Unveiling a Hidden Route to Tropospheric Acetaldehyde: The Water-Assisted Reactivity of Methylhydroxycarbene···Water Dimer

Methylhydroxycarbene (MHC) is formed in the atmosphere via the UV photodissociation of pyruvic acid (PA) and has emerged as an important intermediate in atmospheric acetaldehyde (Ald) formation. While recent findings (J. Am. Chem. Soc. 2025, 147, 211–222) underscore the importance of the MHC + H2O reaction for tropospheric Ald production, here we show that the reaction of the MHC···H2O dimer with an additional water molecule can also be a significant source of Ald. Using high-level electronic structure calculations [CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd)] coupled with master equation analysis, we find that the lowest-energy pathway for this reaction is effectively barrierless, with the kinetics being governed by the initial bimolecular association step involving the MHC···H2O dimer and H2O reactants. At 298 K, the overall rate constant is estimated to be 1.42 × 10–11 cm3molecule–1s–1, which is approximately 5.3 times larger than that for the MHC + H2O reaction. Further, accounting for typical atmospheric abundances, the effective rate of the MHC···H2O + H2O reaction is only ∼6.2 times slower than that for the MHC + H2O pathway. Thus, given the significant concentration of water vapor and MHC expected in the lower troposphere, this water-assisted dimer reaction is likely a significant, overlooked source of tropospheric Ald.