Theories of Unimolecular Gas Reactions at Low Pressures. Ii

Abstract

the chief means of activation at low pressure, and unless the mechanism of transfer of energy is very specific, we would expect the reaction rate to fall off during the course of a run as the products of reaction accumulate but practically this is not the case, though the run falls off in most cases to a very small extent. What the effects of simple local heating might be, and whether such an effect could be combined with our theory to give satisfactory results, we cannot at present decide. SummaryTwo theories based on collision hypotheses of quasi-unimolecular reactions, which we have previously developed, are applied to the decomposition of azomethane. It is possible to explain the falling off of the rate with pressure and to decide between the two theories. The efficiency of activation and deactivation at collision is considered, and the following are discussed: (1) the number of degrees of freedom involved in the various cases; (2) the effect of heat of reaction in the case of azomethane. The decomposition of diethyl ether is also considered briefly.

Electronic effects on melting: Comparison of aluminum cluster anions and cations

“…k͑E , D͒ is the rate constant for unimolecular dissociation of a cluster with dissociation energy D and total internal energy E. We use the quantum Rice-Ramsperger-Kassel ͑RRK͒ model 34 for the reaction rate constant …”

Dynamical hierarchy in transition states: Why and how does a system climb over the mountain?

“…The concept of transition state or dividing surface was introduced by Eyring (1) and Wigner (2). If the system begins in thermal equilibrium, and if conditions justify assuming a quasiequilibrium between the reactants and systems crossing the transition state in the forward direction (i.e., toward the products) along the reaction coordinate q 1 , then the apparatus of ''transitionstate theory'' in any of several forms (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) can be invoked to evaluate the rate coefficient of the reaction. The greater part of the effort in using any of the specific methods is establishing the hypersurface dividing reactant from product so that it is as free as possible from the ''recrossing problem.''…”

Phase space geometry of dynamics passing through saddle coupled with spatial rotation

“…Recent theoretical developments on nonlinear dynamics through the saddle have revealed the robust existence of no-return transition state (TS) and the reaction pathway along which all reactive trajectories necessarily follow not in the configuration space but in the phase space. In addition to what chemists have long envisioned as TS, [1][2][3][4][5][6][7][8] it was revealed that there exist another important "building blocks" in the phase space for the understanding of the origin of the reactions: that is, normally hyperbolic invariant manifold (NHIM) and the stable/unstable invariant manifolds [9][10][11][12][13][14][15][16][17] (and their remnants 16,[18][19][20][21] ). An invariant manifold is a set of points in the phase space such that, once the system is in that manifold, the system will stay in it perpetually.…”