Falloff curv

Cobos 0 J, Hippier H and Tree J 1985 High-pressure falloff curves and specific rate constants for the reactions J. Phys. Chem. 89 342-9... 

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)... 

The falloff curve of the quantum yield is explained by the contribution of the internal (mostly rotational) energy to supplement the incident photon energy (550,810), (see Section 1-4.3 for details). Above 4358 A a small ( 0.01) but significant yield of NO was observed, which is attributed to reactions of electronically excited N02 (NO ) by Jones and Bayes (550). 

After the high-pressure limit has been established, the unimolecular rate constants (kum) obtained are plotted versus pressure to generate the experimental falloff curve. The RRKM specific rate constants [703] are calculated from ... 

Reduced falloff curves klk as a function of a "reduced pressure scale" k k being proportional to [M], are identical for dissociation and association reactions. In the simplified mechanism given above, they take the form... 

In the following, we shall discuss the properties of k, k, and of the reduced falloff curves in more detail, trying to identify common and different contributions for ionic and neutral reaction systems. 

It remains to be discussed, whether eq. (17) provides an adequate representation of the falloff curves interpolating between and k and how the parameter Fcenter can be predicted. This question applies as well to neutrals as to ionic systems. The question has a strong collision and a weak collision aspect. If one concludes that (A ) and Pc are similar for ionic and for neutral systems, then the weak collision broadening factor ire TT-irc i-sc can be estimated [1] by... 

The described procedme for modeling the limiting low and high pressure rate coefficients ko and k and the falloff curves interpolating between these values, can be... 

NH3 + + He —> N2H7" He with modeled strong collision falloff curves (lines)... 

Figure 5 compares the experiments from figure 1 with modeled falloff curves for M = N2. The low temperature results are systematically above the... 

Figure 17. Dependence of C2H4 + OH +M on total pressure at 295 K for M = synthetic air (M = Ar not shown). The solid line through the data points is the calculated falloff curve. The error bar represents the average of 2cr errors for all individual data points.

Figure 18. Comparison of literature ki values near room temperatiue with the calculated falloff curves from this work (solid line M = Ar, broken line M = synthetic air. The data points are presented without error limits to simplify the comparison.

Comparison of Pressure Falloff Curves. The assignment of energy transfer models and values has often been based upon comparisons between the curvatures of experimental vs. calculated (ka/ka ) vs. (S/D) plots. This type of comparison obviates the requirement for accurately known ka values in s units, because the ka values are scaled by dividing by ka °. 

The calculated log (ka/ka °) vs. log (S/D) falloff curves for the three transition probability models have been intercompared in Figure 6. Subtle differences in curvature exist for these various hypothetical cascading models over the range 0.001 < (S/D) < 0.4. However, from Figure 6 the slopes of all the calculated falloff plots increase monotonically with decreasing (S/D), and thus also with diminishing pressure. 

Fig. 7.8. Falloff curves for a model calculation on the thermal dissociation of carbon dioxide, considering only first order randomisation processes. In descending order, the curves correspond to aji = oo, 10 , 10 , 10 , 10 , and lO s" and zero, respectively.

Cobos, C.J., Troe, J. Prediction of reduced falloff curves for recombination reactions at low temperatures. Z. Phys. Chem. 217, 1031-1044 (2003)... 

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