Comprehensive Theoretical Study on Four Typical Intramolecular Hydrogen Shift Reactions of Peroxy Radicals:...

Area of Science:

• Computational Chemistry
• Chemical Kinetics
• Atmospheric Chemistry

Background:

• Intramolecular hydrogen shift reactions in peroxy radicals (RO2• → •QOOH) are vital for low-temperature combustion and atmospheric chemistry.
• The multireference character of potential energy surfaces (PES) in these reactions can impact computational accuracy.

Purpose of the Study:

• To systematically assess the multireference character of PES for four typical peroxy radical hydrogen shift reactions.
• To evaluate the performance of single-reference and multireference electronic structure methods for these reactions.
• To develop and validate a new density functional (M06-HS) for accurate calculations of these reactions.

Main Methods:

• Systematic assessment of multireference character using diagnostics like T1, %TAE, M, and C02.
• Comparison of PES calculations using CASPT2/CBS, CCSD(T)-F12, WMS, and various Kohn-Sham (KS) density functional theory methods.
• Development of the M06-HS functional based on benchmark CASPT2/CBS and WMS results.
• Calculation of rate coefficients and thermochemical properties using multi-structural variational transition-state theory (MS-CVT/SCT) and multi-structural torsional (MS-T) methods.

Main Results:

• Mild-to-moderate multireference character was widely present in the studied peroxy radical hydrogen shift reactions.
• Single-reference methods (CCSD(T)-F12, WMS) and KS-DFT methods showed significant deviations from multireference CASPT2/CBS results.
• The newly developed M06-HS functional demonstrated superior performance with an averaged MUD of 0.34 kcal/mol against benchmark PESs, outperforming 38 other KS functionals.
• High-pressure-limit rate coefficients and thermochemical properties were calculated using MS-CVT/SCT and MS-T methods with the M06-HS functional.

Conclusions:

• The multireference character of peroxy radical hydrogen shifts necessitates accurate computational approaches.
• The M06-HS functional provides a reliable and accurate method for studying these important reactions.
• The calculated kinetic and thermochemical data using M06-HS can improve low-temperature combustion and atmospheric chemistry models.