20 June 2008 - reactive intermediates

A German/American/Hungarian team has recently isolated hydroxymethylene (HOCH), a singlet carbene and formaldehyde tautomer, although only in an argon matrix at 11 K (Schreiner et al. DOI). Ordinarily carbenes are too energetic for isolation but in this particular case the oxygen lone pair is able to provide stabilization just as in a N-heterocyclic carbene.

The compound was prepared by flash pyrolysis at 1000 °C of glycolic acid with extrusion of carbon dioxide, trapped at 11 K and analysed by infrared spectroscopy and UV/VIS spectroscopy. A big surprise came when hydroxymethylene was found to convert to formaldehyde. Surprisingly because the conversion rate appears to be independent of temperature, the activation energy is supposed to be very high (in silico) and because the deuterated compound (HCOD) does not rearrange at all. The effect is attributed to a tunneling effect similar to the one observed in the kinetic isotope effect.

The researchers state that their findings are bad news for astronomers who have claimed to have spotted hydroxymethylene through their telescopes: the molecule is just too unstable.

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