1. Institut für Organische Chemie der Justus-Liebig-Universität, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany
   
2. Center for Computational Chemistry and Department of Chemistry, University of Georgia, Athens, Georgia 30602, USA
   
3. Laboratory of Molecular Spectroscopy, Institute of Chemistry, Eötvös University, H-1518 Budapest 112PO Box 32, Hungary

Correspondence to: Peter R. Schreiner¹Wesley D. Allen² Correspondence and requests for materials should be addressed to P.R.S. (Email: prs@org.chemie.uni-giessen.de) or W.D.A. (Email: wdallen@uga.edu).

Singlet carbenes exhibit a divalent carbon atom whose valence shell contains only six electrons, four involved in bonding to two other atoms and the remaining two forming a non-bonding electron pair. These features render singlet carbenes so reactive that they were long considered too short-lived for isolation and direct characterization. This view changed when it was found that attaching the divalent carbon atom to substituents that are bulky and/or able to donate electrons produces carbenes that can be isolated and stored¹. N-heterocyclic carbenes are such compounds now in wide use, for example as ligands in metathesis catalysis². In contrast, oxygen-donor-substituted carbenes are inherently less stable and have been less studied. The pre-eminent case is hydroxymethylene, H–C–OH; although it is the key intermediate in the high-energy chemistry of its tautomer formaldehyde^{3, 4, 5, 6, 7}, has been implicated since 1921 in the photocatalytic formation of carbohydrates⁸, and is the parent of alkoxycarbenes that lie at the heart of transition-metal carbene chemistry, all attempts to observe this species or other alkoxycarbenes have failed⁹. However, theoretical considerations indicate that hydroxymethylene should be isolatable¹⁰. Here we report the synthesis of hydroxymethylene and its capture by matrix isolation. We unexpectedly find that H–C–OH rearranges to formaldehyde with a half-life of only 2 h at 11 K by pure hydrogen tunnelling through a large energy barrier in excess of 30 kcal mol^–1.