Conferences can be intense, and this one is no exception. After five days, saturation is in danger of setting in. But before it does, I include two more (very) brief things I have learnt.
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Sason Shaik introduced a theme he first investigated years ago, but for which no experiment had been devised for verification. He revived his theme when a journalist contacted him last year to report exactly such an observation, which I now recount. A Diels-Alder adduct was captured between a flat layer of gold atoms and the tip of a scanning-tunneling microscope. With the molecule exactly oriented, a strong external electric field (OEEF) was applied, in both senses of polarisation. This is exactly the model studied by Sason, who had argued thus. A Diels-Alder reaction can be modelled using VB theory as the avoided crossing of a covalent ground state with ionic excited states at the transition state. Depending on the polarisation of an applied external electric field and the orientation of the molecule, one of these ionic states can stabilized or destabilised by about 8 kcal/mol, thus either stabilising or destabilising the transition state itself by mixing with the covalent state.
And so it was that the oriented molecule caught between a gold layer and an STM probe could be persuaded to undergo a retro-Diels-Alder far more easily than it would thermally. The technique can even be tuned to selecting between endo and exo isomers. Sason held out the prospect that the toolbox of the synthetic chemist, which already includes Δ, hν and ? (ultrasound) as reagents, might be extended using OEEF. He called this a smart reagent since it can be tuned to the reaction required (as of course can light). At the moment this technique can only be applied to one molecule at a time, but it might be just a matter of designing a suitable apparatus!
- Pavel Hobza talked about non-covalent interactions, an occasional theme on this blog. Amongst many interesting observations was that the DNA helix is not stabilised as such by the hydrogen bonding between the base pairs but by the π-Ï€ stacking between them. One of these examples caught my eye, the known weak “hydrogen bonded” weak complex between benzene and chloroform in the gas phase. The C-H hydrogen points directly to the ring centroid and the C-H vibrational wavenumber is blue shifted by 12 cm-1. At the time this (experimental) observation caused consternation, since all known hydrogen bonds (both strong and weak) were routinely characterised by the magnitude of their red shift (up to ~100 cm-1). In fact, as Pavel showed, this interaction is less electrostatic in nature and more like dispersion attraction. Accurate calculations including dispersion also predict a blue shift for this system. A question from the audience suggested that as many π-facial “hydrogen bonds” in the crystal state tend to point not to the ring centroid but to the ring edge, what would happen if the chloroform H were to slide across the surface of the ring until it reached the edge; would the CH shift invert to become red, implying a change from dispersion interaction to whatever is implied by a hydrogen bond?
Apologies to all those who gave fascinating talks which are unrecorded here. I hope some tiny and selective flavour nevertheless emerges of WATOC 17.
