Addition channel rate

We have calculated the addition channel rate constant using the RRKM approach to unimolecular reaction rate theory, as formulated by Troe ( ) to match RRKM results with a simpler computational approach. The pressure dependence of the addition reaction (1) can be simply decribed by a Lindemann-Hinshelwood mechanism, written most conveniently in the direction of decomposition of the stable adduct ... 

Acoustic-chemical coupling, 151-54 Activation energy, 112-13 Activation energy, reaction of NH and oxygen, 109 Addition channel rate constant, 249-54... 

As indicated by the involvement of water vapor and an inert third body, this reaction has several channels (see DeMore et al., 1997, for a review). There is both a bimolecular channel, which is pressure independent, and a termolecular channel, which is pressure dependent. In addition, the rate constant increases in the presence of gaseous water, suggesting that the reaction proceeds through a mechanism such as... 

Thus, at 1 atm in air and 298 K, abstraction predominates. The addition channel (45b) would be expected to have a pressure dependence and a negative temperature dependence (see Chapter 5.A.2). Thus is consistent with the observation that the effective overall bimolecular rate constant in 1 atm of air decreases as the temperature increases from 250 to 310 K and that the fraction of the reaction that proceeds via (45a) increases from 0.24 to 0.87 over the same temperature range (e.g., Hynes et al., 1986). 

The subexcitation electrons lose their energy in small portions, which are spent on excitation of rovibrational states and in elastic collisions. In polar media there is an additional channel of energy losses, namely, the dipole relaxation of the medium. The rate with which the energy is lost in all these processes is several orders of magnitude smaller than the rate of ionizaton losses (see the estimates presented in Section II), so the thermalization of subexcitation electrons is a relatively slow process and lasts up to 10 13 s or more. By that time the fast chemical reactions, which may involve the slow electrons themselves (for example, the reactions with acceptors), are already in progress in the medium. For this reason, together with ions and excited molecules, the subexcitation electrons are active particles of the primary stage of radiolysis. 

Barnes, et al. (22) have studied reaction (3) in the presence of O2 using a competitive rate technique which utilizes H2O2 as the OH photolytic precursor. This avoids the use of NOx containing precursors which nave been shown to produce unreliable results in studies of OH reactions with organo-sulfiir compounds (24.251. They find up to 30% yields of dimethylsulfone, (013)2802, and suggest that reaction (4b) is important If reaction (4b) is an elementary reaction and OH is regenerated, then our study of reaction (3) would have obtained values of k s which were too low and, hence, underestimated the importance of the addition channel. One apparent inconsistency between the recent work of Barnes, et al. (231 and our study (2) is that Barnes, et al. observed only DMSO2 as a product of reaction (4) neither DMSO nor MSA was observed. Since we would not have observed an O rate enhancement if the adduct + O2 reaction proceeded entirely via channel (4b), our results imply that an addition channel other than (4b) must be important The only addition pathways which seem reasonable are (4a), (4b), and... 

The results for Pt(5 3 3) (Figs. 11 and 26) are consistent with the increased sticking probability observed for a Maxwellian source of H2 on Pt(9 9 7) over Pt(l 1 1) [81]. It also provides direct evidence for an additional channel to dissociative adsorption through step sites which was invoked to explain the enhanced rate of H2 + D2 exchange reaction at Pt(3 3 2) over that observed on Pt(l 1 1) surfaces investigated using a Maxwellian beam source [82]. 

The majority of the reaction proceeds via the addition channel (b) to give a methyl hydroxycyclohexadienyl radical. Cyclohexadienyl radicals are resonance stabilized and are relatively unreactive. Typical alkyl radicals add O2 rapidly with rate constants of the order of 10 12, in contrast the reaction of cyclohexadienyl and methyl cyclohexadienyl radicals with O2 proceed with rate constants of (2 - 5) x 10 16 cm3 molecule 1 s 1 [67,68]. It can be calculated that in one atmosphere of air the cyclohexadienyl radicals have a lifetime of... 

