Absolute Reactivity Research Articles

The Influence of Optimum Power Level Change on Boration and Dilution Operations

Xenon and samarium concentrations changes occur in a nuclear reactor as a consequence of power level variations. To compensate for the reactivity introduced by these isotopes, the boron concentration in the reactor coolant also needs to be changed. Boron concentration changes result from boration or dilution operations. Both boration and dilution operations have economic effects due to the cost of boric acid and the treatment of the effluent.A method is presented that finds the nuclear power level change that leads to an optimization of the boration and dilution operations. The use of the method for practical applications is demonstrated by comparing the absolute reactivity change for the optimum power ramp and a ramp of ±3%/h.The numerical calculations are very fast. Thus, the method may be implemented in the process computer of any nuclear power plant.

Gas Phase and Gas Surface Kinetics of Transient Silicoin Hydride Species

AbstractThis paper presents a summary of the current state of our understanding of the absolute reactivity of transient silicon hydride species, such as SiH, SiH2and SiH3in the gas phase and at the surface of thin films.

Short-term reproducibility of a mental arithmetic stress test.

1. To evaluate the short-term reproducibility of heart rate, oscillometrically determined blood pressure, antecubital venous plasma catecholamine concentrations and subjective responses to strictly standardized mental arithmetic, we performed two identical tests 1 h apart in 14 young, healthy and normotensive male subjects (age 22-35 years). 2. Heart rate and blood pressure responses to the two stress tests were highly correlated, when expressed both as correlations between levels attained during stress (rs greater than 0.80 throughout) and as absolute reactivity measures (all rs greater than 0.75). Also, subjective stress responses were highly correlated, when considering both levels during stress and reactivity (r = 0.97 and r = 0.85, respectively). Stress levels of catecholamines were correlated, but the change scores (reactivity) were unrelated. 3. The measurement error SD for heart rate was 2.6 and 3.0 beats/min for reactivity and stress levels, respectively. The corresponding SD for blood pressure ranged between 2.7 and 4.4 mmHg. Subjective stress experience showed an SD of a similar magnitude. The responses of plasma catecholamine concentrations were subject to considerable variability. 4. It is concluded that haemodynamic and subjective stress responses and stress levels during the mental arithmetic stress test show acceptable reproducibility and high test-retest correlations. However, stress-induced changes in venous plasma catecholamine concentrations show low reproducibility.

Whether to measure change from baseline or absolute level in studies of children's cardiovascular reactivity: A two-year follow-up

Blood pressure and heart rate were measured at rest and during a television video game in 399 healthy children. At the first assessment, children were in third grade and at the 2-year follow-up, they were in fifth grade. Children were classified according to race (black/white), gender (boy/girl), body mass index (light/heavy), and video game performance (low score/high score). Correlational analysis indicated that irrespective of the classification variable, absolute reactivity values, e.g., a heart rate of 110 bpm, tended to be more stable than change reactivity values, e.g., a heart rate change of 10 bpm. The greater stability of absolute values was most pronounced with systolic blood pressure. Absolute values were at least as stable as resting values and frequently demonstrated greater stability. Mean and maximal reactivities were correlated comparably. We conclude that children's reactivity is a stable characteristic and that, unless indicated by a priori hypotheses, the use of absolute values is preferable to the use of change values.

Patterns of behavior in free cationic copolymerization

The free cationic copolymerization behavior of styrene derivatives in methylene dichloride solution was examined. The copolymerization behavior of all monomer pairs was found to be strongly ideal. The relationships between reactivity ratios of the monomer pairs can be accurately correlated by a number of simple models. Hammett plots give very similar values for reaction constants for the different cations, and the copolymerization behavior can also be correlated by a generalized Hammett relation using an overall reaction constant. This suggests that under the particular reaction conditions of this work, the cations do not differ greatly in their absolute reactivity. This idea is briefly discussed in light of other work that has been done concerning the nature of the propagating cations in the polymerization of styrene derivatives.

