hence the final extent of oxidation, is a function of the time of exposure to the increased temperature.

The mechanism of the catalytic oxidation of sulphur dioxide with a commercial vanadium catalyst: A kinetic study

AbstractThe kinetics of the oxidation of sulphur dioxide with oxygen using a commercial vanadium pentoxide catalyst has been investigated. Experimental conditions have been chosen such that the reaction has been carried out isothermally in a flow‐type reactor at temperatures (360° to 450°) and conversions (up to 10%) where the reverse reaction may be ignored.The results, analysed in conjunction with the relevant adsorption isotherms and rates of adsorption on the clean catalyst, indicate that the rate‐determining step is the rate of reaction between gaseous oxygen and chemisorbed sulphur dioxide.The adsorption isotherms, which are Langmuir in character, indicate the presence of at least two different types of active centres. Both oxygen and sulphur dioxide are reversibly chemisorbed, oxygen being weakly bound with a heat of adsorption of 6·4 kcal. and sulphur dioxide strongly bound with a heat of 28·8 kcal. There is evidence that both adsorbed species possess considerable mobility on the surface.Measurements of the rates of adsorption of SO2 and O2 on the clean surface indicate that oxygen is adsorbed by a second‐order mechanism at a slower rate than the observed rate of reaction, whereas sulphur dioxide is adsorbed by a third‐order mechanism at a much faster rate than the rate of reaction. Adsorption equilibrium of SO2 may be assumed, under the conditions of reaction, without sensible error.Adsorption‐rate measurements of both SO2 and O2 indicate that the number of active centres (defects) increases with temperature, according to a relationship of the type d ln L/dT = A/RT2 where L = no. of active centres, as suggested by Volkenstein (1949).With SO2, reaction between it and an active centre requires no activation energy, the increase in rate with increase in temperature being due entirely to the increase in the number of active centres. With O2, a small activation energy may be required, although the main increase in rate of adsorption with temperature is similarly due to the increase in the number of active centres with temperature.Some interesting comparisons are made between the results of these adsorption measurements and recent work on the semi‐conductivity of oxide catalysts, notably by Bevan & Anderson (1950) and Garner, Gray & Stone (1950). Several points of similarity are noted which indicate lines for further work.Finally, a mechanism for the catalytic oxidation of sulphur dioxide is proposed, the main features of which are: magnified image.

Gold Sensitization of X-Ray Films*

Sensitization of non-screen type x-ray emulsions with aqueous solutions of ammonium or alkali aurous thiocyanates leads to a wave-length dependent increase of their x-ray and gamma-ray sensitivities. For a particular fine-grain emulsion the gain in photographic speed is shown to increase from a factor of 1.5 at 1.3A to a factor of 10 at 0.011A. This wave-length dependence of the sensitizing effect is governed by the range—energy relationship of electrons, by the shape of the energy loss curve of electrons, and the threshold sensitivity of the average grain. The sensitivity of the average grain in an unsensitized and a gold-sensitized emulsion was derived from the number of quanta required to produce the same photographic effects at different wave-lengths. Gold sensitization caused an increase in threshold sensitivity from (6 kev)−1 to (1.2 kev)−1 for the average grain of the emulsion under investigation. A comparison of the quantities of latent image silver formed by light exposure on one hand, and by electrons on the other, indicates that gold sensitization increases the effective radius of action of the sensitivity centers and also the number of active centers at the grain surface.

The Kinetics of Heterogeneous Atom and Radical Reactions. I. The Recombination of Hydrogen Atoms on Surfaces

The data on the kinetics of the recombination of hydrogen atoms on various types of surfaces are analyzed in the light of the theory of absolute reaction rates. Expressions are derived for the recombination coefficient and the first-order rate constant, enabling these quantities to be calculated on the basis of postulated mechanisms. It is shown that the experimental activation energies are consistent with estimates using either Hirschfelder's rule or an expression utilizing the experimental ``desorption temperature.'' For the reaction on dry oxides and dry glass good agreement is found assuming that reaction occurs between a gas-phase hydrogen atom and an adsorbed hydrogen atom, the adsorption being of the van der Waals type at low temperatures and chemisorption at higher ones. The mechanism of surface poisoning by water vapor is shown to be not due to the necessity for desorbing the water molecule: in the high temperature region it is due to an increase in activation energy, the mechanism being S–OH+H→S–O+H2; in the low temperature region to a decrease in the number of active centers, the reaction being between a gaseous atom and a van der Waals adsorbed hydrogen atom above the water layer.

Decay in Brightness of Infra-Red Sensitive Phosphors

An equation is derived for the decay in brightness of infra-red (IR) sensitive phosphors under constant IR stimulation. The derivation is based on a second-order process, but takes into account the absorption of the stimulating IR and the emitted visible light. The theoretical curves obtained differ markedly from the simple second-order decay curve, especially when the IR absorption is greater than the visible absorption. The equation involves three arbitrary constants which depend on the quantum efficiency, the number of active centers, and the absorption coefficients of the phosphor for visible and infra-red radiation. The equation is shown to fit the decay curve of a strontium selenide phosphor for an effective decrease in brightness of ${10}^{5}$. It is shown that theories involving a distribution of electron traps, with or without interaction, lead to conclusions that do not agree with the experimental results. Some experimental evidence is offered indicating that there is no change in the trap distribution as this phosphor is exhausted. The apparent deepening of the traps as the decay process continues can be attributed to the absorption of the infra-red.

On the Kinetics of Complex Reactions

Chemical kinetics is the chemistry of intermediate compounds. These intermediates can be studied directly only with great difficulty but they can be investigated through kinetic measurements themselves. For example during the induction period in the oxidation of hydrogen sulfide the ``active centers'' are withdrawn from the reaction vessel passed through a connecting vessel and introduced into a chamber in which the new reaction is just starting. The ``active centers,'' responsible for shortening the induction period, are stable for several hours at room temperature. The formula previously given for a chain reaction proceeding from a single active center is extended here to include reactions with any number of active centers. Complex reactions of various types are treated mathematically and experimental curves are given. Among the types of complex reactions considered are those which involve the mutual inhibiting or promoting action of chains with quadratic rupture.