Impact of tunneling on hydrogen-migration of the n-propylperoxy radical

Abstract

The kinetics of three unimolecular reactions of the n-propylperoxy radical were studied by canonical variational transition state theory and multidimensional small curvature tunneling (SCT). The reactions studied were 1,5 and 1,4 H-migration, and HO(2) elimination. Benchmark calculations were carried out at the CCSD(T) level in order to determine which density functional to use for SCT calculations for each reaction. For 1,5 and 1,4 H-migration, and HO(2) elimination, the M05-2X, B3LYP and B1B95 functionals, respectively, performed closest to the benchmark when coupled to the 6-311+G(2df,2p) basis set. The SCT tunneling corrections, κ(T), computed here were much larger than those calculated from the Wigner or zero-curvature tunneling treatments at low temperatures, but the asymmetric Eckart method works surprisingly well in these three reactions. Comparison of energy-dependent transmission coefficients, Γ(E), indicates that not only the magnitude, but also the sign, of the error in the Eckart approximation is a function of energy; therefore, the error introduced by using the Eckart approach depends strongly on the steady state energy distribution. These results may provide guidance for future studies of tunneling effects in reactions of other peroxy radicals.

Thermodynamic and kinetic study of ibuprofen with hydroxyl radical: A density functional theory approach

“…The corresponding rate constant using Eckart tunneling correction is 8.48 × 10 9 M −1 s −1 , which is 30.5% higher than the experimental value. It is reported that the Eckart method is often found to overestimate the tunneling corrections . The calculated rate constant using Wigner's method is in a good agreement with the experimental value.…”

Tunneling effect in 1,5 H-migration of a prototypical OOQOOH

“…As is typical, is much broader than the Eckart potential. 8,30 This leads to the expectation that  ZCT will be much smaller than  Eckart . Corner-cutting will tend to increase multi-dimensional tunneling coefficients above  ZCT .…”

“…9 The presence of the -OOH group lowers the barrier to 1,5 Hshift by ~4 kcal/mol as compared to that in 1-propylperoxy radical. 8,9 As indicated in Reaction 1, the radical product of this 1,5 H-migration will fall apart to hydroxyl radical plus a ketohydroperoxide. This radical product is absolutely unstable or only slightly metastable with respect to O-O scission.…”

“…We previously studied tunneling in the reactions of 1propylperoxy radical, which is the prototype for 1,5 H-migration in peroxy radicals. 8 In the present paper we investigate tunneling in the analogous 1,5 H-migration reaction of Reaction (1) is part of the chain propagation/branching pathways indicated in Scheme 1. The competing chain termination reaction to produce HOO• + CH 2 =CHCH 2 OOH has a barrier which is 8-10 kcal/mol higher in energy.…”

Multiconformer transition state theory rate constants for the reaction between OH and ‐dimethoxyfluoropolyethers

“…The third part of the protocol involves the calculation of the tunneling correction, obtained through the Eckart method, which is widely used in atmospheric chemistry studies . To calculate this correction, we started by selecting the TS conformer with the lowest ZPE‐corrected energy and calculating the IRC in both reactants and products direction at the M08‐HX/pcseg‐1 level of theory.…”