Showing papers on "Reaction rate published in 1970"

The hydrated electron

Abstract: In the article " The hydrated electron" by E. J. Hart (2 Oct., p. 19), reaction 11 (column 1, page 20) was omitted. The ratio for the rates of reactions 9 and 10 was printed twice: correctly in place of reaction 11 and incorrectly in its proper place. The correct version follows: The ratio for the rates of reactions 9 and 10, ke- aq +H 2 O 2 /KH+H 2 O 2 =500, attests to the high reactivity of e- aq compared to H atoms in this reaction. Among other reactions showing this difference in reactivity is the effect of metal ions such as Fe 3+ and Cu ++ on the hydrogen yield in irradiated solutions of methanol (3). Hydrogen formed by reaction H+CH 3 OH→H 2 +CH 2 OH (11) is decreased by the addition of Fe 3+ or Cu ++ because the reaction e - aq+Fe 3+ →Fe ++ (12) interferes with H-atom-producing reaction 4.

Low-temperature oxidation

Abstract: Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces “dangling” bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands.

Calorimetric investigation of polymerization reactions. III. Curing reaction of epoxides with amines

Abstract: The curing reactions of epoxy resin with aliphatic diamines and the reaction of phenyl glycidyl ether with butylamine as a model for the curing reactions were investigated with a differential scanning calorimeter (DSC) operated isothermally. The heat of reaction of phenyl glycidyl ether with butylamine is equal to 24.5 ± 0.6 kcal/mole. The rate of reaction was followed over the whole range of conversion for both model and curing reactions. The reactions are accelerated by the hydrogen-bond donor produced in the system. The rate constants based on the third-order kinetics were determined and discussed for the model reaction and for the chemically controlled region of curing reactions. The activation energies for these rate constants are 13-14 kcal/mole. At a later stage of conversion, the curing reactions become controlled by diffusion of functional groups. The final extent of conversion is short of completion for most isothermally cured and even for postcured samples because of crosslinking. It was quantitatively indicated that the final conversion of isothermal cure corresponds to the transition of the system from a viscous liquid to a glass on the basis of the theory of glass transition temperature of crosslinked polymer systems.

Composition containing gel

Abstract: A new plastic composition which is slow dissolving in aqueous media and can be used to slowly release medication or other additive ingredients in an aqueous environment, which plastic composition is prepared by cross-linking a hydrophilic colloid such as carboxymethyl cellulose gum or alginate gum with a crosslinking agent, usually a polyol such as propylene glycol, in a relatively nonreactive water soluble carrier such as glycerol. The total composition, including the carrier, sets up into a plastic gel, the consistency of which can be varied by varying proportions of ingredients. The reaction can be speeded or driven to a more completely cross-linked state by using a catalysts such as aluminum or calcium salts. Complexing agents can be included for complexing the catalysts and releasing it slowly throughout the reaction to slow the reaction down where desired. Generally, the reaction proceeds faster in a close to neutral or basic medium at pH''s ranging up to 10 or 11 and slower in an acid medium at pH of 5 and below so the rate of reaction can be controlled by adjusting the pH. Usually the composition will contain medication or cosmetic additives or ingredients and it can be molded by extrusion or the like into any predetermined form, e.g., in the form of a ring. In a preferred form, the composition has excellent pressure sensitive adhesive properties and these properties are very pronounced where the hydrophilic colloid used is carboxymethyl cellulose.

Centrifugal effects in reaction rate theory

Abstract: Early theories of unimolecular reaction rates have considered the role of overall molecular rotations by using two ostensibly different approaches ; the results of both approaches are identical, however. Eyring1n2& postulated that all degrees of freedom of the reactants are in thermodynamic equilibrium with those of the activated complex (except that special considerations attach to that degree of freedom associated with the reaction coordinate). In this simple formulation, and without specific considerat of the conservation of angular momentum, rotationq contribute a factor to the high-pressure specific rate constant, kmuni, for a unimolecular reaction which is the ratio of the square root of the moments of inertia of the rotational partition functions of the activated complex and the molecule, i.e., (ZA+ZB+ZO+/ZAZBZC)’”. Another approach that was applied even earlier to a diatomic approximation for unimolecular decomposition reactions is that of Rice and Gershinowitz (RG).8 They pointed out that angular momentum conservation requires that the rotational quantum number does not change when the “activated complex” configuration at the top

The Reaction of Oxygen with Radiation-induced Free Radicals in DNA and Related Compounds

Abstract: SummaryThe transient absorption spectra produced on pulse radiolysis of aqueous solutions of DNA and related biochemicals, saturated with mixtures of nitrous oxide and oxygen, have been measured. Spectra observed immediately after the radiation pulse have been assigned to the respective adducts formed by the reaction of hydroxyl radicals. Except in the cases of 5-bromouracil, guanylic acid and DNA, the absorptions decayed rapidly and first order in oxygen. Absolute rate constants for the reaction of oxygen with the adducts of several pyrimidine bases have been determined and approach those of diffusion-controlled reactions. Analogous rates of reaction for the related nucleotides and nucleosides are approximately 50 per cent slower. Absolute rate constants for the reaction of oxygen with the hydroxy-adducts of tryptophan and histidine have also been determined.

