Kinetic Investigations of the OH-Initiated Reactions of Methyl Cyclohexene Isomers in the Troposphere: An Experimental and Computational Study.

Kinetic investigations of two isomers of methyl cyclohexene (1-methyl cyclohexene, 1MCHXE, and 4-methyl cyclohexene, 4MCHXE) with OH radical over the temperature range 263-363 K employing experimental and theoretical methodologies were performed in this study. The pulsed laser photolysis-laser-induced fluorescence technique was employed for the experimental studies. The obtained temperature-dependent rate coefficients for the OH-initiated reactions of 1MCHXE and 4MCHXE are k(263-363 K) = (2.09 ± 0.65) × 10^-11 exp{(388 ± 96)/T} and k(263-363 K) = (6.49 ± 1.34) × 10^-12 exp{(462 ± 63)/T} cm³ molecule^-1 s^-1, respectively. Computational studies were conducted using CVT/SCT/ISPE with the aid of Gaussian 16 and POLYRATE 17-C software. 1MCHXE exhibited higher reactivity than 4MCHXE, both experimentally and theoretically. The reason behind this increase in the rate coefficient is discussed in terms of the position of the methyl group with respect to the unsaturated double bond. The thermodynamic and kinetic feasibilities of different reaction pathways were investigated in detail. Branching ratios and Fukui functions for radical attack were calculated to support the site-specific reactivity followed by the OH-initiated reaction pathways. Since methyl cyclohexene is released from a variety of natural and manmade sources and serves as a model for a large number of biogenic terpene compounds, atmospheric implication parameters such as lifetime and photochemical ozone creation potential of its isomers were determined.