Cymenes are monoterpene derivatives composed of a benzene ring with iso-propyl and methyl substituents, and the proximity of the two alkyl groups enables relevant analysis into the ortho-effect of polysubstituted aromatics, in which low-temperature autoignition is more facile for ortho-substitution. The initial steps of ROO-related reactions from Cl-initiated oxidation of ortho-, meta- and para-cymene were studied at low pressure (8 Torr) over the temperature range 450-750 K using multiplexed photoionization mass spectrometry (MPIMS). Ratios of cyclic ether formation (related to chain-propagation and OH formation) relative to HO2-loss (related to chain-termination) were measured to characterize the ortho-effect as a function of temperature. The main results are twofold: Cyclic ether measurements indicate significant chain-propagation below 700 K only in o-cymene oxidation; above 700 K, chain-propagation of the three cymene isomers converge.The competition between chain-propagation channels stemming from resonance- and non-resonance-stabilized initial cymene radicals changes significantly with temperature, with chain-propagation near 700 K arising predominantly from non-resonance-stabilized R radicals. Cyclic ether yields in m- and p-cymene oxidation are negligible below 650 K, indicating minimal chain-propagation. In contrast, o-cymene exhibits significant cyclic ether formation, attributed to the favorable 6- and 7-membered-ring transition states in the formation of hydroperoxyalkyl (QOOH) intermediates from peroxy radicals (ROO). The cyclic ether/HO2-loss ratio in o-cymene oxidation is defined as unity at 450 K and decreases to ∼0.10 at 700 K. The ratio converges to ∼0.10 at 700 K for the three cymene isomers, indicating an upper limit of temperature for the ortho-effect. Photoionization spectra of cyclic ether formation from o-cymene oxidation indicate a competition between both resonance- and non-resonance-stabilized initial radicals at the lower range of temperature. With increasing temperature, cyclic ether formation shifts from pathways involving resonance-stabilized initial radicals towards pathways solely from non-resonance-stabilized initial radicals because of back-dissociation of weakly bound ROO adducts.
Combined with a Herriott-type multi-pass slow flow reactor, high-resolution differential direct absorption spectroscopy has been used to probe, in situ and quantitatively, hydroxyl (OH), hydroperoxy (HO 2 ) and formaldehyde (CH 2 O) molecules in fuel oxidation reactions in the reactor, with a time resolution of about 1 micro-second. While OH and CH 2 O are probed in the mid-infrared (MIR) region near 2870nm and 3574nm respectively, HO 2 can be probed in both regions: near-infrared (NIR) at 1509nm and MIR at 2870nm. Typical sensitivities are on the order of 10 10 - 10 11 molecule cm -3 for OH at 2870nm, 10 11 molecule cm -3 for HO 2 at 1509nm, and 10 11 molecule cm -3 for CH 2 O at 3574nm. Measurements of multiple important intermediates (OH and HO 2 ) and product (CH 2 O) facilitate to understand and further validate chemical mechanisms of fuel oxidation chemistry.