Rate constant high-pressure limiting

In equation (C), A() (or A 111 as used earlier) is the low-pressure limiting rate constant and Ay is the high-pressure limiting rate constant. Fc is known as the broadening factor of the falloff curve its actual value depends on the particular reaction and can be calculated theoretically. Troe (1979) suggests that for reactions under atmospheric conditions, the value of Aft will be 0.7-0.9, independent of temperature. However, values as low as 0.4 are often observed. The NASA evaluations of stratospheric reactions (DeMore et al., 1997) take Aft = 0.6 for all reactions. The IUPAC evaluation (Atkinson et al., 1997a,b) does not restrict Fc to 0.6. However, it is important to note that the values of A0 and Ay will depend on the value of Fc used to match the experimental data. For example, for reaction (11)... 

However, under conditions typical of the lower stratosphere ( 20 km), the temperature and pressure are much lower. Let us calculate k m for conditions where the temperature is 219 K and the total pressure is 39 Torr. First we need to recognize that [M] changes not only with pressure but also with temperature according to the ideal gas law [M] = P/RT. Under these conditions, [M] = 1.7 X 10 X molecules cm 3. The low-and high-pressure limiting rate constants at this temperature are given by Eqs. (D) and (E) ... 

In summary, rate constants for addition reactions in the atmosphere can be estimated as a function of temperature and pressure if values are available for the low- and high-pressure limiting rate constants as a... 

High-pressure limiting rate constants (k,) except for C2H4 and C3H() d From Atkinson (1994). 

The only significant loss of alkynes is reaction with OH, for which a pressure dependence is observed. Table 6.15 gives the high-pressure limiting rate constants for the OH reactions with acetylene, propyne, 1-butyne, and 2-butyne. The reaction of acetylene approaches the high-pressure limit at several thousand Torr (see Problem 5). However, for the larger alkynes, the reactions are essentially at the high-pressure limit at 1 atm (and room temperature). 

TABLE 6.15 High-Pressure Limiting Rate Constants (k,) for the Reaction of OH Radicals with Alkynes at 298 K ... 

The vinoxy radical reacts rapidly with 02 with a high-pressure limiting rate constant kx = (1.9 0.2) X 10 13 cm3 molecule- 1 s- 1 at P > 400 Torr (Zhu and Johnston, 1995). While the reaction leads to glyoxal... 

However, the formation of the dimer in the ter-molecular reaction is sufficiently fast under stratospheric conditions that the bimolecular reactions are not important. For example, using the recommended termolecular values (DeMore et al., 1997) for the low-pressure-limiting rate constant of /c,3()0 = 2.2 X 10-32 cm6 molecule-2 s-1 and the high-pressure-limiting rate constant of k3()0 = 3.5 X 10-12 cm3 molecule-1 s-1 with temperature-dependent coefficients n = 3.1 and m = 1.0 (see Chapter 5), the effective rate constant at 25 Torr pressure and 300 K is 1.6 X 10-14 cm3 molecule-1 s-1, equal to the sum of the bimolecular channels (Nickolaisen et al., 1994). At a more typical stratospheric temperature of 220 K and only 1 Torr pressure, the effective second-order rate constant for the termolecular reaction already exceeds that for the sum of the bimolecular channels, 2.4 X 10-15 versus 1.9 X 10-15 cm3 molecule-1 s-1. 

In the gas phase, the high-pressure limiting rate constants for methyl [52] and for ethyl radical [53] have the value 1.3 x 109, resp. 2.6 x 109 which are comparable with the rate constants in the liquid medium. 

The high-pressure limiting rate constants also correlate with the ionization potential of the alkyl radical the lower the ionization potential of the alkyl radicals, the higher is their rate constant with molecular oxygen [55, 56]. 

Harding, L.B., Troe, J., and Ushakov, V.G. (2000) Classical trajectory calcirlations of the high pressure limiting rate constants. Phys. Chem. Chan. Phys. 2.631-642. 

The high-pressure limiting rate constants for the respective CF3OX dissociation reactions obtained by Brudnik et al.135 via the theoretical equilibrium constant can be expressed as... 

Experiments performed at room temperature and at SF6 pressures of 250-1000 mbar show that the kinetics of decay of the C2F5 radicals does not depend on the total pressure. The reaction of the oxygen atoms produced in the hydroxyl radical self-reaction with the ethyl and hydroxyl radicals is not important in this pressure range and has been omitted in the reaction scheme. The value of k,+2 = (1.1 0.2) x 10"10 cm3molecule"1s"1 estimated by Fagerstrom et al.203 can be considered as an upper limit of the high-pressure limiting rate constant kl oo for reaction (1). 

One version of the statistical adiabatic channel models, i.e. the maximum free energy method,20 was applied in the theoretical analysis of Fagerstrom et al,203 The calculated high-pressure limiting rate constant... 

The theoretical analysis based on the maximum free energy method20 predicts a weak temperature dependence of the high-pressure limiting rate constant, ki,, as... 

The predicted low- and high-pressure limiting rate constants can be expressed by ... 

Once formed, an alkyl radical reacts rapidly with O2 to form the corresponding peroxy radical. With high-pressure limiting rate constants in the range of 10" cm s" the atmospheric lifetime of an alkyl radical is expected to be on the order of 1 /rs or less, and no other removal mechanism... 

In this section, the basic components of a canonical FTST theory are reviewed. A more detailed derivation can be found in paper I. The canonical variational transition state theory expression for the high pressure limiting rate constant is (with p =... 

Since no conserved frequencies or shape changes have yet been studied, Eq. (12) for k(T) applies. The final computed high pressure limiting rate constants. 

Canonical TST was used to predict the temperature dependence of the rate constants. Accordingly, the high-pressure limit rate constants, k T), were... 

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