Tunneling in the 1,5 H-migration reaction of the prototypical O_2QOOH: ·OOCH_2CH_2CH_2OOH

摘要

Autoignition of diesel fuel depends on a sequence of reactions leading to radical chain propagation and chain branching. The precursor to chain branching is the 1,5 H-migration in O_2QOOH, e.g., ·OOCH_2CH_2CH_2OOH → HOOCH_2CH_2CH=O + OH (1) Multidimensional tunneling is used to examine the extent of tunneling in this reaction, and its dependence on assumptions about the stability of a putative HOOCH_2CH_2·HOOH intermediate. Tunneling corrections to the transition state theory (TST) rate constant were carried out in the framework of the small curvature tunneling (SCT) approximation, which is a semiclassical approach that gives accurate results for unimolecular H-migration reactions. In addition, the quantum mechanical rate constant was directly computed using the fully coupled anharmonic semi-classical transition state theory (SC-TST) approach of Miller, as recently implemented in the MultiWell program suite. This approach has a technical advantage over SCT in that, like TST, it only requires characterization of the reactant and transition state, rather than characterization of a significant section of the reaction path. SCTST calculations indicate significant effects of coupled anharmonicity at relevant temperatures, leading to increases in the rate constant of a factor of 8-10 versus that computed without anharmonicity. SC-TST tunneling coefficients were inferred from comparing SC-TST rate constants with classical rate constants computed by omitting coupling of the reaction coordinate with other modes. Agreement was good at temperatures relevant to combustion and autoignition. Unfortunately, agreement was very poor at room temperature. Assumptions about the stability of the putative HOOCH_2CH_2C·HOOH intermediate have little effect on the predicted tunneling corrections at relevant temperatures. Tunneling corrections, κ(T), computed using the inexpensive Eckart approximation closely approximates SCT predictions at temperatures relevant to autoignition and combustion.