Collisional efficiency

It is often the case that certain species have enhanced collisional efficiencies. For example, for reaction 9.74 it is found that when H2O is the third body collision partner the rate of reaction is increased by a factor of 15 over the rate when other species participate as the collision partner M.2 Thus a collisional efficiency akl is introduced. Such enhanced collision efficiencies result in an effective concentration of third bodies that can be higher than the actual concentration [M]. 

Since (1) and (2) will have different collisional efficiencies, the result will be a composite relaxation time for A, given by... 

Several anomalies appear to exist in these data. First, the temperature dependence of rate seems excessive for the last group. Second, the diethyl ketone rates are much higher than those in the ethylene system and led Heller and Gordon to attribute greatly reduced collisional deactivation efficiency to the ketone relative to ethylene. If anything, however, we believe that the efficiency inequality should be reversed. Finally the experimental k values were calculated on the assumption that hydrogen or deuterium gas present in the mixture was completely inefficient as a collisional deactivator. We believe that this assumption is too extreme and that a reasonable lower estimate17 of their collisional efficiency would be 0.20. On this basis, all rate constants would be doubled or tripled. 

Further work has been reported by Johnston on the low-pressure rate in the presence of different inert gases. It is found that the collisional efficiencies of various gases in activating N2O6 at low pressures (reaction 1) relative to N2OB itself are N2O6, 1 Ar, 0.135 He, 0.124 Ne, 0.090 Kr,... 

NXv, the value of N (e) when + = kT, was used since A (e) is not a strongly varying function of e+. The collisional efficiencies of He and SF were assumed to be 0.15 and 1.0, as for HNO3, and the collision diameter of HNO2 was taken to be 5.2 A. The experimental and predicted values of ks and k a are shown in table 2. The rate coefficients calculated with model B" agree more closely with the experimental values than those computed with A". At 80 Torr He, A 5[He] = 1.2 x 10 cm molecule- s (model B") or 0.47x 10 cm molecule" s (model A"). Clearly the assumption that this recombination reaction is in its third order region when / hc < 80 Torr is fully justified. 

Fig. 4-3 Collisional efficiencies for energization of methyl isocyanide at 280.5°C by various gases relative to CH3NC. Filled points represent coincident points for the stated number of gases (two if not stated). [Adapted from P. J. Robinson and K. A. Holbrook, Unimolecular Reactions, Fig. 10-7. Copyright 1972, Wiley (Interscience), New York. Used by permission of John Wiley Sons, Inc. The data were obtained from Chan et al. (19)]...

The second term in this expression is reasonable for a three body combination reaction between molecules of the complexity of H02- However, there is no chemical or spectroscopic evidence for the formation of a stabilised H2O4 species and estimates of the stability of the H2O4 molecule are based on an 0-0 chain structure which would decompose to + 2Q2 rather than H2P2 + O2. The negative temperature coefficient at low pressures again suggests a reaction proceeding over an attractive potential surface, albeit with a low collisional efficiency. 

Kahlert, M. Hofmann, K. P. (1991). Reaction rate and collisional efficiency of the rhodopsin system in intact retinal rods. Biophysical Journal, 59, 375- 6. 

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