Rate constant low-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, 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. 

Fig. 23. The observed150,151 and calculated156 rate constants for the reaction HCHO + OH. The solid line denotes the calculated rate constant k, whereas the dashed line corresponds to the values of the strong-collision, low-pressure limiting rate constant sckADi,o calculated for the formation of the pre-reaction adduct.

Low-pressure limiting rate constants for other bath-gases were also derived by Jodkowski et al.212 by fitting the theoretical fall-off curve to available... 

The role of FNO in atmospheric chemistry is related to the destruction of stratospheric ozone.244 246 The reaction F + NO has been extensively studied using different experimental techniques, including mass spectrometry, chemiluminescence, ESR, IR, and UV spectroscopy.241,247 251 The pressure dependence of the kinetics of FNO formation was recently studied by Pagsberg et al.25i in order to obtain the fall-off curve and the high- and low-pressure limiting rate constants. The reaction was initiated by pulse radiolysis of a SF6/NO gas mixture. In the presence of NO, the decay of the formed... 

Analysis of the experimental data of Pagsberg et al262 shows that the experiments were made at total pressures, where the kinetics of the association reaction corresponds to the third-order process F + 02 (+ M) —> F02 (+ M), with M = Ar. The low-pressure limiting rate constant derived by Pagsberg et al.262... 

The Variflex program [35] was employed to calculate the dissociation rate constants. The predicted high- and low-pressure limiting rate constants for the decomposition reaction CIO4 S-CIO3 + O can be expressed by ... 

The slightly different low pressure limit rate constant ko of reaction (5.1) has been obtained in laboratory for O2 and N2 acting as M (Lin and Leu 1982 Hippier et al. 1990). The recommended values of the lUPAC subcommittee (Atkinson et al. 2004) are. 

Measurements conducted between 2 and 5 Torn The rate coefficient begins to show a pressure dependence at temperatures above 335 K. An Arrhenius expression of Atg = (2.5 7.4) x 10 exp[(4,150 l,150)/7] cm molecule" was obtained for the low pressure-limiting rate constant. The value quoted for 300 K was obtained in 5 Torr total pressure of helium diluent containing 5-15% O2. 

With the exception of the work by Louge and Hanson, none of the studies surveyed corrected their data for pressure effects to obtain the low-pressure limiting rate constant (Chapter 4). We can estimate part of the error incurred by this neglect from the Lindemann-Hinshelwood relation... 

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