On the absolute reactivity of aryl cations: Selectivity toward halide ions as a function of viscosity

The selectivities toward bromide and chloride ions observed in the dediazoniation of three arenediazonium salts are found to be independent of viscosity. It is inferred that the capture of aryl cations by these halide ions is diffusion controlled.

Comparative study of the absolute reactivity of vinyl monomers: Kinetics of polymerization of vinyl monomers initiated by Mn3+–thiodiglycolic acid redox system

AbstractThe kinetics and mechanism of polymerization of methacrylic acid (MAA) and ethyl acrylate (EA) initiated by the redox system, Mn3+–thiodiglycolic acid (TDGA) were investigated in the 15–35°C temperature range. The polymerization kinetics of both the monomers followed the same mechanism, viz., initiation by primary radical and termination by Mn3+–thiodiglycolic acid complex. The rate coefficients ki/k0 and kp/kt were related to the monomer reactivity and polymer radical reactivity, respectively. It was observed that both monomer reactivity and polymer radical reactivity followed the same order, viz., EA > MAA. The polymer radical reactivity varied inversely with the Q values of the monomers.

Alloying and entropy effects in predicting metal/compound–semiconductor interface reactivity

A previous bulk thermodynamic model, which used enthalpies of compound and alloy formation to predict metal/compound–semiconductor interface reactivity, is extended to include entropy. It is shown that, for most metals on CdTe, GaAs, GaSe, InP, and MoS2, solid-state reactions are energetically favored up to semiconductor dissociation temperatures and, consequently, entropy effects are minimal. Gold and silver, with their small enthalpies of metal–semiconductor anion compound formation, can be exceptions. Even here, the results for gold/III–V systems favor solid-state reaction at room temperature, but at higher temperatures entropy drives the reaction via vapor-phase production of the group V element. The binary phase bulk thermodynamic model is not sufficient to predict absolute reactivity, but can rank the reactivity of the various metal–semiconductor combinations successfully, as long as possible alloy formation is included. It is suggested that the limitations of the model are due to the specific effects of the interface.

Singlet-triplet interconversion of diphenylmethylene. Energetics, dynamics and reactivities of different spin states

A combination of picosecond and nanosecond laser spectroscopy measurements, chemical quenching experiments and triplet sensitization experiments has allowed the determination of the rapid singlet to triplet and slower triplet to singlet intersystem crossing rates for diphenylmethylene in fluid solution at room temperature. It is shown that under the conditions of the kinetic measurements, singlet and triplet diphenylmethylene ( 1DPM and 3DPM, respectively) are in rapid equilibrium relative to reactions, so that knowledge of the values of k ST and k TS allows determination of the equilibrium constant and change in free energy for the 1DPM 〈 3DPM process. The absolute reactivity of 1DPM toward a series of alcohols has been determined and is discussed in terms of other current investigations of carbene reactivity.

On the mechanism of the potassium carbonate catalysed gasification of activated carbon: the influence of the catalyst concentration on the reactivity and selectivity at low steam pressures

In this study a detailed description is given of reactivities, selectivities and kinetics in the potassium carbonate catalysed gasification of highly pure activated carbon with low pressures of steam. It is shown that the absolute reactivity increases at increasing initial catalyst loading and at increasing burn off, the latter leading to a maximum value between 40 and 80% burn off, dependent on the initial loading. The reactivity per unit weight of carbon reacting increases linearly with the catalyst loading up to 40 weight %, either initially or created by partial gasification. This implies that the catalyst is mobile under reaction conditions. Activation of the catalyst in the region up to 50% burn off is probably due to breaking up of K-O-C bonds. The lack of activity at low catalyst concentrations is explained by intercalate formation, which makes the catalyst inaccessible for the gaseous reactant. The results suggest that both CO and CO 2 are primary reaction products from different reaction pathways, in which potassium catalysis is basically controlled by the catalyst-carbon interaction. The reaction can be described by a redox mechanism in which the reduction step becomes more important at higher catalyst loadings. It is finally shown, that the assumption of water gas shift equilibrium is doubtful during the potassium catalysed low pressure steam gasification of activated carbon. A Langmuir type rate equation appears to be operative with an adsorption constant for water increasing with the catalyst loading.

