Isomer-Resolved Unimolecular Dynamics of Transient Hydroperoxyalkyl Intermediates (•QOOH) in Isopentane Oxidation

Transient carbon-centered hydroperoxyalkyl intermediates (•QOOH) in isopentane oxidation are characterized by their time- and energy-resolved unimolecular dissociation dynamics to hydroxyl (OH) and cyclic ether products. Two distinct •QOOH isomers are examined with radical sites at a primary carbon of one of the methyl groups (b-Me) or a secondary carbon (b-Et) of the ethyl group. Energy-dependent unimolecular rates are obtained from the time-dependent appearance of OH products for the two isomers and compared with statistical microcanonical rates computed using RRKM theory, including heavy atom tunneling, based on high level theoretical calculations. A benchmark-corrected approach is utilized to compute high accuracy stationary point energies, most importantly, transition state barriers, for the •QOOH_Me and •QOOH_Et isomers in isopentane oxidation building on higher level reference calculations for oxidation of ethane (C2H5O2) and propane (C3H7O2), respectively. The measured rates are compared with RRKM calculations incorporating the benchmark-corrected transition state parameters, a vibrationally adiabatic multidimensional hindered‑rotor treatment of key torsions, and quantum tunneling. Agreement between experiment and theory validates the statistical description and shows faster decay for •QOOH_Et due to its lower barrier. Both β-QOOH isomers decay almost exclusively to OH + cyclic ether products under the conditions studied.