Measurements of thermal-energy ion-neutral reaction rate coefficients for rare-gas ions

Abstract: A steady-state flowing afterglow method has been used to measure rate coefficients for ion-neutral reactions involving rare-gas ions. In particular the technique has been extended to reactions with neon ions. The rate coefficients are for collisions of He+ with N2, O2, CO2, CO, NO, H2, Ar, Kr, Xe, NH3 and CH4; Ar+ with NH3 and CH4 and Ne+ with O2. The method of analysis of the experimental data is given in some detail.

Kinetic studies of the reactions of oxide, hydroxide, alkoxide, phenyl, and benzylic anions with methyl chloride in the gas phase at 22.5.deg.

Abstract: As part of a program directed toward an understanding of the intrinsic nature of anion-molecule re- actions involving organic constituents, rate constants have been measured, reaction channels have been identified, and intrinsic reaction probabilities (the ratios of the experimentally determined rate constants to the theoretical collision rate constants) have been calculated for the reactions of oxide, hydroxide, alkoxide, phenyl, and benzylic anions with methyl chloride. All measurements were made in the gas phase at 22.5" under thermal equilibrium conditions using the flowing afterglow technique. The highly charge-localized anions were found to react rapidly with methyl chloride with rate constants larger than 8 x 10-10 cm3 molecule-1 sec-1 whereas the charge-delocalized anions reacted only slowly with rate constants less than 3 x 10-11 cm3molecule-1 sec-l. The formation of chloride ion was the dominant reaction channel observed in all cases. Calculated reaction probabilities were larger than 0.20 for the reactions involving highly charge-localized anions and less than 0.03 for the charge-delocalized anion re- actions. These results obtained in a rarified medium provide evidence for a correlation well known from solution studies, namely, a correlation between the reactivity of an anion and the nature of the distribution of its charge. Solvated alkoxide ions could be established as the dominant negative ions in a low-pressure helium afterglow. This allowed an investigation of the effect of weak solvation on the rate of reaction of alkoxide ions with methyl chloride. Upon the association of one molecule of the conjugate acid to the alkoxide ion the specific rate for reaction decreased by at least a factor of three. n our continuing studies of the intrinsic kinetics and I energetics of anion-molecule reactions involving organic constituents, we have turned our attention to the intrinsic behavior of nucleophilic substitution reactions of the type

Absolute Rates of the Reactions H+C2H4 and H+C2H5

Abstract: The hydrogen atom–ethylene system was studied at 298°K employing the methods of resonance fluorescence and absorption by hydrogen atoms of Lyman α radiation at 1216 A. The contribution of hydrogen atom–radical reactions was evaluated under varying experimental conditions, and the rate of disappearance of H atoms in ethylene was measured under conditions where stoichiometric corrections became significant. Measurements in the literature of reaction rates for H+C2H4 at low total pressure are now in good agreement; however the limiting high‐pressure absolute rate constants thus far reported differ depending on the assignment of stoichiometric factors. Our results indicate that stoichiometric factors obtained under low‐pressure conditions may not be applicable to high pressure. Furthermore, extrapolations based on plots of inverse rate constant vs inverse pressure may be in error due to significant curvature in such plots. Our high‐pressure limiting rate constant for H+C2H4, extrapolated from data at pressure...

Stereospecific reaction of isopropyl methylphosphonofluoridate (sarin) with α‐cyclodextrin: A model for enzyme inhibition

Abstract: The rates of alkaline hydrolysis of the enantiomers of isopropyl methylphosphonofluoridate (Sarin) have been determined without and with varying concentrations of α-cyclodextrin (cyclohexaamylose) in aqueous 0.1M/-KCl solution at pH 9.0 and 25°. The pseudo first-order rate constants are reported. The observed kinetics and product analysis are consistent with a reaction scheme in which complex formation precedes the nucleophilic reaction of α-cyclodextrin with Sarin. Dissociation constants (Kdiss) of the inclusion complexes and rate constants (k2) for the nucleophilic reaction are reported. The results indicate a stereospecific formation of the inclusion complexes, (S)-(+)-Sarin forming a more stable complex than (R)-(-)-Sarin, as well as a stereospecific rate of reaction, (R)-{-)-Sarin reacting faster than (S)-(+)-Sarin. The α-cyclodextrin-Sarin system represents the first model reaction for enzyme inhibition showing typical properties as complex formation, saturation effects and stereospecificity.

