Abstraction rate coefficient

As abstractions involve the reaction of one radical species with a stable substrate (as opposed to radical-radical reactions) it is relatively easy to isolate these reactions for detailed study. Section 2.3.2 examines an elegant method for measuring hydroxyl abstraction rate coefficients using laser-flash photolysis coupled with laser-induced fluorescence detection of the OH radical. 

Absolute values of abstraction rate coefficients and their temperature... 

Williams et al. (2001) and Atkinson et al. (2004). For die calculation of the branching ratio between addition and abstraction for this work, values for the abstraction rate coefficients were taken, for comparison piuposes, both from the expression reported by Hynes et al. (1986) and also from the new recommendations of Atkinson et al. (2004). 

C3H7O. Only very limited data exist on the abstraction of H atoms by i-C3H70 radicals. Batt and Milne (1977a), in their study of the thermal decomposition of 1-C3H7O radicals, found that at 160 C, 1-C4H10 at 10 M could remove —25% of the radicals. If we take our recommended value for the decomposition rate coefficient of 10 °exp —17.5// 7 sec , then the abstraction rate coefficient is 2.0 x lO Af -sec at 160°C. 

The proposed reaction scheme was employed to successfully model the kinetics of the DTBA-mediated MMA polymerization in the early reaction phase via PREDICI simulations (see Figure 6), resulting in a hydrogen abstraction rate coefficient of fetr = 3.0xl0 L-moE -s and a reinitiation rate of the alkylsulfanyl radical of fejeini = 7.8 L moP The... 

Taatjes, G.A., L.K. Christensen, M.D. Hurley, and T.J. WaUington (1999), Absolute and site-specific abstraction rate coefficients for reactions of Cl with CH3CH2OH, CH3CD2OH, and CD3CH2OH between 295 and 600 K, J. Phys. Chem. A, 103, 9805-9814. 

A few results have been reported on the oxidation of cyclohexanol by acidic permanganate In the absence of added fluoride ions the reaction is first-order in both alcohol and oxidant , the apparent first-order rate coefficient (for excess alcohol) at 25 °C following an acidity dependence k = 3.5-1-16.0 [H30 ]sec fcg/A , depends on acidity (3.2 in dilute acid, 2.4 in 1 M acid) and D2o/ H20 is f-74. Addition of fluoride permitted observation of the reaction for longer periods (before precipitation) and under these conditions methanol is attacked at about the same rates as di-isopropyl ether, although dioxan is oxidised over twenty times more slowly. The lack of specificity and the isotope effect indicates that a hydride-ion abstraction mechanism operates under these conditions. (The reactivity of di-isopropyl ether towards two-equivalent oxidants is illustrated by its reaction with Hg(II).) Similar results were obtained with buffered permanganate. 

The remarkable inertness of dialkyl ethers to one-equivalent oxidants is good evidence that the readier oxidation of alcohols involves more than simple electron abstraction. Di-isopropyl ether is oxidised by Co(III) in CH3CN-H2O mixtures with complicated kinetics individual runs show first-order decay of Co(III) but the rate coefficients increase with increasing [Co(III)], and the order with respect to substrate is less than one but is neither fractional nor of a Michaelis-Menten type. The main product is acetone and the following reaction sequence is proposed... 

Rate-determining enolisation is discounted by the authors on the grounds of the appearance of cupric ion in the rate law and of the value of the rate coefficient, which shows oxidation to be faster than enolisation. However, it is known that Cu(II) catalyses enolisation and an intermediate radical (CH3)2CCHO could abstract chlorine from CUCI2. 

Data for the specific rate coefficients for abstraction from CH bonds have been derived from experiments with hydrocarbons with different distributions of primary, secondary, and tertiary CH bonds. A primary CH bond is one on a carbon that is only connected to one other carbon, that is, the end carbon in a chain or a branch of a chain of carbon atoms. A secondary CH bond is one on a carbon atom connected to two others, and a tertiary CH bond is on a carbon atom that is... 

Since radical and atom metathesis reactions generally have low activation energies, their rate coefficients are expected to exhibit non-Arrhenius behavior because of the increased importance of the term noted previously. In Fig. 9, rate coefficient data for H-atom abstractions from CH4... 

Heicklen, J The Correlation of Rate Coefficients for H-Atom Abstraction by HO Radicals with C-H Bond Dissociation Enthalpies, hit, J. Chem. Kinet., 13, 651-665 (1981). 

A more complicated reaction scheme is proposed by the authors to include the formation of the by-products acetonitrile, acetaldehyde and ethylene. However, appropriate rate coefficients cannot be given as the reactions appear to be partially homogeneous gas phase reactions, implying that factors like the reactor geometry are also involved. Regarding the oxidation mechanism, the authors assume that two hydrogen atoms are first abstracted from propene, followed by reaction with surface oxygen or NH species. 

