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

Based on a small test system, (R)-CH(OH)(OO·)CH2CHO, 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.