Predicting pressure-dependent unimolecular rate constants using variational transition state theory with multidimensional tunneling combined with system-specific quantum RRK theory: a definitive test for fluoroform dissociation

Abstract

Understanding the falloff in rate constants of gas-phase unimolecular reaction rate constants as the pressure is lowered is a fundamental problem in chemical kinetics, with practical importance for combustion, atmospheric chemistry, and essentially all gas-phase reaction mechanisms. In the present work, we use our recently developed system-specific quantum RRK theory, calibrated by canonical variational transition state theory with small-curvature tunneling, combined with the Lindemann-Hinshelwood mechanism, to model the dissociation reaction of fluoroform (CHF3), which provides a definitive test for falloff modeling. Our predicted pressure-dependent thermal rate constants are in excellent agreement with experimental values over a wide range of pressures and temperatures. The present validation of our methodology, which is able to include variational transition state effects, multidimensional tunneling based on the directly calculated potential energy surface along the tunneling path, and torsional and other vibrational anharmonicity, together with state-of-the-art reaction-path-based direct dynamics calculations, is important because the method is less empirical than models routinely used for generating full mechanisms, while also being simpler in key respects than full master equation treatments and the full reduced falloff curve and modified strong collision methods of Troe.

Hydrogen shift isomerizations in the kinetics of the second oxidation mechanism of alkane combustion. Reactions of the hydroperoxypentylperoxy OOQOOH radical

“…It includes the contributions from all the conformational structures of the reactant (R) and the transition state (TS) and is computed by the coupled-potential MS-T method as described previously [27][28][29]35]. Please notice that, in this work we are using the complete MS-T method, but for larger systems with many of internal torsions, the dual-level MS-T method could be much more efficient [36].…”

“…Unimolecular dissociation with multiple parallel dissociation reactions can be described by using the SS-QRRK method [27] with the following thermal activation mechanism [28][29][30]:…”

“…These rate calculations include multiple-structure anharmonicity and torsional potential anharmonicity. To estimate the pressure effects, we use system-specific quantum Rice−Ramsperger−Kassel (SS-QRRK) theory [27] with the thermal activation mechanism [28][29][30] to compute the thermal rate constants as functions of pressure. The thermal activation mechanism assumes that OOQOOH is thermally equilibrated before it reacts (as opposed to reaction of hot nascent molecules, which would be the chemical activation mechanism).…”

Relative Rates of Hydrogen Shift Isomerizations Depend Strongly on Multiple-Structure Anharmonicity

“…To compute the pressure dependence of the thermal rate constants, we use system-specific quantum Rice−Ramsperger−Kassel (SS-QRRK) theory 37 with the thermal activation mechanism. [38][39][40]…”

Barrierless association of CF 2 and dissociation of C 2 F 4 by variational transition-state theory and system-specific quantum Rice–Ramsperger–Kassel theory

“…Then, after the calculation of the high-pressure rate constant, the pressuredependent dissociation rate constants are computed from the high-pressure rate constants using our recently proposed system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory with the Lindemann-Hinshelwood mechanism (30)(31)(32). SS-QRRK theory is able to effectively incorporate variational effects, anharmonicity, and multidimensional tunneling into the microcanonical rate constants needed to calculate pressure effects with negligible additional computational cost over and above the previous step of calculating high-pressure unimolecular rate constants.…”