(121h) Variable Reaction Coordinate Transition State Theory for Computing Rate Constants of Barrierless Reactions on Catalytic Surfaces

Harmonic Transition State Theory (HTST) has long served as a reliable method for calculating reaction rate constants in systems where a well-defined energy barrier exists. However, its accuracy decreases significantly for reactions without a first-order saddle point (barrierless reactions) such as gas-phase homolysis or surface desorption processes. To address this limitation, Variational Transition State Theory (VTST) was developed. VTST offers improved flexibility by allowing the transition state to vary along the reaction coordinate, enabling more accurate predictions.

In this study, we focus on the Variable Reaction Coordinate Transition State Theory (VRC-TST), a form of VTST that is especially useful for treating multi-dimensional transition states. We introduce ROTDpy, an open-source Python software package designed to compute rate constants with high accuracy. Originally developed for radical–radical recombination reactions in the gas phase, we have extended the methodology and the software to handle barrierless adsorption and desorption reactions on various surfaces.

We applied ROTDpy to calculate rate constants for the barrierless desorption of carbon monoxide (CO) and nitrogen monoxide (NO) from a platinum surface. We then compared our results with those obtained using the Potential of Mean Force (PMF) approach that has been used by Doren and Tully. The strong agreement between the two approaches supports the accuracy of our computational framework. This result also highlights the potential of ROTDpy as a flexible and powerful tool for studying surface reactions that are not well described by the conventional transition state theory.[1,2]

[1] Doren D. J., Tully J. C., Dynamics of precursor-mediated chemisorption. J. Chem. Phys. 94, 8428 (1991).

[2] Doren D. J., Tully J. C., Precursor-mediated adsorption and desorption: A theoretical analysis. Langmuir 4, 256 (1988).