Rate theory for correlated processes: Double-jumps in adatom diffusion

We study the rate of activated motion over multiple barriers, in particular the correlated double-jump of an adatom diffusing on a missing-row reconstructed Platinum (110) surface. We develop a Transition Path Theory, showing that the activation energy is given by the minimum-energy trajectory which succeeds in the double-jump. We explicitly calculate this trajectory within an effective-medium molecular dynamics simulation. A cusp in the acceptance region leads to a sqrt{T} prefactor for the activated rate of double-jumps. Theory and numerical results agree

Low-temperature reactions: Tunnelling in space.

International audienceChemical reactions with activation barriers generally slow to a halt in the extreme cold of dense interstellar clouds. Low-temperature experiments on the reaction of OH with methanol have now shown that below 200 K there is a major acceleration in the rate that can only be explained by enhanced quantum mechanical tunnelling through the barrier

A theoretical analysis of the CH{sub 3} + H reaction : isotope effects, the high pressure limit, and transition state recrossing.

The reaction of methyl radicals with hydrogen atoms is studied with a combination of ab initio quantum chemistry, variational transition state theory, and classical trajectory simulations. The interaction between the two radicals, including the umbrella mode of the methyl radical, is examined at the CAS+1+2 level using an augmented correlation consistent polarized valence triple zeta basis set. The implementation of an analytic representation of the ab initio data within variable reaction coordinate transition state theory yields predictions for the zero-pressure limit isotopic exchange rate constants that are about 15% greater than the available experimental data. Trajectory simulations indicate that the transition state recrossing factor for the capture process is 0.90, essentially independent of temperature and isotope. The dynamically corrected theoretical prediction for the CH{sub 3} + H high pressure rate coefficient is well reproduced by the expression 1.32 x 10{sup -10}T{sup 0.153}exp(-15.1/RT) cm{sup 3}molecule{sup -1}s{sup -1}, where R = 1.987 cal mole{sup -1} K{sup -1}, for temperatures between 200 and 2400 K. This prediction is in good agreement with the converted experimental data for all but the one measurement at 200 K. Calculations for the triplet abstraction channel suggest that it is unimportant. Methyl umbrella mode variations have surprisingly little effect on the predicted rate coefficients

EStokTP: Electronic Structure to Temperature- and Pressure-Dependent Rate Constants-A Code for Automatically Predicting the Thermal Kinetics of Reactions

A priori rate predictions for gas phase reactions have undergone a gradual but dramatic transformation, with current predictions often rivaling the accuracy of the best available experimental data. The utility of such kinetic predictions would be greatly magnified if they could more readily be implemented for large numbers of systems. Here, we report the development of a new computational environment, namely, EStokTP, that reduces the human effort involved in the rate prediction for single channel reactions essentially to the specification of the methodology to be employed. The code can also be used to obtain all the necessary master equation building blocks for more complex reactions. In general, the prediction of rate constants involves two steps, with the first consisting of a set of electronic structure calculations and the second in the application of some form of kinetic solver, such as a transition state theory (TST)-based master equation solver. EStokTP provides a fully integrated treatment of both steps through calls to external codes to perform first the electronic structure and then the master equation calculations. It focuses on generating, extracting, and organizing the necessary structural properties from a sequence of calls to electronic structure codes, with robust automatic failure recovery options to limit human intervention. The code implements one or multidimensional hindered rotor treatments of internal torsional modes (with automated projection from the Hessian and with optional vibrationally adiabatic corrections), Eckart and multidimensional tunneling models (such as small curvature theory), and variational treatments (based on intrinsic reaction coordinate following). This focus on a robust implementation of high-level TST methods allows the code to be used in high accuracy studies of large sets of reactions, as illustrated here through sample studies of a few hundred reactions. At present, the following reaction types are implemented in EStokTP: abstraction, addition, isomerization, and beta-decomposition. Preliminary protocols for treating barrierless reactions and multiple-well and/or multiple-channel potential energy surfaces are also illustrated

Roaming radicals in the thermal decomposition of dimethyl ether: Experiment and theory

The thermal dissociation of dimethyl ether has been studied with a combination of reflected shock tube experiments and ab initio dynamics simulations coupled with transition state theory based master equation calculations. The experiments use the extraordinary sensitivity provided by H-atom ARAS detection with an unreversed light source to measure both the total decomposition rate and the branching to radical products versus molecular products, with the molecular products arising predominantly through roaming according to the theoretical analysis. The experimental observations also provide a measure of the rate coefficient for H + CH3OCH3. An evaluation of the available experimental results for H + CH3OCH3 can be expressed by a three parameter Arrhenius expression as, k = 6.54 x 10-24T4.13 exp(-896/T)cm3 molecule-1s-1(273-1465 K) The potential energy surface is explored with high level ab initio electronic structure theory. The dynamics of roaming versus radical formation is studied with a reduced dimensional trajectory approach. The requisite potential energy surface is obtained from an interpolative moving least squares fit to wide-ranging ab initio data for the long-range interactions between methyl and methoxy. The predicted roaming and radical micro-canonical fluxes are incorporated in a master equation treatment of the temperature and pressure dependence of the dissociation process. The tight (i.e., non-roaming) transition states leading to a variety of additional molecular fragments are also included in the master equation analysis, but are predicted to have a negligible contribution to product formation. The final theoretical results reliably reproduce the measured dissociation rate to radical products reported here and are well reproduced over the 500–2000 K temperature range and the 0.01–300 bar pressure range by the following modified Arrhenius parameters for the Troe falloff format: k1,∞(T)= 2.33 x 1019T-0.661exp(-42345/T)s-1 k1,0(T) = 2.86 x 1035T-11.4exp(-46953/T)cm3 molecule-1s-1 Fcent(T)= exp(-T/880) The experimentally observed branching ratio of 0.19 ± 0.07 provides a direct measure of the contribution from the roaming radical mechanism. The theoretical analysis predicts a much smaller roaming contribution of 0.02

First-Principles Chemical Kinetic Modeling of Methyl <i>trans</i>-3-Hexenoate Epoxidation by HO₂

The design of innovative combustion processes relies on a comprehensive understanding of biodiesel oxidation kinetics. The present study aims at unraveling the reaction mechanism involved in the epoxidation of a realistic biodiesel surrogate, methyl <i>trans</i>-3-hexenoate, by hydroperoxy radicals using a bottom-up theoretical kinetics methodology. The obtained rate constants are in good agreement with experimental data for alkene epoxidation by HO₂. The impact of temperature and pressure on epoxidation pathways involving H-bonded and non-H-bonded conformers was assessed. The obtained rate constant was finally implemented into a state-of-the-art detailed combustion mechanism, resulting in fairly good agreement with engine experiments