Cl+HBr Pulsed Chemical Laser: A Theoretical and Experimental Study

Abstract: The most powerful and efficient pulsed chemical lasers produced to date have resulted from the simple exothermic reactions between halogen atoms and hydrogen halides. These reactions have been initiated by flash photolysis. One such reaction is Cl+HBr→HCl+Br, − ΔH = 15.5 kcal/mole. A kinetic model has been developed for this laser system which calculates the intensity profile of the laser pulse as a function of time. The input data for the model include estimates of the reaction rate into the zeroth and first vibrational level of HCl and cross sections for collisional deactivation by the different chemical species in the laser medium. The rate of reaction into the second vibrational level of HCl is assumed to be zero. Any small finite rate into the second level would have a negligible effect on the calculated performance of the laser which operates only on the V(1 → 0) fundamental transition of HCl. The theoretical model also requires as input data the photolysis flash intensity profile, the cavity geomet...

Three-Body Recombination and Dissociation of Nitrogen: A Comparison between Theory and Experiment

Abstract: The modified phase‐space theory of reaction rates is used to calculate the over‐all recombination and dissociation rate coefficients of nitrogen in a heat bath of argon atoms. Substantial quantitative agreement is obtained between the theoretical predictions and the low‐temperature (90–611°K) “discharge‐flow‐tube” measurements of the recombination rate coefficient and the high‐temperature (8000–15 000°K) “shock‐tube” measurements of the dissociation rate coefficient. The success of the theory in correlating the experimental measurements over such a wide temperature range clearly illustrates the importance of the weak attractive forces between the nitrogen and argon atoms for recombination at low temperatures, the marked reduction in the rates at high temperatures due to nonequilibrium distributions in the vibrational state populations of the molecules, and the major contribution to the over‐all reaction rate coefficients due to reaction progress via the first electronically excited molecular state of nitrogen over the entire temperature range. The working relationships required for applying the modified phase‐space theory to predict the dissociation and recombination rate coefficients of other diatomic molecules in the presence of weakly attracting collision partners, such as argon atoms, are summarized.

Ion-molecule reaction rate studies in a flowing afterglow system

Abstract: A steady-state flowing afterglow has been utilized to measure the rate coefficients for the following reactions: He++ N2→ N+; N+2; He., He++ O2→ O+; O+2; He., N+2+ O2→ O+2; N2., N++ O2→ NO+; O+2; N. The data have been analyzed both with regard to the decay of the parent ion and the changing ratio of the product to parent ion with increasing reactant addition or reaction time. The experimental conditions have been varied over a limited range of pressures and flow velocities in order to minimize any systematic errors associated with a single method of data analysis. The resulting rate coefficients are consistent and agree well with previous estimates for these reactions.

On the photodecomposition of chlorophyll in vitro. I. Reaction rates.

Abstract: — Chlorophyll solutions are irreversibly bleached by light in the presence of oxygen. The action spectra parallel the absorption spectra for both chlorophyll a and b. The reaction is of second order with a Q10 of 1.26. The reaction rates for chlorophylls a and b are of the same order of magnitude. Depending upon the light source, the initial rate for chlorophyll a is slightly higher, by a factor of 1.15 to 1.30. The rate for pheophytin is lowe 3 by several orders of magnitude. No pheophytin has been detected in the reaction products of the irradiated chlorophyll solutions in the absence of water.

Flame structure and flame reaction kinetics - V. Investigation of reaction mechanism in a rich hydrogen+nitrogen+oxygen flame by solution of conservation equations

