Mechanism and kinetics of atmospheric degradation of menthol initiated by hydroxyl radical

Abstract

Menthol is a naturally occurring compound belonging to the cyclic monoterpene family and is one of the main constituents of peppermint oil, recognized worldwide as a cooling agent. This study is the first report on the reaction mechanism of menthol with OH radical by exploring the hydrogen abstraction mechanism using theoretical methods like M06-2X and wB97X-D/6-311+G(d,p) level of theory. Menthol has three chiral centres with cyclohexane, isopropyl and methyl groups; all the reactive sites have been studied to identify the dominant route. The H atom abstracted from the isopropyl group of menthol is found to be more energetically favourable than the entire reactive site. The results acquired are compared with high level G3B3 for the most prominent pathway. Further, the rate coefficients of all initial reactions are calculated using canonical variational transition state theory (CVT) with Wigner tunnelling correction. The most favourable pathway shows the rate coefficient of 9.83 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (298 K) at the M06-2X level of theory and the results are consistent with the experimental value (1.48 ± 0.31 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹). The formation of a peroxy radical and its reaction with atmospheric species is analyzed theoretically. The stable product 2-(1-hydroperoxy-1-methyl-ethyl)-5-methyl-cyclohexanol is formed via reaction of the peroxy radical with HO₂. The thermochemistry of the entire reaction channel shows both exothermic and exergonic nature. The atmospheric  life time of menthol in the presence of OH radical is found to be 1.13 hours indicates the inevitable contribution of the present study at local scale.