Atmospheric Oxidation Reactions of Methyl Salicylate as Green Leaf Volatiles by OH Radical: Theoretical Kinetics and Mechanism

Abstract

Green leaf volatile (GLV) organic compounds, released from vegetation, act as plants’ multifunctional weapon and are known to be a source of secondary organic aerosols (SOAs). In the present study, the mechanism of oxidation reaction of methyl salicylate (MeSa), a known GLV, by OH radical has been investigated by means of density functional theory (DFT). Canonical transition state (CTST) and Rice‐Ramsperger‐Kassel‐Marcus theories (RRKM) with Wigner and Eckart tunneling correction were applied to calculate the transition pressure, the total rate constant, and the individual rate constants for every channel of this oxidation reaction of MeSa with OH radical. The calculated overall bimolecular rate constant with Eckart tunneling correction for the title reaction is 1.18×109 and 1.01×109 L mol−1s−1 at Mn15‐L/aug‐cc‐pVTZ and MN15‐L/MG3S levels of theory in the gas phase at 298 K which is comparable to the experimental reported value of 6.66×109 L mol−1s−1, indicating that the results of MN15‐L/aug‐cc‐pVTZ level of theory is more accurate. Investigation of transition pressure and fall‐off curve of unimolecular channels at proposed mechanism for oxidation reaction of MeSa revealed that the TST breaks down slightly to estimate the high pressure limit of reaction rate and the reaction become second order (bimolecular reaction). Furthermore, the atmospheric lifetime of MeSa is about 2.95 and 3.44 days calculated at those two levels of theory, respectively, which indicates that MeSa can be considered as a medium‐lifetime organic volatile compound.

Green Leaf Volatiles in the Atmosphere—Properties, Transformation, and Significance

“…The dominating addition channel was the OH addition at para position (Scheme 16). On the contrary, Seif et al [190] theoretically found that the addition at the ipso position had the largest branching ratio and the addition at the para position, the smallest one. The rate constants k i for the individual addition channels calculated at 298 K and 1 atm using RRKM theory at MN15-L/aug-cc-pVTZ level with Eckart tunneling were 4.65, 2.02, 1.01, 0.24, 3.27, and 0.57 10 8 M −1 s −1 , respectively for the positions from 1 to 6.…”

“…V RKKM theory at MN15-L/aug-cc-pVTZ level with Eckart tunneling correction [190] VI DFT theory using B3LYP, M06-2X, and MPW1K functionals with 6-311++G(d,p) basis set. [189] Most of the reactions have negative activation energies, as their rate decreases with increasing temperature.…”