Journal of Physical Chemistry A, Vol.107, No.30, 5743-5754, 2003
Kinetics and product study of the OH- and NO2-initiated oxidation of 2,4-hexadiene in the gas phase
The kinetics and products of the OH- and NO2-initiated oxidation of 2,4-hexadiene have been investigated at 296 K and 700 Torr by long-path Fourier transform infrared (FTIR) spectroscopy. Relative rate methods were employed for the photolysis of 2,4-hexadiene/CH3ONO/NO/air mixtures to measure k(OH + 2,4-hexadiene) = (1.76 +/- 0.58) x 10(-10) cm(3) molecule(-1) s(-1). The OH-initiated oxidation of 2,4-hexadiene crave acetaldehyde, trans-crotonaldehyde, and 2,5-dimethylfuran, with molar yields of (30.6 +/- 5.0)%, (12.9 +/- 2.0)%, and (5.6 +/- 1.0)%, respectively. These products suggest that the OH-initiated oxidation of 2,4-hexadiene proceeds, at least partially, by pathways analogous to those reported for 1,3-butadiene and isoprene. From the pseudo-first-order decay of 2,4-hexadiene in the presence of excess NO2, a value of k(NO2 + 2,4-hexadiene) = (3.11 +/- 0.18) x 10(-19) cm(3) molecule(-1) s(-1) was derived. The reaction stoichiometry and products were investigated in the absence Of O-2, in the presence Of O-2, and in the presence of O-2 and NO. Reaction mechanisms consistent with the observations are presented. In the presence of NO and O-2, the NO2-initiated chemistry leads to NO-to-NO2 conversion, and the formation of HOx radicals in significant yield, such that 2,4-hexadiene removal occurs by reaction with both NO2 and OH. The results are consistent with HOx yields of up to 0.75 +/- 0.05, depending on the experimental conditions. The results also suggest that the nitrate forming channels account for (25 5)% of the reactions of NO with peroxy radicals formed from the reactions of both OH and NO2 with 2,4-hexadiene. The implications of the results are discussed within the context of the atmospheric chemistry of conjugated dienes. As part of this work, rate coefficients were determined for the reactions of NO2 with 1,3-butadiene and styrene and for NO with 2,4-hexadiene: k(NO2 + 1,3-butadiene) = (2.54 +/- 0.15) x 10(-20), k(NO2 + styrene) = (1.4 +/- 0.3) x 10(-20), and k(NO + 2,4-hexadiene) less than or equal to 2 x 10(-21) cm(3) molecule(-1) s(-1).