| Autor: |
Qian Y; Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19103-6323, United States., Roy TK; Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19103-6323, United States., Valente DS; Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19103-6323, United States., Cruz EM; Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19103-6323, United States., Kozlowski MC; Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19103-6323, United States., Della Libera A; Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States.; Department of Chemistry, Materials and Chemical Engineering 'G. Natta', Politecnico di Milano, Piazza Leonardo da Vinci 32, MI, Milano 20133, Italy., Klippenstein SJ; Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States., Lester MI; Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19103-6323, United States. |
| Abstrakt: |
A transient carbon-centered hydroperoxyalkyl intermediate (•QOOH) in the oxidation of cyclopentane is identified by IR action spectroscopy with time-resolved unimolecular decay to hydroxyl (OH) radical products that are detected by UV laser-induced fluorescence. Two nearly degenerate •QOOH isomers, β- and γ-QOOH, are generated by H atom abstraction of the cyclopentyl hydroperoxide precursor. Fundamental and first overtone OH stretch transitions and combination bands of •QOOH are observed and compared with anharmonic frequencies computed by second-order vibrational perturbation theory. An OH stretch transition is also observed for a conformer arising from torsion about a low-energy CCOO barrier. Definitive identification of the β-QOOH isomer relies on its significantly lower transition state (TS) barrier to OH products, which results in rapid unimolecular decay and near unity branching to OH products. A benchmarking approach is utilized to compute high-accuracy stationary point energies, most importantly TS barriers, for cyclopentane oxidation (C 5 H 9 O 2 ), building on higher level reference calculations for ethane oxidation (C 2 H 5 O 2 ). The experimental OH product appearance rates are compared with computed statistical microcanonical rates using RRKM theory, including heavy-atom tunneling, thereby validating the computed TS barrier. The results are extended to thermal unimolecular decay rate constants at temperatures and pressures relevant to cyclopentane combustion via master-equation modeling. The various torsional and ring puckering states of the wells and transition states are explicitly considered in these calculations. |