The calculated rate coefficients at 298 K are 1.10 and 1.45 x 10 L/(cm s) for formaldehyde and acetaldehyde, respectively [27]. These values are in excellent agreement with the experimental data. The structure of the transition states are shown in Figure 12.4. Both are early transition states consistent with reactions with very low barriers. The transition state of OH hydrogen abstraction from acetaldehyde is earlier than the one of formaldehyde. Additionally, the possibility of the addition channel was excluded due to the large barrier associated to this process. The temperature dependence of the rate coefficient was not studied. In a more recent work [105] the level of the calculations was increased, the temperature dependence of the rate coefficient and the role of direct abstraction were studied. All the main conclusions from these articles are now accepted in recent works [106-108], and it is well known that the hydrogen abstraction is the main reaction channel if not unique [108,109]. 

The results of the rate constant calculations by d Anna et al,156 seem to confirm this reaction mechanism. In Fig. 25 is shown the temperature dependence of the observed and calculated rate constants. The rate constant k describes the rate of formation of the post-reaction adduct under the assumption that the pre-reactive adducts are not stabilized by collisions, whereas kadd describes the kinetics of formation of the stable pre-reactive complexes at a total pressure of 1 bar. Thus the overall rate constant for the decay of reactants (denoted in the figure by a solid line) is given by the sum k + k. The values of k predicted by d Anna et al.156 distinctly underestimate the reaction rate at low temperatures, but they approach the results of measurements at temperatures above 700 K. The limiting rate constants kadd, and kadd,0 for the addition channels were analyzed in terms of statistical unimolecular rate theory. Results of the calculations show a fall-off behavior of the reaction kinetics under typical atmospheric conditions corresponding to a total pressure of 1 bar. Therefore, all kadd values were derived from the... 

The reverse processes of (LVI) and (LVII), which proceed at similar rates, represent additional channels for the pumping up of vibrational energy. 

Our results taken together with these lower temperature measurements (, l y raise questions which we have attempted to answer using unimolecular and bimolecular reaction rate theory. The first is, simply, are the results at low and high temperature consistent Further, what is the expected behavior in the intermediate region and under what conditions does the direct or addition channel dominate ... 

Alkyl nitrates, formed in a secondary channel of the reaction of RO2 with NO, are another "stable" form of NOy. LACTOZ has provided a comprehensive data set for the formation of nitrates, mainly from the higher molecular weight VOCs. In addition, the rates of degradation of organic nitrates, by photolysis and by their reaction with OH radicals, can be determined from LACTOZ results, although some uncertainties remain in the production and loss of alkyl nitrates. 

Rate constants for the reactions of OH radicals with n-nitroalkanes, n-alkyl nitrates and n-alkyl nitrites have been determined at 298 K and 1 atmosphere total pressure using both pulse radiolysis combined with kinetic spectroscopy and a conventional relative rate method. In order to provide more mechanistic information for these reactions, rate constants for the reaction of Cl atoms with these compounds were determined using the relative rate method. The data indicate that the reaction of OH radicals with these nitrogen-containing compounds involves both an abstraction and an addition channel. These results are discussed in terms of reactivity trends and compared with the data from the literature. 

Part of the energy transferred from the molecule to the excitation of plasmon is actually dissipated within the metal nanoparticle. This is an additional channel of de-excitation of the molecule that is not leading to photon emission, and it is not included in Prad- The energy flux method calculates the decay rate associated to this quenching by the net flux of S (equal to the absorbed power Pahs ) through a closed surface that contains the metal nanoparticle but not the molecule (see Fig. 5.4). The non-radiative decay -/nr is then calculated as for... 

In the case of adsorption on a metal surface, there may be an additional channel of relaxation through the transfer of adsorbate energy to electron-hole pairs. The number of accessible electron states in the metal increases linearly with the transition frequency, coq, and so does the decay rate through... 

ViUano SM, Carstensen HH, Dean AM. Rate rules, branching ratios, and pressure dependence of the HO2 + olefin addition channels. J Phys Chem A. 2013 ... 

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