(449g) Analysis of the Reactivity On the C10H10 and C10H11 Potential Energy Surfaces

The investigation of the
reactivity on potential energy surfaces characterized by the presence of a
large number of wells interlinked by shallow energy barriers is a complex task.
This is determined by the many possible reaction pathways and by the fact that
the estimation of accurate kinetic constants requires the integration of the
master equation over the whole portion of the PES accessible to the reactants,
which is often considerable and, in many cases, mostly unknown. In this work we
report about a systematic approach, summarized in Figure 1, that we have
developed and used to study the C₁₀H₁₀ and C₁₀H₁₁
PESs and estimate channel specific rate constants.

Figure 1. Systematic approach
used to study the reactivity on the C₁₀H₁₀ and C₁₀H₁₁PESs.

The pillars on which the
proposed approach rests are the estimation of energy and kinetic constants of
elementary reactions at a high level of theory, the availability of a master
equation/RRKM integrator, and the possibility to compare the computed kinetic
constants with experimental data through kinetic simulations. To make the
problem more tractable, we have divided the approach in two steps. In the first
an approximate evaluation of an upper limit of the kinetic constant is
performed by constructing a tentative PES through ab initio calculations,
determining the bottleneck of the reaction fluxes, and calculating the kinetic
constant assuming equilibrium between reactants and bottleneck transition
state. If the insertion of the calculated kinetic constant in a detailed
reaction mechanism allows reproduction of experimental data through a
modification of the kinetic constant value that is consistent with the
uncertainty of the ab initio calculations (usually 2-3 kcal/mol) and the
oversimplified description of the system reactivity (a factor of 3 at high
pressures), then a detailed estimation of the rate constant is performed.
Failures in comparison with experimental data lead back to the re-examination
of the PES.

The results of the application
of this approach to the study of the reactivity of the cyclopentadienyl radical
with cyclopentadiene (1) and with a second cyclopentadienyl radical, two
reactions that have been the object of many studies in the recent scientific
literature, will be the subject of this presentation. In particular it will be shown
how this approach allowed to eliminate some pathways that on the basis of an
energetic analysis appeared feasible as well as to identify some previously
unexplored key reaction routes.

1) C. Cavallotti, D. Polino, A. Frassoldati, E. Ranzi, J.
Phys. Chem. A, 116(13), 3313-3324 (2012).