The Role of Functional Groups in the Atmospheric Oxidation of Vanillin: Mechanism and Kinetic Study

Vanillin (4-hydroxy-3-methoxybenzaldehyde), a phenolic compound with multifunctional groups (aldehyde, hydroxyl, and ether), acts as a free radical scavenger in the atmosphere and is the objective of the present study focusing on the effect of substitution. Previous theoretical studies show significant discrepancies in the rate coefficients of vanillin's initial oxidation by OH compared to similar substituted aromatic volatile organic compounds (VOCs). Using high-level quantum chemical methods (DLPNO-CCSD(T)/aug-cc-pVQZ//M05-2X-D3/6-311++G(d,p)) combined with Rice-Ramsperger-Kassel-Marcus/master equation simulations, we theoretically investigate the vanillin + OH reaction and its subsequent oxygenation, analyzing the unimolecular and bimolecular reactions of primary peroxy species to understand their roles in secondary organic aerosol (SOA) formation. The total rate coefficient for the vanillin + OH reaction at 300 K and 1 atm was computed as 1.0 × 10^-10 cm³ molecule^-1 s^-1. The branching ratios of H-abstraction and OH-addition are 58.9 and 41.1%, respectively, with the 4-hydroxy-3-methoxybenzoyl radical (abs12) being the primary product (57.7%). Further oxidation of abs12 leads to the formation of 4-hydroxy-3-methoxybenzoyl peroxide (add7OO). Wavefunction analysis on three key species (vanillin, abs12, and add7OO) reveals the formyl group's susceptibility to nucleophilic reactions, driving the formation of oxidized products. We also analyze the unimolecular and bimolecular reactions of add7OO in the atmosphere, which lead to the formation of small inorganic molecules (CO₂, NO₂, NO₃, O₂, OH) and oxygenated ring-retained aromatics, impacting SOA formation. Keeping in view the substitution pattern, this finding focuses on vanillin's atmospheric degeneration, shedding light on the fate of VOCs with multifunctional groups.