| Summary: | Monoterpenes are the most essential reactive biogenic volatile organic compounds. Their removal from the atmosphere leads to the formation of oxygenated compounds, such as nopinone (C<sub>9</sub>H<sub>14</sub>O), one of the most important first-generation β-pinene oxidation products that play a pivotal role in environmental and biological applications. In this study, experimental and theoretical rate coefficients were determined for the gas-phase reaction of nopinone with hydroxyl radicals (OH). The absolute rate coefficient was measured for the first time using a cryogenically cooled cell along with the pulsed laser photolysis–laser-induced fluorescence technique at 298 K and 7 Torr. The hydrogen abstraction pathways were found by using electronic structure calculations to determine the most favourable H-abstraction position. Pathway 5 (bridgehead position) was more favourable, with a small barrier height of −1.23 kcal/mol. The rate coefficients were calculated based on the canonical variational transition state theory with the small-curvature tunnelling method (CVT/SCT) as a function of temperature. The average experimental rate coefficient (1.74 × 10<sup>−11</sup> cm<sup>3</sup> molecule<sup>−1</sup> s<sup>−1</sup>) was in good agreement with the theoretical value (2.2 × 10<sup>−11</sup> cm<sup>3</sup> molecule<sup>−1</sup> s<sup>−1</sup>). Conclusively, the results of this study pave the way to understand the atmospheric chemistry of nopinone with OH radicals.
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