A comparison of the absolute reactivity of vinyl monomers. I. Kinetic studies on radical polymerization of vinyl monomers initiated by a Mn3+–diglycolic acid redox system

AbstractThe kinetics of polymerization of acrylamide (AM), acrylic acid (AA), and acrylonitrile (AN) initiated by the redox system Mn3+–diglycolic acid (DGA) was studied. All three systems followed the same mechanism; namely, initiation by an organic free radical arising from the oxidation of diglycolic acid and termination by the interaction of polymer radicals with Mn3+ ion. The rate coefficients ki/k0 and kp/kt were related to monomer and polymer radical reactivity, respectively. An inverse relation between monomer and polymer radical reactivity was observed. Monomers with higher Q values gave higher ki/k0 values but lower kp/kt values. The e values of the monomers were important in determining the reactivities of monomers with nearly the same Q values.

Regulation of immune reactivity to collagen in human beings.

Denaturated beef collagen was tested for its ability to induce the production of leukocyte inhibition factor among the peripheral blood mononuclear cells from patients with rheumatoid arthritis and normal individuals. Responsiveness, defined as the production of leukocyte inhibition factor sufficient to cause greater than 20% inhibition of leukocyte migration, was significantly (P less than 0.001, X2 = 31.1) associated with HLA-DR4. All HLA-DR4 positive individuals, including subjects without any evidence of synovitis, were collagen responders. There was no significant (P = 0.3) difference in the absolute reactivity of HLA-DR4+ versus HLA-DR4- individuals to respond to another antigen, Candida albicans. Collagen reactivity required interactions between macrophages and T cells and was directed against determinants inherent in the linear polypeptide, (Gly-Pro)n. In 5 normal HLA-DR4- nonresponders tested, absence of discernable reactivity to collagen was associated with the presence of antigen-specific, radiosensitive suppressive T cells. These studies suggest that during the physiologic metabolism of collagen all individuals are exposed to Gly-Pro determinants normally buried in the interstices of the collagen triple helix. In individuals whose major histocompatibility complex contains genes linked to those coding for HLA-DR4, this results in the activation of reactive T cells. Conversely, in individuals lacking these genes, collagen-specific suppressive cells predominate.

Reactivity of cyclic and acyclic olefinic hydrocarbons in acid-catalyzed hydration

The rates of hydration of ethylene, propene, 1-hexene, cyclohexene, 1-methylcyclohexene, 1-methylcyclopentene, and norbornene catalyzed by H/sub 2/SO/sub 4/ and D/sub 2/SO/sub 4/, and the hydration of cyclohexene catalyzed by HClO/sub 4/ and DClO/sub 4/, all at 25/sup 0/C, were measured. Ethylene is only a factor of 24 less reactive than p-nitrostyrene at H/sub 0/ = -7.37, but has a very steep slope of log k/sub obsd/ vs. H/sub 0/ of -1.54 so that ethylene rates extrapolated to low acidities appear quite small relative to other alkenes. All of the substrates are interpreted to react by rate-determining protonation on carbon to give the corresponding carbonium ions (the A/sub SE/2 mechanism). Solvent isotope effects k/sub H/sup +///k/sub D/sup +// range from 2.82 for propene to 1.06 for cyclohexene, and are only tenuously related to absolute reactivity. Norbornene is 770 times more reactive than cyclohexene, and at least part of this difference is attributed to the unsymmetrical ..pi.. bond in this molecule endowing it with enhanced nucleophilicity on the exo face.

A comparison of the absolute reactivity of monomers, 1. Kinetic study of radical polymerization of acrylamide and methacrylamide, initiated by manganese(III) acetate

AbstractThe kinetics of polymerization of acrylamide (AAM) and methacrylamide (MAM) with manganese(III) acetate as thermal initiator was studied. In the temperature range between 303 and 313 K, the initial rate of polymerization follows the relation: The rate of initiation of the polymerization was determined from the rate of decomposition of manganese(III) acetate. The reactivity of the two monomers under the same conditions was compared.

