Efficient quantum-classical method for computing thermal rate constant of recombination: application to ozone formation

Efficient method is proposed for computing thermal rate constant of recombination reaction that proceeds according to the energy transfer mechanism, when an energized molecule is formed from reactants first, and is stabilized later by collision with quencher. The mixed quantum-classical theory for the collisional energy transfer and the ro-vibrational energy flow [M. Ivanov and D. Babikov, J. Chem. Phys. 134, 144107 (2011)] is employed to treat the dynamics of molecule + quencher collision. Efficiency is achieved by sampling simultaneously (i) the thermal collision energy, (ii) the impact parameter, and (iii) the incident direction of quencher, as well as (iv) the rotational state of energized molecule. This approach is applied to calculate third-order rate constant of the recombination reaction that forms the (16)O(18)O(16)O isotopomer of ozone. Comparison of the predicted rate vs. experimental result is presented.     

Features of the temperature dependence for the rates of gas-phase reactions of combustion

The specific character of the dependence for the rate of most gas-phase reactions of combustion on temperature and reagent concentrations is found to be determined by the ratio of the rates of the multiplication and death of active intermediate particles. It is shown that the stepwise acceleration of the process and the transition to the combustion mode upon raising the initial temperature by 1–2 K and crossing over a critical value is due to a change in the character of the rate’s temperature dependence as a result of the sign changing from negative to positive to reflect the difference between the rates of chain branching and termination. It is concluded that this difference’s change in sign upon varying the content of reagents imposes limits on concentration.     

Velocity dependence of polarised rotational transfer rates: close-coupled calculations on model systems

Expressions for the laboratory-frame rate constants for polarised rotational energy transfer, as a function of molecular velocity, have been derived for atom-diatom systems. These equations require the kinetic energy dependence of the collision-frame cross sections, which have been obtained by close-coupled calculations using four different N₂-rare gas potentials over the temperature range 0 to 400 K. The energy dependence of multipolar cross sections is presented as are results for spherical and cylindrical velocity distributions. These model calculations indicate strongly that information on intermolecular potentials may be obtained from polarisation ratio velocity dependence measurements.     

Pressure and temperature dependence of the recombination of p-fluorobenzyl radicals

The rate constants of the recombination reaction of p-fluorobenzyl radicals, p-F-C6H4CH2 + p-F-C6H4CH2 (+M) --> C14H12F2 (+M), have been measured over the pressure range 0.2-800 bar and the temperature range 255-420 K. Helium, argon, and CO2 were employed as bath gases (M). At pressures below 0.9 bar in Ar and CO2, and 40 bar in He, the rate constant k1 showed no dependence on the pressure and the nature of the bath gas, clearly indicating that it had reached the limiting high-pressure value of the energy-transfer (ET) mechanism (k(1,infinity)ET). A value of k(1,infinity)ET(T) = (4.3 +/- 0.5) x 10(-11) (T/300 K)(-0.2) cm3 molecule(-1) s(-1) was determined. At pressures above about 5 bar, the k1 values in Ar and CO2 were found to gradually increase in a pressure range where according to energy-transfer mechanism, they should remain at the constant value k(1,infinity)ET. The enhancement of the recombination rate constant beyond the value k(1,infinity)ET increased in the order He < Ar < CO2, and it became more pronounced with decreasing temperature. The dependences of k1 on pressure, temperature, and the bath gas were similar to previous observations in the recombination of benzyl radicals. The effect of fluorine-substitution of the benzyl ring on k1 values is discussed. The present results confirm the significant role of radical complexes in the recombination kinetics of benzyl-type radicals in the gas-liquid transition range. The observations on a rate enhancement beyond the experimental value of k(1,infinity)ET at elevated densities up to the onset of diffusion-control are consistently explained by the kinetic contribution of a "radical-complex" mechanism which is solely based on standard van der Waals interaction between radicals and bath gases.     

The pressure and temperature dependence of the rate constant for methyl radical recombination over the temperature range 296–577 K

The rate constant for methyl radical recombination has been measured over the temperature range 296–577 K and at pressures between 5 and 500 Torr using laser flash photolysis, coupled with absorption spectroscopy at 216.36 nm. Analysis of the fall-off curves gives k_∞ = (2.78 ± 0.18) × 10^?11 exp(154 ± 22 K/T) cm³ molecule^?1 s^?1 and k₀ = (6.0 ± 3.3) × 10^?29 exp(1680 ± 300 K/T) cm⁶ molecule^?2 s^?1. The quoted errors (two standard deviations) do not include the present uncertainty in the absorption cross section, which is a major source of error (± 30%).     

