Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory

Embargo End Date
2021-07-14

Type
Article

Authors
Grajales Gonzalez, Edwing
Monge Palacios, Manuel
Sarathy, Mani

KAUST Department
Chemical Engineering Program
Clean Combustion Research Center
Combustion and Pyrolysis Chemistry (CPC) Group
Physical Science and Engineering (PSE) Division

KAUST Grant Number
OSR-2016-CRG5-3022

Online Publication Date
2020-07-14

Print Publication Date
2020-08-06

Date
2020-07-14

Abstract
The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory [J. Am. Chem. Soc. 2016, 138, 2690] is suitable to determine rate constants below the high-pressure limit. Its current implementation allows incorporating variational effects, multi-dimensional tunneling, and multi-structural torsional anharmonicity in rate constant calculations. Master equation solvers offer more rigorous approach to compute pressure-dependent rate constant, but several implementations available in the literature do not incorporate the aforementioned effects. However, SS-QRRK theory coupled with a formulation of the modified strong collision model underestimates the value of unimolecular pressure-dependent rate constants in the high temperature regime for reactions involving large molecules. This underestimation is a consequence of the definition for collision efficiency, which is part of the energy transfer model. The selection of the energy transfer model and its parameters constitute a common issue in pressure-dependent calculations. To overcome this underestimation problem, we evaluated and implemented in a bespoke Python code two alternative definitions for the collision efficiency using the SS-QRRK theory, and tested their performance by comparing the pressure-dependent rate constants with Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) results. The modeled systems were the tautomerization of propen-2-ol and the decomposition of 1-propyl, 1-butyl, and 1-pentyl radicals. One of the tested definitions, which Dean et al. explicitly derived [Z. Phys. Chem. 2000, 214, 1533], corrected the underestimation of the pressure-dependent rate constants and, in addition, qualitatively reproduced the trend of RRKM/ME data. Therefore, the used SS-QRRK theory with accurate definitions for the collision efficiency can yield results that are in agreement with those from more sophisticated methodologies such as RRKM/ME.

Citation
Grajales-González, E., Monge-Palacios, M., & Sarathy, S. M. (2020). Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory. The Journal of Physical Chemistry A. doi:10.1021/acs.jpca.0c02943

Acknowledgements
This work was supported by King Abdullah University of Science and Technology (KAUST), Office of Sponsored Research (OSR) under Award No. OSR-2016-CRG5-3022. We appreciate the resources of the Supercomputing Laboratory at KAUST.

Publisher
American Chemical Society (ACS)

Journal
The Journal of Physical Chemistry A

DOI
10.1021/acs.jpca.0c02943

Additional Links
https://pubs.acs.org/doi/10.1021/acs.jpca.0c02943

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