Relative rates of hydrogen shift isomerizations depend strongly on multiple-structure anharmonicity
Type
ArticleDate
2018-11-23Online Publication Date
2018-11-23Print Publication Date
2018-12-19Permanent link to this record
http://hdl.handle.net/10754/630174
Metadata
Show full item recordAbstract
Hydroperoxyalkylperoxy species (OOQOOH) are important intermediates that are generated during the autoignition of transport fuels. A key reaction of hydroperoxyalkylperoxy radicals is a [1,5] hydrogen shift, for which kinetics data are experimentally unavailable. Here we study two typical OOQOOH reactions and compare their kinetics to one another and to a previous study to learn the effect of structural variations of the alkyl group on the competition between alternative [1,5] hydrogen shifts of hydroperoxyalkylperoxy species. We use electronic structure calculations to determine previously missing thermochemical data, and we use variational transition state theory (VTST) with multidimensional tunneling (MT), multiple structures, torsional potential anharmonicity, and high-frequency anharmonicity to obtain more accurate rate constants than the ones that can computed by conventional single-structure harmonic transition state theory (TST) and than the empirically estimated rate constants that are currently used in combustion modeling. The calculated temperature range is 298−1500 K. The roles of various factors in determining the rates are elucidated, and we find an especially strong effect of multiple structure anharmonicity due to torsions. Thus, even though there is some cancellation between the anharmonicity of the reactant and the anharmonicity of the transition state, and even though the reactants are very similar in structure, differing only by a methyl group, the effect of multiple structure anharmonicity has a large effect on the relative rates – as large as a factor of 17 at room temperature and as large as a factor of 7 at 1500 K. This has broad implications for the estimation of reaction rates in many subfields of chemistry, including combustion chemistry and atmospheric chemistry, where rates of reaction of complex molecules are usually estimated without explicit consideration of this fundamental entropic effect. In addition, the pressure-dependence of the rate constants is modelled by system-specific quantum RRK theory.Citation
Xing L, Bao JL, Wang Z, wang xuetao, Truhlar DG (2018) Relative rates of hydrogen shift isomerizations depend strongly on multiple-structure anharmonicity. Journal of the American Chemical Society. Available: http://dx.doi.org/10.1021/jacs.8b09381.Sponsors
This work was supported in part by Key Science Foundation of Higher Education of Henan (19A480002), by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award Number DE-SC0015997. The authors are grateful to Feng Zhang for helpful discussions.Publisher
American Chemical Society (ACS)Additional Links
https://pubs.acs.org/doi/10.1021/jacs.8b09381ae974a485f413a2113503eed53cd6c53
10.1021/jacs.8b09381