The pressure and temperature dependence of methane decomposition

Single-channel hindered Gorin model RRKM calculations were performed on reaction (1). (1) Good agreement between theory and experiment was obtained for the temperature and pressure dependence of reaction (1). Isotopic data for the reverse association reaction, (?1), reported previously, are consistent with the model. Rate constants were cast in the form of an analytical expression and appropriate parameters were tabulated.     

An approximate theory of the ozone isotopic effects: rate constant ratios and pressure dependence

The isotopic effects in ozone recombination reactions at low pressures are studied using an approximate theory which yields simple analytic expressions for the individual rate constant ratios, observed under "unscrambled" conditions. It is shown that the rate constant ratio between the two competing channels XYZ-->X+YZ and XYZ-->XY+Z is mainly determined by the difference of the zero-point energies of diatomic molecules YZ and XY and by the efficiency of the deactivation of the newly formed excited ozone molecules, whereas the mass-independent fractionation depends on a "nonstatistical" symmetry factor eta and the collisional deactivation efficiency. Formulas for the pressure effects on the enrichment and on the rate constant ratios are obtained, and the calculated results are compared with experiments and more exact calculations. In all cases, ratios of isotope rates and the pressure dependence of enrichments, the agreement is good. While the initial focus was on isotope effects in the formation of O(3), predictions are made for isotope effects on ratios of rate constants in other reactions such as O+CO-->CO(2), O+NO-->NO(2), and O+SO-->SO(2).     

Experimental evidence to support viscosity dependence of rates of Diels-Alder reactions in solvent media

This note is aimed at ascertaining whether rates of Diels-Alder reactions depend on the viscosity of solvent media in which the reactions are performed. On the basis of the data collected from the literature and in this laboratory, it is seen in general that the rates increase in the solvents with their viscosities ranging up to approximately 1.2 cP. In solvents possessing viscosities above 1.2 cP, a drop in the reaction rates is observed in all cases. The effect of temperature on the above phenomena is also examined.     

Dipole-bound CH3CN- ions: temperature dependence of ion production rates and lifetimes

The formation of long-lived (tau less, similar10 mus) dipole-bound CH(3)CN(-) ions through electron transfer in K(14p)CH(3)CN collisions is investigated as a function of target temperature. The rate for their formation is observed to decrease steadily with increasing target temperature. The results are consistent with earlier suggestions that only target molecules in the ground vibrational state and low-lying rotational states can form long-lived dipole-bound anions. For CH(3)CN, the data indicate that creation of long-lived ions requires that the target molecules be in states with rotational quantum numbers j less, similar20. The measurements further demonstrate that the lifetime of the longest-lived (tau greater, similar50 mus) ions is limited by blackbody-radiation-induced photodetachment.     

The application of high pressure to some difficult Wittig reactions

Wittig reactions, previously shown to be favourably accelerated under pressure have been carried out between the stabilised ylides carboethoxymethylenetriphenylphosphorane and carboethoxy- ethylidinetriphenylphosphorane with a variety of ketones at 9–10 kbar pressure. Successful formation of tri- and tetra-substituted ethylenes is reported.     

A statistical/collisional approach to rates of bimolecular reactions at low pressure

An expression for the rate constant of gas‐phase bimolecular reactions at low pressure is derived in the framework of collisional theory. The key feature of the proposed model is the calculation of the energy‐dependent rate constant in terms of the collisional cross section and the probability of reaction, expressed as the ratio of the volume in phase space that leads to product over the total volume. The contribution of the internal energy of the reacting fragments is taken into account, as well as the relative translational energy. The resulting formulation is able to account for both negative and positive temperature dependences of the rate constants of neutral and charged species. The dependence of temperature of the bimolecular rate constant is given both for reactions with and without potential energy barriers. The performance of the proposed model is tested against experimental rate constant for three well‐studied reactions by fitting the parameters of the model to experimental data at various temperatures. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 233–242, 2005     

Solvent and external pressure effects on the ratio of the cyanoisopropyl radical recombination and disproportionation rates

A GC-MS analysis of the azobisisobutyronitrile thermal decomposition products of in solutions at 80°C showed that the ratio of recombination and disproportionation rates of the cyanoisopropyl radical does not depend on the medium viscosity, but increases when the internal pressure of the solvent increases according to the log(k _dispr/k _rec) = ?1.25 + 0.096 P _int ^0.5 law. This means that the activation volume corresponding to recombination is larger than that corresponding to disproportionation. It follows from the relationship log(k _dispr/k _rec) = (ΔV _rec ^≠ ? Δv _dispr ^≠ )ΔP/RT that, for the decomposition of the substrate in benzene under a pressure of 0.5–4.0 kbar, the difference between the activation volumes is ΔV _rec ^≠ ? ΔV _dispr ^≠ = 8 cm³/mol.     

