283502 Analysis of the Reactivity On the C10H10 and C10H11 Potential Energy Surfaces

Wednesday, October 31, 2012: 10:30 AM
320 (Convention Center )
Daniela Polino and Carlo Cavallotti, Dept. Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, Milano, Italy

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).


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