Frederic W. Patureau Angew. Chem. Int. Ed.
Synthetic chemistry is a very practical discipline, often relying on the intuition of the chemist based on fairly simple rules of molecular behaviour. As such, organic chemists rarely encounter the more esoteric consequences of quantum mechanics.
However, in a highlight for Angewandte Chemie, Frederic Patureau discusses studies of a fascinating behaviour of organic molecules: atom tunneling.
The phenomena was first observed in the isomerisation of formic acid at 8 K. Lundell and co-workers showed that (E)-formic acid isomerises within a few minutes to the more stable (Z)-formic acid. However, calculations showed that the rate of this isomerisation by rotation of the C-O bond should be about 13 orders of magnitude slower at this temperature. They concluded that the proton was shifting from the (E) configuration to the (Z) by quantum tunneling, where the atom disappears from its current location and simultaneously appears in another.
This conclusion was backed up by the observation that (E)-[D1]-formic acid isomerises more slowly than (E)-formic acid by three orders of magnitude; a difference that would not be observed if C-O bond rotation was rate-determining.
Further studies by Schreiner et al. looked at substituent effects on isomerisation in (E)-[D1]-benzoic acids. In this case, isomerisation of (E)-benzoic acid is too rapid for measurement even at 11 K; the deuterated analogue was a more suitable substrate. The increased rate of isomerisation in benzoic acid with respect to formic acid is due to the slightly shorter distance between the locations of the proton/deuteron to be transversed.
Shorter half-lives for isomerisation were found with electron deficient substituents on the aromatic ring. Interestingly, the rate of isomerisation is unaffected by changes in temperature (11-20 K), again ruling out a rotational pathway transition state.
Perhaps the most interesting form of atom tunneling observed is found in the study of hydroxy carbenes. In this case, atom tunneling occurs across atoms; the proton hops from oxygen to carbon to form the more stable aldehyde. The paper goes on to discuss further competition experiments and substituent effects in the isomerisation of hydroxy carbenes.
Of course, atom tunneling effects are swamped by standard molecular dynamics at ambient temperatures, but understanding this phenomenon may lead to interesting applications in catalytic reactions.