Comment on "Influence of Multiple Conformations and Paths on Rate

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Comment on "Influence of Multiple Conformations and Paths on Rate Constants and Product Branching Ratios. Thermal Decomposition of 1-Propanol Radicals" by Ferro-Costas et al. Judit Zádor, and James A. Miller J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12016 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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The Journal of Physical Chemistry

Comment on "Influence of Multiple Conformations and Paths on Rate Constants and Product Branching Ratios. Thermal Decomposition of 1‑Propanol Radicals" by Ferro-Costas et al.

Judit Zádor1* and James A. Miller2 1

Combustion Research Facility, MS 9055, Sandia National Laboratories, Livermore, CA

94551-0969, USA 2

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne,

IL 60439, USA

In their paper titled "Influence of Multiple Conformations and Paths on Rate Constants and Product Branching Ratios. Thermal Decomposition of 1‑Propanol Radicals", FerroCostas et al.1 raised the important question of the role of conformers, and especially of

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configurational entropy, in theoretical kinetics calculations. They calculated and characterized part of the potential energy surface of the propene + OH reaction and compared the effect of various treatments of the conformers on the phenomenological rate coefficients. As a point of reference and comparison, they used the results from the papers of Zádor et al.2-3 on the same system, propene + OH. In this comment we would like to correct some errors in the way our work is described in the article by Ferro-Costas et al.

1. Ferro-Costas et al.1 describe an approach termed "1W", which means that only the lowest energy conformer of each chemically identical stationary point (well or saddle point) is considered in a rigid-rotor (external) and harmonic-oscillator framework to calculate sums and densities of states, as phrased in the paper: "taking into account only the most stable stationary points (reactant and transition state) of each path".1 The paper states that our published calculations are based on such a "1W" approach and makes extensive comparisons in the text and in tables with this assumption in mind. This is a misunderstanding that we would like to correct. As a direct quote from our original paper:3 "In this work the low-frequency torsional modes were treated as hindered rotors using the Pitzer–Gwinn4 approach applied to state densities". This means that we scanned all torsional angles separately (relaxed scans) starting from the lowest energy conformer each time, recorded the resulting potential energy curves, and fitted them with a Fourier-like series using 6 sine and 6 cosine terms. In Variflex the classical density of


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states is calculated with numerical integration.5 This is then corrected by the ratio of the quantum and the classical partition functions for the harmonic oscillator that corresponds to the hindered rotor at the lowest energy conformer. The frequency (or frequencies, if there are multiple rotors) corresponding to the hindering motions are removed from the set of harmonic frequencies. Our results2-3,

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show that this methodology is suited to

represent this complex multi-species and multi-conformer potential energy surface in a tractable way. When applied correctly and in conjunction with high-level ab initio data, the comparison between experiments and theory shows the power of this method.2-3, 6 In effect, this approach largely captures the other conformers via these hindering potentials for systems with not too many rotors that are not too strongly coupled. We agree that our methodology from a decade ago can be improved upon by more explicit representations of the conformers.

2. Another aspect of our previous work is also mischaracterized in the paper of FerroCostas et al.: "As mentioned before, they calculated 1W rate constants with the Variflex program, using an RRKM-based master equation and performing fittings to the species time profiles obtained from the master equation."1 We would like to correct this second misunderstanding of our previous work. We have not obtained the rate coefficients by fitting; rather, we diagonalized the transition matrix of the master equation and related the eigenvectors and eigenvalues, in a rigorous framework, to phenomenological rate coefficients.7-11 Fitting the time profiles of species is a method that we especially do not


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think is appropriate for multiwell systems particularly at high temperatures and/or low pressures.12

Both these issues are critical to the accurate theoretical prediction of phenomenological rate coefficients of “elementary” reactions. They continue to be topics of active discussion in the literature. Consequently, it is important to us to correct the record on our handling of these points.

AUTHOR INFORMATION Corresponding Author * Corresponding author. E-mail: [email protected], 1 (925) 294-3603, Address: 7011 East Ave, Livermore, CA 94550, USA. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was funded by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). JZ was funded under DOE BES, the Division of Chemical Sciences,


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Geosciences, and Biosciences. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DENA0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. REFERENCES 1. Ferro-Costas, D.; Martínez-Núñez, E.; Rodríguez-Otero, J.; Cabaleiro-Lago, E.; Estévez, C. M.; Fernández, B.; Fernández-Ramos, A.; Vázquez, S. A., Influence of Multiple Conformations and Paths on Rate Constants and Product Branching Ratios. Thermal Decomposition of 1-Propanol Radicals. J. Phys. Chem. A 2018, 122 (21), 4790-4800. 2. Zádor, J.; Miller, J. A., Unimolecular dissociation of hydroxypropyl and propoxy radicals. Proc. Combust. Inst. 2013, 34, 519-526. 3. Zádor, J.; Jasper, A. W.; Miller, J. A., The reaction between propene and hydroxyl. Phys. Chem. Chem. Phys. 2009, 11 (46), 11040-11053. 4. Pitzer, K. S.; Gwinn, W. D., Energy levels and thermodynamic functions for molecules with internal rotation. I. Rigid frame with attached tops. J. Chem. Phys. 1942, 10 (7), 428-440. 5. Klippenstein, S. J.; Wagner, A. F.; Dunbar, R. C.; Wardlaw, D. M.; Robertson, S. H.; Miller, J. A. VARIFLEX, 2.0m; 2008. 6. Kappler, C.; Zádor, J.; Welz, O.; Fernandes, R. X.; Olzmann, M.; Taatjes, C. A., Competing channels in the propene + OH reaction: Experiment and validated modeling over a broad temperature and pressure range. Z. Phys. Chem. 2011, 225, 1271-1293. 7. Miller, J. A.; Klippenstein, S. J., Master equation methods in gas phase chemical kinetics. J. Phys. Chem. A 2006, 110 (36), 10528-10544. 8. Miller, J. A.; Klippenstein, S. J., From the multiple-well master equation to phenomenological rate coefficients: Reactions on a C3H4 potential energy surface. J. Phys. Chem. A 2003, 107 (15), 2680-2692. 9. Miller, J. A.; Klippenstein, S. J.; Robertson, S. H.; Pilling, M. J.; Green, N. J. B., Detailed balance in multiple-well chemical reactions. Phys. Chem. Chem. Phys. 2009, 11 (8), 1128-1137. 10. Klippenstein, S. J.; Miller, J. A., From the time-dependent, multiple-well master equation to phenomenological rate coefficients. J. Phys. Chem. A 2002, 106 (40), 9267-9277.


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11. Miller, J. A.; Klippenstein, S. J., Determining phenomenological rate coefficients from a time-dependent, multiple-well master equation: “species reduction” at high temperatures. Phys. Chem. Chem. Phys. 2013, 15 (13), 4744-4753. 12. Miller, J. A.; Klippenstein, S. J.; Robertson, S. H.; Pilling, M. J.; Shannon, R.; Zádor, J.; Jasper, A. W.; Goldsmith, C. F.; Burke, M. P., "Comment on “When Rate Constants Are Not Enough” by John R. Barker, Michael Frenklach, and David M. Golden". J. Phys. Chem. A 2016, 120, 306-312.


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Comment on "Influence of Multiple Conformations and Paths on Rate

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