Publications

Predictive simulation for designing efficient engines requires detailed modeling of combustion chemistry, for which the possibility of unknown pathways is a continual concern. Here, we characterize a low-lying water elimination pathway from key hydroperoxyalkyl (QOOH) radicals derived from alcohols. The corresponding saddle-point structure involves the interaction of radical and zwitterionic electronic states. This interaction presents extreme difficulties for electronic structure characterizations, but we demonstrate that these properties of this saddle point can be well captured by M06-2X and CCSD(T) methods. Experimental evidence for the existence and relevance of this pathway is shown in recently reported data on the low-temperature oxidation of isopentanol and isobutanol. In these systems, water elimination is a major pathway, and is likely ubiquitous in low-temperature alcohol oxidation. These findings will substantially alter current alcohol oxidation mechanisms. Moreover, the methods described will be useful for the more general phenomenon of interacting radical and zwitterionic states. © 2013 American Chemical Society.

Unimolecular pressure- and temperature-dependent decomposition rate coefficients of radicals derived from n- and i-propanol by H-atom abstraction are calculated using a time-dependent master equation in the 300-2000 K temperature range. The calculations are based on a C3H7O potential energy surface, which was previously tested successfully for the propene + OH reaction. All rate coefficients are obtained with internal consistency with particular attention paid to shallow wells. After minor adjustments very good agreement with the few available experimental results is obtained. Several interesting pathways are uncovered, such as the catalytic dehydration, well-skipping reactions and reactions forming enols. The results of the calculations can be readily used in CHEMKIN simulations or to assess important channels for higher alcohols. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Theoretical methods to obtain rate coefficients are essential to fundamental combustion chemistry research, yet the associated uncertainties are largely unexplored in a systematic manner. In this paper we focus on the study of parametric uncertainties for a hydrogen-atom-abstraction reaction, CH 3CH(OH)CH3 + OH → CH3C (OH)CH3 + H2O, which bears significant importance in low-temperature alcohol combustion and especially in autoignition models. After identifying the parameters causing significant uncertainty in the rate-coefficient calculations, Bayesian inference is employed to determine the joint probability density function (PDF) thereof using the experimental data of Dunlop and Tully (1993) [6] on isopropanol + OH. The inferred PDFs are compared to the various parameter values obtained from high-level electronic-structure calculations in order to assess the limitations of current methodologies. To gain insight on modeling the kinetic isotope effect (KIE), the reaction of the hydroxyl radical with deuterated isopropanol is also investigated. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Optimization of new transportation fuels and engine technologies requires the characterization of the combustion chemistry of a wide range of fuel classes. Theoretical studies of elementary reactions - the building blocks of complex reaction mechanisms - are essential to accurately predict important combustion processes such as autoignition of biofuels. The current bottleneck for these calculations is a user-intensive exploration of the underlying potential energy surface (PES), which relies on the "chemical intuition" of the scientist to propose initial guesses for the relevant chemical configurations. For newly emerging fuels, this approach cripples the rate of progress because of the system size and complexity. The KinBot program package aims to accelerate the detailed chemical kinetic description of combustion, and enables large-scale systematic studies on the sub-mechanism level.

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Optimization of new transportation fuels and engine technologies requires the characterization of the combustion chemistry of a wide range of fuel classes. Theoretical studies of elementary reactions — the building blocks of complex reaction mechanisms — are essential to accurately predict important combustion processes such as autoignition of biofuels. The current bottleneck for these calculations is a user-intensive exploration of the underlying potential energy surface (PES), which relies on the “chemical intuition” of the scientist to propose initial guesses for the relevant chemical configurations. For newly emerging fuels, this approach cripples the rate of progress because of the system size and complexity. The KinBot program package aims to accelerate the detailed chemical kinetic description of combustion, and enables large-scale systematic studies on the sub-mechanism level.

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Although the propene+OH reaction has been in the center of interest of numerous experimental and theoretical studies, rate coefficients have never been determined experimentally between ∼600 and ∼ 750 K, where the reaction is governed by the complex interaction of addition, back-dissociation and abstraction. In this work OH time-profiles are measured in two independent laboratories over a wide temperature region (200-950 K) and are analyzed incorporating recent theoretical results. The datasets are consistent both with each other and with the calculated rate coefficients. We present a simplified set of reactions validated over a broad temperature and pressure range, that can be used in smaller combustion models for propene+OH. In addition, the experimentally observed kinetic isotope effect for the abstraction is rationalized using ab initio calculations and variational transition-state theory. We recommend the following approximate description of the OH+C 3H6 reaction: C3H6+OH⇄C 3H6OH (R1a,R-1a) C3H6+OH→C 3H5+H2O (R1b) k1a(200K ≤ T ≤ 950 K;1 bar ≤ P) = 1.45×10-11 (T/K)-0.18e 460K/Tcm3 molecule-1s-1 k -1a(200 K ≤ T ≤ 950 K; 1 bar ≤ P) = 5.74×10 12e-12690K/Ts-1 k1b(200 K ≤ T ≤ 950 K) = 1.63×10-18 (T/K)2.36e -725K/T cm3 molecule-1s-1. © by Oldenbourg Wissenschaftsverlag, München.

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