Limonene: A scented and versatile tropospheric free radical deactivator

The reactions of limonene with various free radicals (^•OCH₃, ^•OBr, ^•SH, ^•OOH, and ^•OOCH₃) were investigated along the 273.15–312.15 K temperature range. To that purpose the density functional theory was used, at the M06-2X/6–311+g(d,p) level. Two reaction mechanisms, hydrogen atom transfer (HAT) and radical adduct formation (RAF) were considered. It was found that the relative reactivity of the studied radicals toward limonene is: ^•SH > ^•OBr > ^•OCH₃ > ^•OOH > ^•OOCH₃. HAT was identified as the dominant mechanism for ^•OOH and ^•OOCH₃, while RAF contributes the most to the reactions involving ^•OCH₃, ^•OBr, and ^•SH. The obtained Arrhenius expressions are: k(^•OCH₃) = 1.58 × 10⁻¹³ e^−1.59/RT, k(^•OBr) = 3.55 × 10⁻¹² e^+1.82/RT, k(^•SH) = 3.30 × 10⁻¹¹ e^+0.79/RT, k(^•OOH) = 1.33 × 10⁻¹⁵ e^−5.99/RT, and k(^•OOCH₃) = 5.88 × 10⁻¹⁷ e^−6.26/RT. According to them, the reactions of ^•OBr and ^•SH become slower as temperature rises from 273.15 to 312.15 K, while for the other radicals the reactions rate increases with temperature. The subsequent tropospheric fate of the most abundant ^•OBr adduct was also investigated in the same temperature range, considering O₂ addition to this radical (step 2) and the reaction of the peroxyl radical yielded in this step 2 with NO. The latter is predicted to take place in two steps: the NO addition (3a) and the NO₂ elimination (3b). The corresponding Arrhenius expression are k₂ = 3.56 × 10⁻¹⁵ e^+1.43/RT and k_3b = 1.35 × 10¹⁴ e^−31.65/RT. Step 3a was found to be barrierless. To our best knowledge, all the data provided here is reported for the first time. Thus, it would hopefully contribute to enhance the knowledge necessary for the full understanding (and accurate modeling) of the troposphere.