https://doi.org/10.1007/s100530050197
On the analysis of exit-channel effects in three-atom unimolecular reactions
Laboratoire de Physicochimie Moléculaire (URA 5803) ,
Université Bordeaux 1 and CNRS,
33405 Talence Cedex, France
Corresponding author: a speedy@lpct.u-bordeaux.fr
Received:
18
February
1998
Accepted:
4
May
1998
Published online: 15 November 1998
The key problem of exit-channel effects in unimolecular
reactions, which make Transition State Theory (TST) generally unsuitable
for the calculation of product state distributions, is analyzed in the
triatomic case ABC 
AB + C for a total angular momentum equal to
zero. The vibrational energy of AB is supposed to be quasi conserved on
the way from the transition state (TS) to the products. Moreover, classical
mechanics is used for the description of rotational and translational
motions. In this frame, batches of trajectories are run on model potential
energy surfaces from the TS to the products. Their initial conditions on the
dividing surface associated with the TS are not distributed at random but
instead, they form curves the shapes of which are guided by physical
considerations. The reflection of these curves on hypersurfaces orthogonal
to the reaction path provides worthwhile information about the nature of
exit-channel effects. It is shown that the modulus of the rotational angular
momentum of AB is more likely to decrease than to increase, the
amplitude of the variation being larger on the average in the first than in
the second case. As a consequence, exit-channel effects cause the
rotational state distribution to be colder in the products than at the TS, as
observed in the reaction 
. In addition to
that, a slight improvement of a model recently developed by the authors
allows the description of exit-channel effects in a satisfying way which
might be included in TST in order to go beyond the Phase Space Theory
(PST) of product state distributions.
PACS: 34.10.+x – General theories and models of atomic and molecular collisions and interactions (including statistical theories, transition state, stochastic and trajectory models, etc.) / 34.50.-s – Scattering of atoms, molecules, and ions / 82.20.-w – Chemical kinetics
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 1998
