Guillaume Reinisch

Hindered rotor models with variable kinetic functions for accurate thermodynamic and kinetic predictions

We present an extension of some popular hindered rotor (HR) models, namely, the one-dimensional HR (1DHR) and the degenerated two-dimensional HR (d2DHR) models, allowing for a simple and accurate treatment of internal rotations. This extension, based on the use of a variable kinetic function in the Hamiltonian instead of a constant reduced moment of inertia, is extremely suitable in the case of rocking/wagging motions involved in dissociation or atom transfer reactions. The variable kinetic function is first introduced in the framework of a classical 1DHR model. Then, an effective temperature and potential dependent constant is proposed in the cases of quantum 1DHR and classical d2DHR models. These methods are finally applied to the atom transfer reaction SiCl(3)+BCl(3)→SiCl(4)+BCl(2). We show, for this particular case, that a proper accounting of internal rotations greatly improves the accuracy of thermodynamic and kinetic predictions. Moreover, our results confirm (i) that using a suitably defined kinetic function appears to be very adapted to such problems; (ii) that the separability assumption of independent rotations seems justified; and (iii) that a quantum mechanical treatment is not a substantial improvement with respect to a classical one.

Measurement of pyrolysis products from phenolic polymer thermal decomposition

Batch pyrolysis of phenolic polymer was performed using a step-wise heating procedure in a 50 K increment from room temperature up to 1250 K. A phenolic-polymer sample of 50 mg was loaded in a reactor assembly speci cally designed and built for this study. The mass loss was measured after each 50 K step and the production of gas-phase species was quanti ed using gas-chromatography techniques. The overall mass loss reached about 35%. Water was found to be the dominant product below 800 K. Yields of permanent gases such as hydrogen, methane, carbon monoxide, and carbon dioxide increased with temperature up to 900 K and then decreased at higher temperatures. The yields of light hydrocarbons, such as C2 to C4 hydrocarbons, increased with reaction temperature up to 1000 K and dropped subsequently. Yields of aromatic products, including benzene, toluene, and xylene, were signi cant between 700 and 850 K. The quantitative molar production of species versus temperature is made available for the development of detailed phenolicpolymer pyrolysis models.

An Efficient and Accurate Formalism for the Treatment of Large Amplitude Intramolecular Motion.

We propose a general approach to describe large amplitude motions (LAM) with multiple degrees of freedom (DOF) in molecules or reaction intermediates, which is useful for the computation of thermochemical or kinetic data. The kinetic part of the LAM Lagrangian is derived using a Z-matrix internal coordinate representation within a new numerical procedure. This derivation is exact for a classical system, and the uncertainties on the prediction of observable quantities largely arise from uncertainties on the LAM potential energy surface (PES) itself. In order to rigorously account for these uncertainties, we present an approach based on Bayesian theory to infer a parametrized physical model of the PES using ab initio calculations. This framework allows for quantification of uncertainties associated with a PES model as well as the forward propagation of these uncertainties to the quantity of interest. A selection and generalization of some treatments accounting for the coupling of the LAM with other internal or external DOF are also presented. Finally, we discuss and validate the approach with two applications: the calculation of the partition function of 1,3-butadiene and the calculation of the high-pressure reaction rate of the CH3 + H → CH4 recombination.

Theoretical study of the decomposition of BCl3 induced by a H radical.

We report on a theoretical study of the gas-phase decomposition of boron trichloride in the presence of hydrogen radicals using ab initio energetic calculations coupled to TST, RRKM, and VTST-VRC kinetic calculations. In particular, we present an addition-elimination mechanism (BCl(3) + H → BHCl(2) + Cl) allowing for a much more rapid consumption of BCl(3) than the direct abstraction reaction (BCl(3) + H → BCl(2) + HCl) considered up to now. At low temperatures, T ≤ 800 K, our results show that a weakly stabilized complex BHCl(3) is formed with a kinetic law compatible with the consumption rate measured in the former experiments. At higher temperatures, this complex is not stable and then easily eliminates a chlorine atom. Our work also shows that a very similar mechanism, involving the same intermediate and sharing the same transition state, allows for the elimination of HCl. A dividing coefficient between these two elimination pathways is obtained from both a potential energy surface based statistical analysis and an ab initio molecular dynamics transition path sampling simulation. It finally allows partitioning of the global consumption rate of BCl(3) in terms of the formation of (i) BHCl(3), (ii) BHCl(2) + Cl through a H addition/Cl elimination mechanism, (iii) BCl(2) + HCl through a H addition/HCl elimination mechanism, and (iv) BCl(2) + HCl through direct abstraction.

Reaction mechanism for the thermal decomposition of BCl3/CH4/H2 gas mixtures.

This paper presents an ab initio study of the B/C/Cl/H gas phase mechanism, featuring 10 addition-elimination reactions involving BH(i)Cl(j) (i + j ≤ 3) species and a first description of the chemical interaction between the carbon-containing and boron-containing subsystems through the three reactions BCl(3) + CH(4) ⇌ BCl(2)CH(3) + HCl, BHCl(2) + CH(4) ⇌ BCl(2)CH(3) + H(2), and BCl(2) + CH(4) ⇌ BHCl(2) + CH(3). A reaction mechanism is then proposed and used to perform some illustrative equilibrium and kinetic calculations in the context of chemical vapor deposition (CVD) of boron carbide. Our results show that the new addition-elimination reaction paths play a crucial role by lowering considerably the activation barrier with respect to previous theoretical evaluations; they also confirm that BCl(2)CH(3) is an important species in the mechanism.