In 2008, the previously elusive hydroxycarbene, H-C-OH was finally reported[1] as having been captured by matrix isolation, accompanied by the observation that “we unexpectedly find that H–C–OH rearranges to formaldehyde with a half-life of only 2h at 11K by pure hydrogen tunnelling through a large energy barrier in excess of 30 kcal mol–1. A subsequent theoretical study of this tunnelling in 2017[2] reported that “the half-life calculation after monodeuteration is 2.97 × 1016 hours, which is extremely longer than before monodeuteration that is only 2.5 hours using the same calculation methods“; in other words a kinetic isotope effect kH/kD of ~1016, which is by far the largest ever suggested.[3] In 2011, the original study was extended to methylhydroxycarbene[4], again arguing for “Tunneling Control of a Chemical Reaction.” In this post,† I explore an alternative mechanism for rearrangement of hydroxycarbene to formaldehyde using a “double hydrogen transfer” via a dimeric transition state (Figure 1).
References
- P.R. Schreiner, H.P. Reisenauer, F.C. Pickard IV, A.C. Simmonett, W.D. Allen, E. Mátyus, and A.G. Császár, "Capture of hydroxymethylene and its fast disappearance through tunnelling", Nature, vol. 453, pp. 906-909, 2008. https://doi.org/10.1038/nature07010
- N.D. Aisyah, R.N. Fadilla, H.K. Dipojono, and F. Rusydi, "A Theoretical Study of Monodeuteriation Effect on the Rearrangement of Trans-HCOH to H 2 CO via Quantum Tunneling with DFT and WKB Approximation", Procedia Engineering, vol. 170, pp. 119-123, 2017. https://doi.org/10.1016/j.proeng.2017.03.024
- H. Rzepa, "Reinvestigating the reported transition state structure of a concerted triple H-tunneling mechanism.", 2025. https://doi.org/10.59350/qgwfn-rsc92
- P.R. Schreiner, H.P. Reisenauer, D. Ley, D. Gerbig, C. Wu, and W.D. Allen, "Methylhydroxycarbene: Tunneling Control of a Chemical Reaction", Science, vol. 332, pp. 1300-1303, 2011. https://doi.org/10.1126/science.1203761