The rate constants for the reactions between OH and a range of ethers and hydroxy ethers have been reported at 298 K233 as well as those for reactions between dimethyl ether and methyl f-butyl ether over the range 295-750 K.234 Data from the former study show deviations from simple structure-activity relationships which were postulated to arise due to H-bonding in the reaction transition states.233 The atmospheric lifetime of methyl ethyl ether has been determined to be approximately 2 days.235 Theoretical studies on the H-abstraction from propan-2-ol (a model for deoxyribose) by OH have been reported using ab initio methods (MP2/6-31G ).236 The temperature dependence (233-272 K) of the rate coefficients for the reaction of OH with methyl, ethyl, n-propyl, n-butyl, and f-butyl formate has been measured and structure-activity... 

Values of k+ are known from gas phase studies for a variety of unsaturated compounds91 and with these non-polar solvents it may be assumed that the values will be the same in the liquid phase. Hence it has been possible to determine values of the rate coefficients for a wide series of hydrogen atom abstraction reactions in solution. On the basis of these, it has been possible to determine values of kA for other scavengers92. Some of these rate coefficients are shown in Table 11. It has however, been suggested that alternative physical mechanisms may be responsible for the decrease in hydrogen yield observed in the presence of scavengers93. 

This is a case for which only stabilization is of interest. It has not been treated explicitly as an excitation reaction. Its rate has been measured directly103-105 at low temperatures (300-450 °C) as have those for several other radical combinations. Without doubt, the combination of methyl radicals is the most important reaction of this type for chemical kinetics. It serves as a reference standard for the measurement of rates of many hydrogen abstraction reactions, but very little is known about the temperature dependence of its rate coefficient. 

Heicklen, J. 1981. The correlation of rate coefficients for H-atom abstraction by HO radicals with C-H bond dissociation enthalpies. Int. J. Chem. Kinet. 13 651-665. 

Abstract. In this chapter we discuss approaches to solving quantum dynamics in the condensed phase based on the quantum-classical Liouville method. Several representations of the quantum-classical Liouville equation (QCLE) of motion have been investigated and subsequently simulated. We discuss the benefits and limitations of these approaches. By making further approximations to the QCLE, we show that standard approaches to this problem, i.e., mean-field and surface-hopping methods, can be derived. The computation of transport coefficients, such as chemical rate constants, represent an important class of problems where the QCL method is applicable. We present a general quantum-classical expression for a time-dependent transport coefficient which incorporates the full system s initial quantum equilibrium structure. As an example of the formalism, the computation of a reaction rate coefficient for a simple reactive model is presented. These results are compared to illuminate the similarities and differences between various approaches discussed in this chapter. 

Absolute measurements 124.251 of the rate coefficient for the N03/DMS reaction are in reasonably good agreement. A temperature-independent value of 1 x 10 12 cm3 molecule 1 s 1 has been adopted in the model. The products of the reaction have yet to be established. On the basis that Tyndall et al. (24) saw no evidence for DMSO production, hydrogen-atom abstraction has been assumed to occur (reaction 4). 

Table II summarises the parameters employed in the standard simulations. Cloudless conditions are assumed throughout. The modelling runs performed are listed in Table III. A lower rate coefficient of 4.2 x 10 n cm3 molecule 1 s 1 and a 50 50 abstraction/addition ratio for the OH/DMS reaction were adopted in run b . In run e the triple plume was considered as a single one with corresponding larger initial cross-section.

A reaction sequence analogous to that in Eq. 4.40 can also be developed for the specific adsorption of bivalent metal cations (e.g., Cu2+, Mn2 or Pb2+) by metal oxyhydroxides.21 In this application the abstract scenario in the first row of Table 4.3 is realized with A = =Al-OH, B = M2+, C = =Al-OH - - M2+, D = = Al-OM+, and E = H where M is the metal complexed by an OH group on the surface of an aluminum oxyhydroxide. Analysis of pressure-pulse relaxation kinetics data leads to a calculation of the second-order rate coefficient kf, under the assumption that the first step in the sequence in Eq. 4.40 is rate determining. Like k(l, the rate coefficient for the dissolution of a metal-containing solid (Section 3.1 cf. Fig. 3.4), measured values of k, correlate positively in a log log plot with kw,. , the rate coefficient for water exchange on the metal... 

By means of analogies, the number of independent kinetic constants is strongly reduced. For example, the Arrhenius parameters of any single H abstraction are assumed to be dependent only on the nature of the attacking radical (hydrogen, methyl, etc.) and on the hydrogen atom abstracted (primary, secondary, allylic, etc.) in the same way, reactions of allyl radicals with ethylene and propylene giving cyclopentenes are assumed to have the same rate coefficients, etc. 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

Intramolecular interactions in the transitions states (TS) are also relevant to properly predict or reproduce experimental rate constants, since they directly affect the TSs energies and small variations in reaction barriers have relative large impact on k since they enter exponentially in the rate constant equation. A detailed discussion on such interactions, in the TSs of different H abstraction paths, for 2-propanol -I- OH reacfion has been provided by Luo et al. [85]. These authors have also discussed the influence of fhe inferactions on the reaction barriers and rate coefficients. 

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