Comprehensive Theoretical Study on Four Typical Intramolecular Hydrogen Shift Reactions of Peroxy Radicals: Multireference Character, Recommended Model Chemistry, and Kinetics

Abstract

Intramolecular hydrogen shift reactions in peroxy radicals (RO 2 • → • QOOH) play key roles in the low-temperature combustion and in the atmospheric chemistry. In the present study, we found that a mild-to-moderate multireference character of a potential energy surface (PES) is widely present in four typical hydrogen shift reactions of peroxy radicals (RO 2 • , R = ethyl, vinyl, formyl methyl, and acetyl) by a systematic assessment based on the T 1 diagnostic, %TAE diagnostic, M diagnostic, and contribution of the dominant configuration of the reference CASSCF wavefunction (C 0 2). To assess the effects of these inherent multireference characters on electronic structure calculations, we compared the PESs of the four reactions calculated by the multireference method CASPT2 in the complete basis set (CBS) limit, single-reference method CCSD(T)-F12, and single-reference-based composite method WMS. The results showed that ignoring the multireference character will introduce a mean unsigned deviation (MUD) of 0.46−1.72 kcal/mol from CASPT2/CBS results by using the CCSD(T)-F12 method or a MUD of 0.49−1.37 kcal/mol by WMS for three RO 2• reactions (R = vinyl, formyl methyl, and acetyl) with a stronger multireference character. Further tests by single-reference Kohn−Sham (KS) density functional theory methods showed even larger deviations. Therefore, we specifically developed a new hybrid meta-generalized gradient approximation (GGA) functional M06-HS for the four typical H-shift reactions of peroxy radicals based on the WMS results for the ethyl peroxy radical reaction and on the CASPT2/CBS results for the others. The M06-HS method has an averaged MUD of 0.34 kcal/mol over five tested basis sets against the benchmark PESs, performing best in the tested 38 KS functionals. Last, in a temperature range of 200−3000 K, with the new functional, we calculated the high-pressure-limit rate coefficients of these H-shift reactions by the multi-structural variational transition-state theory with the small-curvature tunneling approximation (MS-CVT/SCT) and the thermochemical properties of all of the involved key radicals by the multi-structural torsional (MS-T) anharmonicity approximation method.

Stereo-electronic factors influencing the stability of hydroperoxyalkyl radicals: transferability of chemical trends across hydrocarbons and ab initio methods

“…A recent study on four intramolecular H-shift reactions of peroxy radicals also found MN15L and MN12L to perform better compared to other DFT methods. 77 However, in our study, MN12L shows a large MAD of about 6 kcal mol −1 without dressed-atom corrections. For an overall performance considering both the reaction energies and reaction barriers, G4 has the lowest error of 0.88 kcal mol −1 among cWFTs.…”

“…This journal is © the Owner Societies 2023 combination with the VDZ-F12 basis set, which has been extensively used to study species involved in low-temperature combustion. 20,[77][78][79] The F12 method is an explicitly correlated version of the canonical CCSD(T) method involving a factor that depends on the explicit two-electron distance, r 12 .…”

“…This observation corroborates the extensive use of CCSD(T)-F12a/cc-pVDZ-F12 in the studies of combustion reactions by others. 20,[77][78][79] The performance of other methods is described below in detail.…”

Hydrazine Hydrate-Initiated GO/PAN Composite Fiber Cyclization Reaction to Achieve Chemical Stabilization of PAN

“…The Gaussian 09 set of programs package was used for all of the calculations. The geometries of reactants, transition states, and products were fully optimized using density functional theory (DFT) method at the level of Becke3LYP (B3LYP) with a 6-31+G (d, p) basis set. − Vibrational frequency analysis was used to confirm that the reactant or product has no imaginary frequency, while the transition state exhibits only one imaginary frequency. Intrinsic reaction coordinate (IRC) was calculated to verify that transition states can link substrates and products.…”