Isomerization of Methylhydroxycarbene in Isolation and Mediated by Its Interactions with Ammonia and Formic Acid

Computational chemistry is used to examine the unimolecular isomerization kinetics of isolated methylhydroxycarbene (MHC) and compare with that of its catalyzed bimolecular isomerization in the presence of ammonia (NH₃) and formic acid (FA) over the 180 K-380 K temperature range. The two catalysts, one being a base and the other being an acid, both exhibit hydrogen bonding. For all the three processes studied, the potential energies along the reaction path are calculated at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level. For the unimolecular isomerization of isolated MHC, canonical variational transition state theory (CVTST) with small curvature tunneling (SCT) corrections reveals that while acetaldehyde (Ald) is the main product at low temperatures, vinyl alcohol (VA) becomes the dominant product above 310 K. The computed barrier heights for VA and Ald formation are ∼23.1 and ∼28.8 kcal/mol, respectively, and are consistent with earlier findings. For the NH₃-catalyzed isomerization of MHC, the energetically most favorable path produces solely the Ald product via a double hydrogen atom transfer (DHAT) mechanism. This reaction path is effectively barrierless, with the transition state (TS) being ∼0.47 kcal/mol below the total energy of the isolated MHC + NH₃ reactants. By contrast, the singlet MHC insertion mechanism is negligible for the NH₃-catalyzed reaction. The FA-catalyzed reaction is found to proceed through three distinct paths (channel-I, channel-II, and channel-III) instead of the two (channel-I and channel-II), reported previously. The newly identified channel-III is effectively barrierless and involves a DHAT mechanism. All three channels in the FA catalyzed reaction predominately produce the Ald product. The temperature-dependent kinetics using the master equation solver for multienergy well reactions (MESMER) reveals that the overall rate constant for Ald production via both the NH₃- and FA-catalyzed reactions is ∼10^-12 cm³ molecule^-1 s^-1 across the examined temperature range. A comparison of the relative rates for the unimolecular isomerization of isolated MHC versus that for the NH₃- and FA-catalyzed isomerization suggests that even for relatively low catalyst concentrations, 10 ppbv, the bimolecular isomerization dominates over the unimolecular isomerization process.