Tropospheric Fate of Methylhydroxycarbene and the Ability of a Single Water Molecule to Efficiently Promote Its Isomerization into Acetaldehyde.

The ultraviolet (UV) photodissociation of pyruvic acid through the absorption of solar actinic flux generates methylhydroxycarbene (MHC) in the atmosphere. It is recognized that isolated MHC can undergo unimolecular isomerization to form acetaldehyde and vinyl alcohol. However, the rates and mechanism for its possible bimolecular reactions with atmospheric constituents, which can occur in parallel with its unimolecular reaction, is not well understood. Here we investigate the energetics, kinetics, and mechanism of the reaction of MHC with three ubiquitous atmospheric molecules N, O, and HO over the 160 K-380 K temperature range. Our study, at the CCSD(T)/6-311++G(3df,3pd)//M06-2/6-311++G(3df,3pd) level, reveals that the MHC + N encounter is nonreactive, while the MHC + O reaction, which leads to CHCO + HO formation, has a rate that is significantly different from previous estimates. For the MHC + HO reaction, we find that a single HO molecule is very effective in catalyzing the isomerization of MHC to form predominantly acetaldehyde. An analysis of the computed rate for this reaction indicates that it will be an important source of tropospheric acetaldehyde ̵ a major pollutant and precursor for atmospheric reactive intermediates. Our findings are in sharp contrast to current assessments in the literature that the MHC + HO reaction is minor. Furthermore, in the MHC + HO reaction system, we find that due to the presence of the OH group on MHC, the concerted insertion mechanism, which is typically dominant in reactions involving singlet carbenes, is suppressed relative to a hydrogen bond mediated double hydrogen atom transfer mechanism.