A computational study of the atmospheric oxidation of nopinone

Electronic structure calculations are used to derive the overall rate coefficient for hydrogen atom abstraction by the hydroxyl radical from a typical volatile organic compound, nopinone. The branching ratios for abstraction from the seven possible different positions are also obtained. Abstraction from the bridgehead position 1 is found to be important, with a branching ratio of 23%. This prediction differs from that derived using a structure-activity relationship, which suggests much less oxidation in this position, but is in agreement with available experimental evidence, showing formation of significant amounts of products such as 1-hydroxynopinone during terpene oxidation. Calculated rate coefficients are derived from standard transition state theory, with energy barriers, vibrational frequencies and rotational constants for reactants and transition states obtained using density functional theory with the KMLYP functional. This approach was calibrated by calculating the well-known rate coefficients for the simpler volatile organic compounds methane, ethane, propane, cyclobutane and acetone. High-level G3 calculations are possible and were carried out for these simpler systems, giving barrier heights in good agreement with KMLYP. Transition state theory gives surprisingly good results for the rate coefficients, probably in part due to error cancellation. This validates the use of the same relatively low level of theory for exploring reactivity and selectivity in oxidation of complex molecules such as nopinone.