Lindemann condition/criterion

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. 

Each of the walls confining the film contains two (111) layers of an fee crystal. The axes are chosen so that the layers lie in the x-y plane and the film thickness is measured along the z-axis. Periodic boundary conditions are imposed in the x-y plane. Each wall atom is tied to a lattice site with a spring. These are stiff enough that thermal displacements are less than the Lindemann criterion for melting (20,27). 

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