Chemical kinetics Lindemann mechanism

We continue our study of chemical kinetics with a presentation of reaction mechanisms. As time permits, we complete this section of the course with a presentation of one or more of the topics Lindemann theory, free radical chain mechanism, enzyme kinetics, or surface chemistry. The study of chemical kinetics is unlike both thermodynamics and quantum mechanics in that the overarching goal is not to produce a formal mathematical structure. Instead, techniques are developed to help design, analyze, and interpret experiments and then to connect experimental results to the proposed mechanism. We devote the balance of the semester to a traditional treatment of classical thermodynamics. In Appendix 2 the reader will find a general outline of the course in place of further detailed descriptions. 

We discuss first collisions that induce vibrational vibrational energy transfer, first in diatomics and then in larger molecules. Such processes can have a low gap and therefore occur fairly efficiently. Then we return to a topic that played a key role throughout the history of chemical kinetics the collisional acquisition (or loss) of vibrational energy of larger molecules. We know from the Lindemann mechanism that such processes can drive a molecule all the way to isomerization or dissociation. But they are also important in any chemical system in disequilibrium. 

It has been seen (p. 92) that, on energetic grounds, a radical non-chain process may be excluded except in very special cases, and so no further consideration need be given to this mechanism. This leaves the decision to be made between the radical chain and the unimolecular mechanisms. There is, at the present time, no criterion which is both necessary and sufficient to prove that a given reaction is proceeding by a unimolecular mechanism. Necessary conditions for a unimolecular mechanism are (a) first-order kinetics at high pressures, (b) Lindemann fall-off at low pressures, (c) absence of induction periods, (d) lack of effect of inhibitors, and (e) an Arrhenius A factor of the order of 1013 sec-1. An additional useful test, though neither a necessary nor a sufficient condition, is the absence of stimulation of the reaction in the presence of atoms or radicals. Finally, the effects of structural alterations on the rates of those related reactions that are claimed to be unimolecular should be capable of interpretation within the framework of current chemical theory. 

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