Hydrogen collisions! deactivation

As mentioned earlier, practically all reactions are initiated by bimolecular collisions however, certain bimolecular reactions exhibit first-order kinetics. Whether a reaction is first- or second-order is particularly important in combustion because of the presence of large radicals that decompose into a stable species and a smaller radical (primarily the hydrogen atom). A prominent combustion example is the decay of a paraffinic radical to an olefin and an H atom. The order of such reactions, and hence the appropriate rate constant expression, can change with the pressure. Thus, the rate expression developed from one pressure and temperature range may not be applicable to another range. This question of order was first addressed by Lindemann [4], who proposed that first-order processes occur as a result of a two-step reaction sequence in which the reacting molecule is activated by collisional processes, after which the activated species decomposes to products. Similarly, the activated molecule could be deactivated by another collision before it decomposes. If A is considered the reactant molecule and M its nonreacting collision partner, the Lindemann scheme can be represented as follows ... 

Figure 3-19. Photodissociation of HI monomers and clusters. The solid traces indicate the substantial discrimination available when using polarized photolysis radiation note the high S/N. Under conditions of such minimal clustering, it is reasonable to assume that most of the clusters are binary. Peaks labeled v = 1 and v = 2 are due to inelastic H + HI collisions within the cluster. The superelastic peak ft is assigned tentatively to secondary photolysis of I HI complexes, in which the escaping hydrogen deactivates the nearby I, (a) Vertical and (b) horizontal polarization of the photolysis radiation relative to the molecular beam. The plenum pressure is 1900 torr.

Two companion papers by Millikan and by Millikan and Switkes [97] describe the latest experimental results on the vibrational relaxation of CO by hydrogen. It is found that for temperatures above 600°K there is no observable difference between the deactivation efficiencies of n-Ha and p-Ha. Below 600°K,/ -Ha is clearly more efficient, and at 300°K the ratio is approximately 2. For temperatures between 600°K and 2700°K, the relaxation time for CO-Ha collisions is given by... 

It is apparent that substitution of fluorine for hydrogen initially causes a reduction in reactivity towards the electrophilic oxy n atom, but tetra-fluoroethylene is anomalous. A further study has indicated that substitution by trifluoromethyl has a strong deactivating effect compared with methyl, which has an activating effect. A study of the reactions of nearly thermal i F atoms, produced by F(n,2n) F and moderated by collisions with an excess of sulphur hexafluoride, with fluoro-oleflns (modes of addition were identified by scavenging the radicals produced with hydrogen iodide) has indicated that F atoms react preferentially with less-fluorinated olefins, and at the less-fluorinated end of a particular olefin. ... 

The total effect of these two modifications is such that e.g. for molecules containing hydrogen atoms (fast rotation, small reduced mass) appears to be much lower than g, and the relevant decrease in 0 in Eq. (13.2) results in a large increase in the transition probability, though fj-ot is small. For instance, in collisions of HCl with Ar the effective mass fjL appears to be 3 or 4 (depending on the interaction potential) instead of the reduced mass [JL = 19. If this value of g in (O /T) / is replaced by the deactivation probability 0) in Fig. 17 shifts to the hatched band corresponding to the BSH model. 

Staircase current response A staircase current is observed when the particles collide with the electrode surface and stick (or adsorb). A steady-state current response is obtained with the following electrocatalytic systems proton reduction, hydrazine oxidation, and hydrogen peroxide reduction. In many cases, the staircase current is distorted by a current decay after collision events. This decay is caused by a deactivation process, such as the adsorption of impurities, and the rate of decay depends on the experimental conditions. 

It is the authors feeling that the above experimental result does not necessarily mean that complete intramolecular vibrational energy redistribution will occur within lO" s in an isolated sec-butyl radical. In the measurements of D/S versus pressure hydrogen is used as the quenching gas M. It is a notoriously inefficient deactivator and may promote intramolecular vibrational energy redistribution by a combination of small impact parameter inefficient collisions and large impact parameter weak collisions. 

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