Gas-phase ozonolysis of β-ocimene: Temperature dependent rate coefficients and product distribution

Abstract

Rate coefficients for the reaction of β-ocimene with O3 molecules have been determined over the temperature range 288–311 K at 750 Torr total pressure of nitrogen using the relative rate technique. The investigations were performed in a large volume reaction vessel using long-path in-situ Fourier transformed infrared (FTIR) spectroscopy to monitor the reactants and products. A value of k(β-ocimene + O3) = (3.74 ± 0.92) × 10−16 cm3 molecule−1 s−1 has been obtained for the reaction at 298 K. The temperature dependence of the reaction is best described by the Arrhenius expression k = (1.94 ± 0.02) × 10−14 exp [(−1181 ± 51)/T] cm3 molecule−1 s−1. In addition, a product study has been carried out at 298 K in 750 Torr of synthetic air and the following products with yields in molar % were observed: formaldehyde (36 ± 2), acetone (15 ± 1), methylglyoxal (9.5 ± 0.4) and hydroxyacetone (19 ± 1). The formation of formaldehyde can be explained by the addition of O3 to the C1[sbnd]C2 double bond of the β-ocimene. Addition of O3 to the C6[sbnd]C7 double bond leads to the formation of acetone and the CH3C·(OO·)CH3 biradical, which can through isomerization/stabilization form methylglyoxal (hydroperoxide channel) and hydroxyacetone. The formed products will contribute to the formation of PAN and derivatives in polluted environments and also the oxidation capacity of the atmosphere.