Kinetic and dynamic investigations of OH reaction with styrene.

The kinetics of hydroxyl radical reaction with styrene has been studied at 240-340 K and a total pressure of 1-3 Torr using the relative rate/discharge flow/mass spectrometry technique. In addition, the dynamics of the reaction was also studied using the ab initio molecular orbital method. The reaction was found to be essentially pressure independent over 1-3 Torr at both 298 and 340 K. At 298 K, the average rate constant was determined, using four different reference compounds, to be kstyrene+OH = (5.80 ± 0.49) × 10(-11) cm(3) molecule(-1) s(-1). At 240-340 K, the rate constant of this reaction was found to be negatively dependent on temperature with an Arrhenius expression determined to be kstyrene+OH = (1.02 ± 0.10) × 10(-11) exp[(532 ± 28)/T] cm(3) molecule(-1) s(-1). Observation of mass spectral evidence of adduct products and their respective fragment ions suggests that the reaction proceeds with addition of the OH to the vinyl carbons of the styrene molecule. Ab initio calculations of both the addition and the abstraction pathways predict that the addition pathways are more energetically favorable because of large exothermicity and essentially barrierless transition state associated with the additions, which is consistent with the experimental observations. Using the styrene + OH rate constant determined at 277 K in the present work, the atmospheric lifetime of styrene was estimated to be 4.9 h.