Mechanistic elucidation and temperature-dependent kinetics of gas phase OH radical reactions with myrtenal: a combined theoretical and experimental investigation.

The gas phase reaction of myrtenal with OH radicals has been studied experimentally and theoretically. Experiments were carried out in a flow reactor, coupled with the pulsed laser photolysis - laser induced fluorescence technique, at around 9 mbar and at temperatures ranging from 279 to 360 K. The total absolute rate constant kr was well fitted using the modified Arrhenius expression: . Electronic structure calculations were performed using density functional theory (DFT) at the M06-2X/6-311+G(d,p) level of theory. Theoretical rate constant calculations were carried out in the temperature range of 278-1000 K, using canonical variational transition state theory with small curvature tunnelling method (CVT/SCT). OH-addition to the more substituted carbon atom of the CC double bond was found to be the dominant reaction pathway, whereas OH-addition to the less substituted carbon atom of the CC double bond becomes significant only at the highest temperatures. H-abstraction from the formyl group accounts for about 25-30% within the explored temperature range. The global theoretical rate constant kCVT/SCT agrees with experimental findings within a factor of 2 to 3 and is well represented by the following expression: . The calculated global rate constant temperature dependence, however, decreases significantly for T ≥ 350 K, depicting a curvature which agrees qualitatively with the experimental results. Further investigation of the fate of OH + myrtenal products in the presence of O2, HO2, NO and NO2 leads to the final production of CO and 6,6-dimethyl-bicyclo[3.1.1]heptane-2,-3-diol. The atmospheric lifetime of myrtenal with OH radicals is estimated to range within 4-9 hours. Indoors, myrtenal could be chemically generated in poorly ventilated spaces subject to significant internal sources of terpenes like α-pinene.