Absolute reactivity in the cationic polymerization of methyl and other alkyl vinyl ethers

The cationic polymerizations of methyl-, 2-chloroethyl-, ethyl-, cyclohexyl- and t-butyl- vinyl ethers initiated by cycloheptatrienyl hexachloroantimonate in methylene chloride solutions have been studied in detail. Reaction rates were measured by an adiabatic calorimetric technique and rate constants for propagation of each of the monomers, k p (obs), were determined by appropriate kinetic analysis of the experimental curves. The results obtained are discussed in terms of current theories regarding ion pair/free ion equilibria in non-aqueous solvents. Although ethyl-, cyclohexyl- and t-butyl- vinyl ethers behave very similarly to isobutyl vinyl ether, and their reactivities are comparable [ k p (obs) ∼ 3 × 10 3M −1sec −1 at 0°C] both methyl- and 2-chloroethyl- vinyl ethers show markedly different characteristics to the others, and in particular exhibit a reactivity approximately one order of magnitude less [ k p (obs) ∼ 2 × 10 2M −1sec −1 at 0°C]. These variations in reactivity are discussed in terms of preferred monomer conformations, and the resulting differences in activation energy which are likely to arise when such conformers are approached by an electrophile.

Absolute reactivity in the cationic polymerization of N-vinylcarbazole

The cationic polymerization of N-vinylcarbazole initiated by tropylium hexachloroantimonate and tropylium perchlorate has been studied in detail. Initiation was rapid and complete and conversion to polymer was quantitative in all cases. Reaction rates were measured by an adiabatic calorimetric technique and half lives were of the order of 1–2 s. Catalyst concentrations employed were sufficiently low (< 10 −5 M for essentially complete dissociation into free ions, as indicated by the ion-pair dissociation constants. The rate coefficient for propagation by free cations in dichloromethane at 0°C is estimated as ∼3 × 10 +5 M −1 s −1, approximately five orders of magnitude greater than that for the corresponding free radical polymerization. Molecular weights were higher when SbCl 6 − was the counter-ion than with ClO 4 − and it is suggested that monomer transfer reactions occur more readily with the small equilibrium concentrations of ion pairs (allowing an effect of counter-ion), whereas propagation occurs predominantly via free cations. Initiation is shown to occur by addition of tropylium ion to the monomer double bond and detailed mechanisms for this, together with propagation and transfer reactions, are considered.

Studies in solvolysis. Part II. Some comments on the ion-pair mechanism for displacements at a primary carbon atom

The implication of Sneen and co-workers (2–5) that all SN2 substitutions at a primary carbon atom probably proceed via the intermediate production of an intimate ion-pair is examined with respect to the reactions of the methyl halides (MeX; X = F, Cl, Br, I) with various nucleophiles (H2O, OH−, F−, Cl−, Br−, I−, CN−, CSN−, S2O32−) in water. By establishing certain rules concerning the behavior of derived reactivity scales (essentially ρ values) as contrasted with absolute reactivity scales (observed rate constants), it is concluded that Sneen and Larsen's mechanistic description is consistent with the experimental facts, and that in such cases the substitution process involves a pre-equilibrium constant, Ke, which is independent of the attacking nucleophile. This is followed by a rate determining bimolecular rate constant, kn, which depends on the ion pair and the nature of the nucleophile. The observed rate (k°) is given by k = Kekn. A method of calculating Ke is described and values of kn for nine nucleophiles attacking the four methyl halide ion-pairs are reported along with a number of confirmatory calculations. It is concluded that the classical Hughes–Ingold SN2 Heitler–London description of these reactions is inadequate. Some further suggestions to place the new mechanistic description on a firmer experimental basis are made.

Absolute reactivity of the oxide radical ion with methanol and ethanol in water

Absolute reactivity of the oxide radical ion with methanol and ethanol in water

Experimental determination of absolute reactivity and other quantities on the basis of statistical weights

Experimental determination of absolute reactivity and other quantities on the basis of statistical weights