Hydrogen collisional deactivation

Gas-phase photolysis of diazoethane results in mixtures of ethylene, acetylene, and cis- and frans-2-butene. A mechanism involving the initial formation of ethylidene followed by formation of activated ethylene [which is collisionally deactivated or decomposes to produce acetylene and hydrogen— Eqs. (11.26(b,c,d)] or alternate attack on diazoethane to produce 2-butene [Eq. 11.26(e)] is proposed ... 

Several mechanisms have been postulated in order to account for ketone-sensitized photodehydrochlorination. Benzophenone and acetophenone have been suggested to act as singlet sensitizers via a collisional deactivation process (13). An alternative mechanism proposed for benzophenone involves abstraction of a methylene hydrogen from PVC by the triplet ketone (Equation 2), followed by 3 scission of a... 

Methylene attacks the double bond and the carbon-hydrogen bonds of olefins. These reactions lead to excited adducts which may undergo isomerization, collisional deactivation, or dissociation ... 

Products which can be ascribed to the intermediate formation of radicals have long been observed in carbene reactions. In the gas phase these products could arise by homolytic decomposition of excited primary products before collisional deactivation rather than from radicals generated in the course of insertion. This is not so in solution. It is found that, in the thermal decomposition of diphenyldiazomethane (Bethell et al., 1965) or photolysis of diphenylketene (Nozaki et al., 1966) in toluene solution, the product of insertion of diphenylmethylene into the benzylic carbon-hydrogen bonds, 1,1,2-triphenylethane, is accompanied by substantial amounts of 1,1,2,2-tetraphenylethane and bibenzyl. This is a strong indication that discrete diphenylmethyl and benzyl radicals are formed, and, taken in conjunction with EPR (Section IIB) and other evidence (Etter et al., 1959) that diphenylmethylene is a ground-state triplet, would support the view that equation (20) is an adequate representation of triplet insertion. 

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. 

In order to observe analytically useful emissions from compounds adsorbed on paper and other solid supports, thorough drying of the sample has been found to be necessary. Moisture competes with the phosphor molecule for hydrogen-bonding sites on the adsorbent, thus increasing the mobility of the phosphor and the chances of collisional deactivation. In most cases, RTP is so sensitive to moisture that this phenomenon may be used in the determination of humidity. 

The rearrangements of 1- and 2-methylbicyclo[2,l,0]pent-2-ene to methylcyclo-pentadienes has been examined in solution and in the vapour phase and a mechanism in which cleavage of the C-1—C-4 bond produces a chemically activated cyclopenta-diene, which then suffers competitive hydrogen shifts and collisional deactivation, has been advanced RRKM calculations support this conclusion. The principle of least motion technique has been applied to this and bicyclobutane rearrangements. ... 

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 ... 

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