Self-Reaction of HO2 and DO2: Negative temperature dependence and pressure effects

The negative temperature dependence, pressure dependence, and isotope effects of the self-reaction of HO₂ are modeled, using RRKM theory, by assuming that the reaction proceeds via a cyclic, hydrogen-bonded intermediate. The negative temperature dependence is due to a tight transition state, with a negative threshold energy relative to reactants, for decomposition of the intermediate to products. A symmetric structure for this transition state reproduces the observed isotope effect. The weak pressure dependence for DO₂ self-reaction is due to the approach to the high-pressure limit. Addition of a polar collision partner, such as ammonia or water vapor, enhances the rate by forming an adduct that reacts to produce deexcited intermediate. A detailed model is presented to fit the data for these effects. Large ammonia concentrations should make it possible to reach the high-pressure limit of the self-reaction of HO₂.     

Mechanism of nitrosylmyoglobin autoxidation: temperature and oxygen pressure effects on the two consecutive reactions

As shown by singular value decomposition and global analysis of the absorption spectra, oxidation of nitrosylmyoglobin, MbFe(II)NO, by oxygen occurs in two consecutive (pseudo) first-order reactions in aqueous air- saturated solutions at physiological conditions (pH 7.0, I=0.16 m (NaCl)). Both reaction steps have a large temperature dependence with the following activation parameters: DeltaS++(1) = 121+/-7 and DeltaS++(1) = 23+/-29; and DeltaS++(2) = 88+/-14 kJ mol(-1) and DeltaS++(2)-63+/-51 J(-1) K(-1) mol(-1) at 25 degrees C for the first and second step, respectively. At physiological temperature, the initial reaction is faster, while at lower temperatures, the first reaction is slower and rate-determining. The rate of the first reaction is linearly dependent on oxygen pressure at lower pressures, while for oxygen pressures above atmospheric, the rate exhibits saturation behaviour. The second reaction is independent of oxygen pressure. The rate of the second reaction increases when oxymyoglobin is added. In contrast, the rate of the first reaction is independent of the presence of oxymyoglobin. The observed kinetics are in agreement with a reaction mechanism in which the nitric oxide that is initially bound to the Fe(II) centre of myoglobin is displaced by oxygen in a reversible ligand-exchange reaction prior to an irreversible electron transfer. The ligand-exchange process is dissociative in nature and depends bond breaking, and nitric oxide is suggested to be trapped in a protein cavity. The absorption spectrum of the intermediate, as resolved from the global analysis, is in agreement with a peroxynitrite complex, and the initial process must involve partial electron transfer.     

Dissociative recombination of the weakly bound NO-dimer cation: cross sections and three-body dynamics

Dissociative recombination (DR) of the dimer ion (NO)(2) (+) has been studied at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm. The experiments were aimed at determining details on the strongly enhanced thermal rate coefficient for the dimer, interpreting the dissociation dynamics of the dimer ion, and studying the degree of similarity to the behavior in the monomer. The DR rate reveals that the very large efficiency of the dimer rate with respect to the monomer is limited to electron energies below 0.2 eV. The fragmentation products reveal that the breakup into the three-body channel NO+O+N dominates with a probability of 0.69+/-0.02. The second most important channel yields NO+NO fragments with a probability of 0.23+/-0.03. Furthermore, the dominant three-body breakup yields electronic and vibrational ground-state products, NO(upsilon=0)+N((4)S)+O((3)P), in about 45% of the cases. The internal product-state distribution of the NO fragment shows a similarity with the product-state distribution as predicted by the Franck-Condon overlap between a NO moiety of the dimer ion and a free NO. The dissociation dynamics seem to be independent of the NO internal energy. Finally, the dissociation dynamics reveal a correlation between the kinetic energy of the NO fragment and the degree of conservation of linear momentum between the O and N product atoms. The observations support a mechanism in which the recoil takes place along one of the NO bonds in the dimer.     

High temperature pyrolysis of methane in shock waves. Rates for dissociative recombination reactions of methyl radicals and for propyne formation reaction

The pyrolysis of 2% CH₄ and 5% CH₄ diluted with Ar was studied using both a single–pulse and time–resolved spectroscopic methods over the temperature range 1400–2200 K and pressure range 2.3–3.7 atm. The rate constant expressions for dissociative recombination reactions of methyl radicals, CH₃ + CH₃ → C₂H₅ + H and CH₃ + CH₃ → C₂H₄ + H₂, and for C₃H₄ formation reaction were investigated. The simulation results required considerably lower value than that reported for CH₃ + CH₃ → C₂H₄ + H₂. Propyne formation was interpreted well by reaction C₂H₂ + CH₃ → P-C₃H₄ + H with ?? = 6.2 × ¹⁰12 exp(?17 kcal/RT) ^cm3 mol^?1 s^?1.