Incorporation of Non-Steady-State Unimolecular and Chemically Activated Kinetics into Complex Kinetic Schemes. 1. Isothermal Kinetics at Constant Pressure

Dissociation-Induced Depletion of High-Energy Reactant Molecules as a Mechanism for Pressure-Dependent Rate Constants for Bimolecular Reactions

In 1922, Lindemann proposed the now-well-known mechanism for pressure-dependent rate constants for unimolecular reactions: reactant molecules with sufficiently high energies dissociate more quickly than collisions can reestablish the Boltzmann distribution...

Atmospheric Reaction of Cl with 4-Hydroxy-2-pentanone (4H2P): A Theoretical Study

The kinetics and the mechanism of the reaction of 4-hydroxy-2-pentanone (4H2P) with Cl atom were investigated using quantum theoretical calculations. Density functional theory, CBS-QB3, and G3B3 methods are used to explore the reaction pathways. Rice-Ramsperger-Kassel-Marcus theory is employed to obtain rate constants of the reaction at atmospheric pressure and the temperature range 278-400 K. This study provides the first theoretical and kinetic determination of Cl rate constant for reactions with 4H2P over a large temperature range. The obtained rate constant 1.47 × 10^-10 cm³ molecule^-1 s^-1 at 298 K is in reasonable agreement with those obtained for C4-C5 hydroxyketones both theoretically and experimentally. The results regarding the structure-reactivity relationship and the atmospheric implications are discussed.

Experimental and theoretical investigation of rate coefficients of the reaction S(P3)+OCS in the temperature range of 298–985K

The reaction S(3P)+OCS in Ar was investigated over the pressure range of 50-710 Torr and the temperature range of 298-985 K with the laser photolysis technique. S atoms were generated by photolysis of OCS with light at 248 nm from a KrF excimer laser; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO2. At pressures less than 250 Torr, our measurements give k(298 K)=(2.7+/-0.5)x10(-15) cm3 molecule-1 s-1, in satisfactory agreement with a previous report by Klemm and Davis [J. Phys. Chem. 78, 1137 (1974)]. New data determined for 407-985 K connect rate coefficients reported previously for T>or=860 and T<or=478 K and show a non-Arrhenius behavior. Combining our results with data reported at high temperatures, we derived an expression k(T)=(6.1+/-0.3)x10(-18) T1.97+/-0.24 exp[-(1560+/-170)/T] cm3 molecule-1 s-1 for 298<or=TK<or=1680. At 298 K and P>or=500 Torr, the reaction rate was enhanced. Theoretical calculations at the G2M(CC2) level, using geometries optimized with the B3LYP6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) calculations agree satisfactorily with experimental observations. According to our calculations, the singlet channel involving formation of SSCO followed by direct dissociation into S2(a 1Deltag)+CO dominates below 2000 K; SSCO is formed via intersystem crossing from the triplet surface. At low temperature and under high pressure the stabilization of OCS2, formed via isomerization of SSCO, becomes important; its formation and further reaction with S atoms partially account for the observed increase in the rate coefficient under such conditions.

Kinetics of the unimolecular decomposition of the 2-chloroallyl radical

The thermal decomposition of the 2-chloroallyl radical, CH(2)CClCH(2) --> CH(2)CCH(2) + Cl (1), was studied using the laser photolysis/photoionization mass spectrometry technique. Rate constants were determined in time-resolved experiments as a function of temperature (720-840 K) and bath gas density ([He] = (3-12) x 10(16), [N(2)] = 6 x 10(16) molecule cm(-3)). C(3)H(4) was observed as a primary product of reaction 1. The rate constants of reaction 1 are in the falloff, close to the low-pressure limit, under the conditions of the experiments. The potential energy surface (PES) of reaction 1 was studied using a variety of quantum chemical methods. The results of the study indicate that the minimum energy path of the CH(2)CClCH(2) dissociation proceeds through a PES plateau corresponding to a weakly bound Cl-C(3)H(4) complex; a PES saddle point exists between the equilibrium CH(2)CClCH(2) structure and the Cl-C(3)H(4) complex. The results of quantum chemical calculations, the rate constant values obtained in the experimental study, and literature data on the reverse reaction of addition of Cl to allene were used to create a model of reactions 1 and -1. The experimental dependences of the rate constants on temperature and pressure were reproduced in RRKM/master equation calculations. The reaction model provides expressions for the temperature dependences of the high-pressure-limit and the low-pressure-limit rate constants and the falloff broadening factors (at T = 300-1600 K): k(infinity)(1) = 1.45 x 10(20)T(-1.75) exp(-19609 K/T) s(-1), k(infinity)(-)(1) = 8.94 x 10(-10)T(-0.40) exp(481 K/T) cm(3) molecule(-1) s(-1), k(1)(0)(He) = 5.01 x 10(-32)T(-12.02) exp(-22788 K/T) cm(3) molecule(-1) s(-1), k(1)(0)(N(2)) = 2.50 x 10(-32)T(-11.92) exp(-22756 K/T) cm(3) molecule(-1) s(-1), F(cent)(He) = 0.46 exp(-T/1001 K) + 0.54 exp(-T/996 K) + exp(-4008 K/T), and F(cent)(N(2)) = 0.37 exp(-T/2017 K) + 0.63 exp(-T/142 K) + exp(-4812 K/T). The experimental data are not sufficient to specify all the parameters of the model; consequently, some of the model parameters were obtained from quantum chemical calculations and from analogy with other reactions of radical decomposition. Thus, the parametrization is most reliable under conditions close to those used in the experiments.

Master equation models for chemical reactions of importance in combustion

The master equation provides a quantitative description of the interaction between collisional energy transfer and chemical reaction for dissociation, isomerization, and association processes. The approach is outlined for both irreversible and reversible dissociation, isomerization, and association reactions. There is increasing interest, especially in combustion, in association reactions that involve several linked potential wells, with the possibility of isomerization, collisional stabilization, and dissociation along several product channels. A major aim of the application of the master equation to such systems is the linking of the eigenvalues obtained by its solution to the rate coefficients for the phenomenological chemical reactions that describe the system and that are used in combustion models. The approach is illustrated by reference to the reactions C2H5 + O2, H + SO2, and the dissociation and isomerization of alkyl radicals.