Subject: Transition States for Reactions with no Activation Barrier

From: [email-masked] (Theis Soelling)
Date: Wed, 8 Dec 1999 12:32:49 +0100

Dear All,

A few days ago Balakrishnan Viswanathan inquired about methods for
characterizing transition states in reactions without an activation
barrier.  One approach to this problem is to use variational transition
state theory (VTST) in which the minimum in flux along the reaction
coordinate is associated with the transition state of the reaction.  A nice
description of the actual procedure can be found in Hase, W. L. Acc. Chem.
Res. 1983, 16, 258 and Baer, T.; Hase, W. L. Unimolecular Reaction
Dynamics; Theory and Experiments, Oxford University Press: Oxford, 1996;
pp. 256.  There are at least three (free) programs that can perform
VTST-calculations: HYDRA, UNIMOL and POLYRATE.

In this manner, I have characterized the transition states of a substantial
amount of unimolecular dissociation reactions of microcanonical ensembles.
I have found that the position of the flux minimum moves towards the
products as I decrease the excess energy of the reactant.  Also, I have
found that when a reactant with a given internal energy dissociates via two
(related) competing exit channels, the flux minimum is found later on the
reaction coordinate for the exit channel with the higher dissociation
energy.  Does anybody know the formal reasons for these observations?

Cheers,

  Theis

 **  **  **      **  Mr. Theis Ivan Solling
 **  **   ** ** **       Department of Chemistry
 ******    ******        University of Copenhagen
 **  **      **          DK-2100 Copenhagen
 **  **      **          Ph. ++45-3532-0187/Fax. ++45-3532-0212