Abstract: A powerful combination of two computational methods has been used to investigate the reaction mechanism in a fuel-rich hydrogen+nitrogen+oxygen flame. The first of these involves the solution of the time-dependent heat conduction and diffusion equations by finite difference methods. It allows a preliminary assessment of reaction mechanisms and rate constants which must be used to reproduce the observed flame velocity. However, the transport fluxes are only represented approximately in this time-dependent model, so that a precise calculation of flame profiles cannot be made. The second computational method uses a Runge–Kutta procedure to calculate the steady-state flame profiles, and is an extension of the methods discussed by Dixon-Lewis (1968). It incorporates detailed transport property calculations, and thus allows computation of detailed flame profiles for comparison with experiment. Application of the methods to the rich hydrogen+nitrogen+oxygen flame and subsequent comparison with experiment has established the participation of hydroperoxyl in the flame mechanism, and has shown the principal reactions in the flame to be: OH + H 2 = H 2 O + H, (i) H + O 2 =OH + O, (ii) O + H 2 =OH + H, (iii) H + O 2 + M = HO 2 + M, (iv) H + HO 2 = OH + OH, (vii) H + HO 2 = H 2 + O 2 , (xii) H+ H + M = H 2 + M. (xv) It was found that the interplay between these reactions is such that it is impossible to use the atmospheric pressure flame for an independent, precise determination of the hydrogenoxygen chain branching-rate constant k 2 . Another property of the mechanism is that the hydrogen atom concentration profile in the flame is not very dependent on the precise rate constants employed, so that the profile itself can be computed probably to better than ±10%. The reaction zone of the very rich flame commences at about 550 K, the maximum overall reaction rate is at about 900 K, and the maximum hydrogen atom concentration is at 1030 to 1040 K. The rate constant ratio k 7 / k 12 is found to lie in the range 5±1, assumed independent of temperature over the reaction zone. Assuming equal efficiencies of all the molecules in the flame as third bodies in the hydrogen atom recombination, the rate constant k 15 is estimated to lie in the range 4.5±1.5 x 10 15 cm 6 mol -2 s -1 .

Gas phase reaction rates of some positive ions with water at 296 K

Abstract: Measurements of the rate constants for the reactions of He+, Ne+, Ar+, Kr+, O+, N+, and N2+ with H2O by the flowing afterglow technique at 296°K gave values of 0.56, 0.74, 1.43, 1.19, 2.33, 2.57, and 2.19 × 10−9 cm3/sec with standard deviation of 7%–15%. They are in good agreement with extrapolated beam results for Ar+, O+, N+, and N2+. Close agreement is also found for all but He+ and Ne+ with estimates based on classical trajectory calculations for similar ion plus dipole systems.

Reaction H+C2H4: Comparison of Three Experimental Techniques

Abstract: The reaction H+C2H4 has been studied as a function of He pressure at room temperature with three independent experimental techniques, and rate constants have been obtained in both the excess ethylene and excess hydrogen‐atom environments. The products of the reaction are CH3, CH4, and C2H6. An experimental value for the third‐body recombination coefficient for H+CH3+M has been obtained. The over‐all stoichiometry of the reaction has been experimentally determined and is found to vary as a function of the initial reactant concentrations. A reaction mechanism is proposed which accounts for all of the experimental data obtained.

The chemistry of flavins and flavoproteins. Aerobic photochemistry

Abstract: 1. When a mixture of FMN and a reducing substrate (e.g. unprotonated amine) is illuminated oxygen is consumed. 2. The rate of oxygen uptake increases as oxygen concentration falls with some substrates (type I reaction), but with other substrates (typically aromatic compounds) the rate falls as the oxygen concentration falls (type II reaction). 3. The kinetics of type I reactions with EDTA, dl-alpha-phenylglycine and diethanolamine are all consistent with a mechanism in which the rate-determining step, hydrogen abstraction by the FMN triplet, is followed by rapid reoxidation of reduced FMN by oxygen. The reaction is faster at low oxygen concentrations because oxygen quenches the triplet. 4. The sensitivity of reaction rates to substituents in dl-alpha-phenylglycine can be described by a Hammett rho value of -0.6. 5. Individual rate constants for quenching and reaction of the FMN triplet with substrate were calculated (2.4x10(8) and 2.1x10(7)m(-1)s(-1) respectively for EDTA) on the assumption that oxygen quenches the triplet in a diffusion-controlled reaction. 6. The pH-dependences of oxygen uptake rates with six natural amino acids as substrates were measured. 7. Photoinactivations of l-glutamate dehydrogenase and d-amino acid oxidase by FMN were demonstrated.

Pulse radiolysis of aqueous solutions. Rate of reaction of OH with OH

Abstract: The results of competition studies of the reactions of hydroxyl radicals with carbonate ion and ethanol or methanol at high concentration in nitrous-oxide- and oxygen-saturated solution at pH 11 are consistent with O– being a product of the reaction of e–aq with N2O, and with the equilibrium (1), OH+OH–⇌ O–+H2O, (1) not being established. On this basis, rate constants for the forward and reverse reactions of this equilibrium have been determined. The values obtained are k1=(1.2±0.3)× 1010 M–1 s–1 and k–1=(9.2±2.0)× 107 s–1.