Resolving Discrepancies between State-of-the-Art Theory and Experiment for HO₂ + HO₂ via Multiscale Informatics

Recent high-level theoretical calculations predict a mild temperature dependence for HO2 + HO2 inconsistent with state-of-the-art experimental determinations that upheld the stronger temperature dependence observed in early experiments. Via MultiScale Informatics analysis of the theoretical and experimental data, we identified an alternative interpretation of the raw experimental data that uses HO2 + HO2 rate constants nearly identical to theoretical predictionsimplying that the theoretical and experimental data are actually consistent, at least when considering the raw data from experimental studies. Similar analyses of typical signals from low-temperature experiments indicate that an HOOOOH intermediateidentified by recent theory but absent from earlier interpretationsyields modest effects that are smaller than, but may have contributed to, the scatter in data among different experiments. More generally, the findings demonstrate that modern chemical theories and experiments have progressed to a point where meaningful comparison requires joint consideration of their data simultaneously

Comment on “When Rate Constants Are Not Enough”

Comment on “When Rate Constants Are Not Enough

Multiscale Informatics for Low-Temperature Propane Oxidation: Further Complexities in Studies of Complex Reactions

The present paper describes further development of the multiscale informatics approach to kinetic model formulation of Burke et al. (Burke, M. P.; Klippenstein, S. J.; Harding, L. B. <i>Proc. Combust. Inst.</i> <b>2013</b>, <i>34</i>, 547–555) that directly incorporates elementary kinetic theories as a means to provide reliable, physics-based extrapolation of kinetic models to unexplored conditions. Here, we extend and generalize the multiscale informatics strategy to treat systems of considerable complexityinvolving multiwell reactions, potentially missing reactions, nonstatistical product branching ratios, and non-Boltzmann (i.e., nonthermal) reactant distributions. The methodology is demonstrated here for a subsystem of low-temperature propane oxidation, as a representative system for low-temperature fuel oxidation. A multiscale model is assembled and informed by a wide variety of targets that include <i>ab initio</i> calculations of molecular properties, rate constant measurements of isolated reactions, and complex systems measurements. Active model parameters are chosen to accommodate both “parametric” and “structural” uncertainties. Theoretical parameters (e.g., barrier heights) are included as active model parameters to account for parametric uncertainties in the theoretical treatment; experimental parameters (e.g., initial temperatures) are included to account for parametric uncertainties in the physical models of the experiments. RMG software is used to assess potential structural uncertainties due to missing reactions. Additionally, branching ratios among product channels are included as active model parameters to account for structural uncertainties related to difficulties in modeling sequences of multiple chemically activated steps. The approach is demonstrated here for interpreting time-resolved measurements of OH, HO₂, <i>n</i>-propyl, <i>i</i>-propyl, propene, oxetane, and methyloxirane from photolysis-initiated low-temperature oxidation of propane at pressures from 4 to 60 Torr and temperatures from 300 to 700 K. In particular, the multiscale informed model provides a consistent quantitative explanation of both <i>ab initio</i> calculations and time-resolved species measurements. The present results show that interpretations of OH measurements are significantly more complicated than previously thoughtin addition to barrier heights for key transition states considered previously, OH profiles also depend on additional theoretical parameters for R + O₂ reactions, secondary reactions, QOOH + O₂ reactions, and treatment of non-Boltzmann reaction sequences. Extraction of physically rigorous information from those measurements may require more sophisticated treatment of all of those model aspects, as well as additional experimental data under more conditions, to discriminate among possible interpretations and ensure model reliability