Cost-Effective Implementation of Multiconformer Transition State Theory for Peroxy Radical Hydrogen Shift Reactions

Abstract

Based on a small test system, (R)-CH(OH)(OO·)CHCHO, we have developed a cost-effective approach to the practical implementation of multiconformer transition state theory for peroxy radical hydrogen shift reactions at atmospherically relevant temperatures. While conformer searching is crucial for accurate reaction rates, an energy cutoff can be used to significantly reduce the computational cost with little loss of accuracy. For the reaction barrier, high-level calculations are needed, but the highest level of electronic structure theory is not necessary for the relative energy between conformers. Improving the approach to both transition state theory and electronic structure theory decreases the calculated reaction rate significantly, so low-level calculations can be used to rule out slow reactions. Further computational time can be saved by approximating the tunneling coefficients for each transition state by only that of the lowest-energy transition state. Finally, we test and validate our approach using higher-level theoretical values for our test system and existing experimental results for additional peroxy radical hydrogen shift reactions in three slightly larger systems.

Atmospheric autoxidation is increasingly important in urban and suburban North America

“…We calculate the rate constants of the H-shift reactions using multiconformer transition state theory (MC-TST) (53)(54)(55)(56). The MC-TST expression for a rate constant is given by the following:…”

“…Here, we use the 1D Eckart tunneling approximation, which takes the forward and reverse barrier height and the imaginary frequency of the TS as input (57). The partition functions, energies, barrier heights, and imaginary frequencies needed to calculate k MC-TST are obtained following the approach described by Møller et al (56). Briefly, ωB97X-D/aug-cc-pVTZ was used for the geometries, frequencies, partition functions, zero-point energy corrections, and relative energies between unique conformers.…”

Assessment of model chemistries for hydrofluoropolyethers: A DFT/M08‐HX benchmark study

“…Note that the treatment of the conformers is performed within the usual framework of MC‐TST . This excludes, for example, the consideration of conformational tunneling, whose evaluation would be computationally daunting to perform.…”

Computational study on the mechanism and kinetics for the reaction between HO2 and n-propyl peroxy radical

“…28,29 Since that the n-C 3 H 7 O 2 has multiple conformers, such complexity should be taken account into the rate constant formula, multiconformer transition state theory (MC-TST) method were carried out for rate constant calculations. [30][31][32][33] Within the framework of the conventional transition state theory (TST), [34][35][36] the MC-TST expression for a rate constant k at temperature T can be written as…”