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

Area of Science:

• Chemical Kinetics
• Theoretical Chemistry
• Computational Chemistry

Background:

• The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory is a method for calculating rate constants below the high-pressure limit.
• Current SS-QRRK implementations can include variational effects, multidimensional tunneling, and multistructural torsional anharmonicity.
• Master equation solvers provide a rigorous approach for pressure-dependent rate constants but often lack these effects.

Purpose of the Study:

• To address the underestimation of unimolecular pressure-dependent rate constants by SS-QRRK theory at high temperatures for large molecules.
• To evaluate and implement alternative collision efficiency definitions within the SS-QRRK framework.
• To compare the performance of modified SS-QRRK theory against established Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) results.

Main Methods:

• Implemented two alternative collision efficiency definitions in a custom Python code based on SS-QRRK theory.
• Applied the modified SS-QRRK theory to model the tautomerization of propen-2-ol and the decomposition of alkyl radicals (propyl, butyl, pentyl).
• Validated the results by comparing pressure-dependent rate constants with RRKM/ME calculations.

Main Results:

• One tested collision efficiency definition, derived by Dean et al., successfully corrected the underestimation of rate constants.
• This improved SS-QRRK approach qualitatively reproduced the trends observed in RRKM/ME data.
• The study demonstrated that SS-QRRK theory, with appropriate collision efficiency models, can achieve results comparable to more complex methods.

Conclusions:

• Accurate definitions of collision efficiency are crucial for the reliability of SS-QRRK theory in pressure-dependent rate constant calculations.
• The modified SS-QRRK theory offers a viable and accurate alternative to RRKM/ME for specific chemical systems.
• This work enhances the applicability of SS-QRRK theory for complex chemical reactions, particularly at elevated temperatures.