Tropospheric\nFate of Methylhydroxycarbene and the\nAbility of a Single Water Molecule to Efficiently Promote Its Isomerization\ninto Acetaldehyde

The ultraviolet (UV)\nphotodissociation of pyruvic acid through\nthe absorption of solar actinic flux generates methylhydroxy­carbene\n(MHC) in the atmosphere. It is recognized that isolated MHC can undergo\nunimolecular isomerization to form acetaldehyde and vinyl alcohol.\nHowever, the rates and mechanism for its possible bimolecular reactions\nwith atmospheric constituents, which can occur in parallel with its\nunimolecular reaction, is not well understood. Here we investigate\nthe energetics, kinetics, and mechanism of the reaction of MHC with\nthree ubiquitous atmospheric molecules N<sub>2</sub>, O<sub>2</sub>, and H<sub>2</sub>O over the 160 K-380 K temperature range. Our\nstudy, at the CCSD(T)/6-311++G(3df,3pd)//M06-2<i>X</i>/6-311++G(3df,3pd)\nlevel, reveals that the MHC + N<sub>2</sub> encounter is nonreactive,\nwhile the MHC + O<sub>2</sub> reaction, which leads to CH<sub>3</sub>CO + HO<sub>2</sub> formation, has a rate that is significantly different\nfrom previous estimates. For the MHC + H<sub>2</sub>O reaction, we\nfind that a single H<sub>2</sub>O molecule is very effective in catalyzing\nthe isomerization of MHC to form predominantly acetaldehyde. An analysis\nof the computed rate for this reaction indicates that it will be an\nimportant source of tropospheric acetaldehyde ̵ a major pollutant\nand precursor for atmospheric reactive intermediates. Our findings\nare in sharp contrast to current assessments in the literature that\nthe MHC + H<sub>2</sub>O reaction is minor. Furthermore, in the MHC\n+ H<sub>2</sub>O reaction system, we find that due to the presence\nof the OH group on MHC, the concerted insertion mechanism, which is\ntypically dominant in reactions involving singlet carbenes, is suppressed\nrelative to a hydrogen bond mediated double hydrogen atom transfer\nmechanism.