Kinetic and reactivity of gas-phase reaction of acyclic dienes with hydroxyl radical in the 273-318 K temperature range

As dienes contain two C&z.dbd;C bonds, theoretically, they are much more chemically reactive with hydroxyl radical (&z.rad;OH) than alkenes and alkanes, and the reaction with &z.rad;OH is one of the main atmospheric degradation routes of dienes during the daytime. In our work, rate coefficients of three types of acyclic dienes: conjugated as 3-methyl-1,3-pentadiene (3M13PD), isolated as 1,4-hexadiene (14HD), and cumulated as 1,2-pentadiene (12PD) reaction with &z.rad;OH were measured in the temperature range of 273-318 K and 1 atm using the relative rate method. At 298 ± 3 K, the rate coefficients for those reactions were determined to be k 3M13PD+OH = (15.09 ± 0.72) × 10 −11 , k 14HD+OH = (9.13 ± 0.62) × 10 −11 , k 12PD+OH = (3.34 ± 0.40) × 10 −11 (as units of cm 3 per molecule per s), in the excellent agreement with values of previously reported. The first measured temperature dependence for 3M13PD, 14HD and 12PD reaction with &z.rad;OH can be expressed by the following Arrhenius expressions in units of cm 3 per molecule per s: k 3M13PD+OH = (8.10 ± 2.23) × 10 −11 exp[(173 ± 71)/ T ]; k 14HD+OH = (9.82 ± 5.10) × 10 −12 exp[(666 ± 123)/ T ]; k 12PD+OH = (1.13 ± 0.87) × 10 −12 exp[(1038 ± 167)/ T ] (as units of cm 3 per molecule per s). The kinetic discussion revealed that the relative position between these two C&z.dbd;C could significantly affect the reactivity of acyclic dienes toward &z.rad;OH. A simple structure-activity relationship (SAR) method was proposed to estimate the reaction rate coefficients of acyclic dienes with &z.rad;OH. The relative position of two C&z.dbd;C on the acyclic dienes could determine its reactivity. The number and type of alkyl group and the number of addition sites on diene could also strongly affect its reactivity toward OH radical.