Infrared Fingerprint\nand Unimolecular Decay Dynamics\nof the Hydroperoxyalkyl Intermediate (•QOOH) in Cyclopentane\nOxidation

A transient carbon-centered hydroperoxyalkyl intermediate\n(•QOOH)\nin the oxidation of cyclopentane is identified by IR action spectroscopy\nwith time-resolved unimolecular decay to hydroxyl (OH) radical products\nthat are detected by UV laser-induced fluorescence. Two nearly degenerate\n•QOOH isomers, β- and γ-QOOH, are generated by\nH atom abstraction of the cyclopentyl hydroperoxide precursor. Fundamental\nand first overtone OH stretch transitions and combination bands of\n•QOOH are observed and compared with anharmonic frequencies\ncomputed by second-order vibrational perturbation theory. An OH stretch\ntransition is also observed for a conformer arising from torsion about\na low-energy CCOO barrier. Definitive identification of the β-QOOH\nisomer relies on its significantly lower transition state (TS) barrier\nto OH products, which results in rapid unimolecular decay and near\nunity branching to OH products. A benchmarking approach is utilized\nto compute high-accuracy stationary point energies, most importantly\nTS barriers, for cyclopentane oxidation (C<sub>5</sub>H<sub>9</sub>O<sub>2</sub>), building on higher level reference calculations for\nethane oxidation (C<sub>2</sub>H<sub>5</sub>O<sub>2</sub>). The experimental\nOH product appearance rates are compared with computed statistical\nmicrocanonical rates using RRKM theory, including heavy-atom tunneling,\nthereby validating the computed TS barrier. The results are extended\nto thermal unimolecular decay rate constants at temperatures and pressures\nrelevant to cyclopentane combustion via master-equation modeling.\nThe various torsional and ring puckering states of the wells and transition\nstates are explicitly considered in these calculations.