Calculations of the rate constants of elementary reactions

Russian Chemical Bulletin -     

Determination of the elementary rate constants of autoacceleration reactions on polymers

Equations describing the course of an autoacceleration reaction on a polymer in the low conversion region were derived and used to obtain relationships between the curvature of the conversion curve at the beginning and the elementary rate constants k₁ and k′₂. Calculations carried out by using our relationships are compared with the procedure suggested by Kawabe and Yanagita, and the applicability of the individual methods for the determination of the elementary rate constants is discussed.     

The rate constants of the elementary reactions of NH3 with O and OH

Two sets of discordant rate constants reported recently for the high-temperature reactions of NH₃ with O and OH were examined by comparing the calculated concentration profiles of NH₃ and OH with experiments by the shock tube technique forNH₃-N₂O-Ar mixtures. The rate constants were confirmed tobe: k₄ = 3.1 × 10¹²exp(−25.5 kJ/RT) cm³ mol⁻¹ s⁻¹ for NH₃ + O → NH₂ + OH and k₅ = 3.2×10¹² exp(−8.4 kJ/RT) for NH₃ + OH → NH₂ + H₂O.     

New method of determining rate constants of elementary reactions of atoms and radicals

Summary A new method is proposed for the determination of absolute values of the rate constants of elementary reactions of atoms and radicals with molecules in the gas phase.In conclusion the authors express their thanks to N. N. Semenov and V. N. Kondrat'ev for valuable advice.     

Determination of rate constants of elementary reactions occurring during compound termination of polymer chains

We analyze the errors in determining the rate constants of elementary reactions that arise when quadratic rather than compound (simultaneous bi- and monomolecular) chain termination is assumed. We show that the value of the chain propagation constant calculated from nonsteady-state kinetic data is independent of the method of calculation and is determined completely by the instrumental experimental error and the accuracy in determining the initiation rate.     

Calculation of the rate constants of diffusion-controlled chemical reactions for reagents of arbitrary shape

A method of describing small-size reactant diffusion in the presence of any amount of arbitrary located sinks is developed. The propagation function of mobile reagent (MR) in such a system is found. The developed method was used to calculate rate constants of bimolecular reactions of MRs with absorbents (traps) having arbitrary shape. The procedure of calculation of the rate constant has been reduced to integral equation for flux density towards a trap which is MRs' absorber. If small parameters exist, the expansion in powers of these parameters is possible. The bimolecular rates were calculated for traps of different shape. The equation was obtained which permits to determine the asymptotic time dependence of rate constants.     

Determination of the elementary reaction rate constants on the basis of the summary values of the rate constants

Conclusions The rate constants of reactions of atomic oxygen with methane, ethane, ethylene, and propylene along different pathways were determined on the basis of the ratios between the concentrations of the products formed and the summary rate constants measured earlier.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2693–2699, December, 1971.     

Calculation of the rate constants of heterogeneous electron-transfer reactions with the participation of metal complexes

Conclusions Numerical estimates of the electronic transition-matrix elements for a number of heterogeneous electron-transfer reactions have been obtained. The main kinetic parameters of homo-geneous and heterogeneous redox systems formed by complexes of transition metals with water and ammonia as ligands have been compared.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1016–1020, May, 1986.     

Evaluation of the rate constants in chemical reactions

This article is concerned with the application of a new method to recover the rate constants in chemical reactions. The method is based on treating the unknown parameters as time dependent. With appropriate experimental data the unknown rate constants are guided from an arbitrary initial condition to their true value at a final time. An explicit equation describing the time evolution of the parameters is obtained by minimizing the error along the trajectory. The method leads to an iterative algorithm which is described in detail. Numerical results with the method indicate that accurate estimates of the rate constants can be obtained directly from experimental data. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 151–159, 1998.     

Temperature dependence of the rate constants of cryochemical reactions

The temperature dependence of the reaction rate constant for tunneling transfer of an atomic particle in solid near absolute zero was studied. Different mechanisms describing the temperature dependence were considered: reorganization of the medium, modulation of parameters of the potential barrier, and under-barrier friction. It was established that for the rate constant (K) at low temperatures the equation InK=InK ₀+C ₄ T ⁴+C ₅ T ⁵+C ₆ T ⁶+C ₈ T ⁸ is valid. Experimental data were compared with the theory. A good agreement is achieved when the quantum nature of the hydrogen crystal is applied under the assumption of a predominant role of reorganization of the medium. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1906–1914, October, 1999.     

Use of stable radicals for the determination of rate constants of elementary processes

A direct method is proposed for determining the concentration of active centers by introducing the stable radical 2, 2, 6, 6-tetramethyl-4-hydroxypiperidine-1-oxyl at the commencement, and during the course of, the polymerization process of tetrafluoroethylene. On the basis of the data obtained, the rate constants of the elementary processes are calculated.