September 14, 2012
Chemistry in the Quantum Vacuum. No, Really.
Posted by Derek
When I was clearing a space on my desk the other day, I came across this paper, which I'd printed several months ago to read later. Later's finally here! A brief look at the manuscript will make clear why I didn't immediately dig into it - it's titled "Modifying Chemical Landscapes by Coupling to Vacuum Fields", and it's about as physics-heavy as anything that Angewandte Chemie would be willing to publish. The scary part is, this is one of a pair of papers from the same group (Thomas Ebbesen's at Strasbourg), and it's the other one that really gets into the physics. (If you can't get the first paper, here's a summary of it, the only mention I've been able to find of this work).
But it's worth a bit of digging, because this is very strange and interesting work. So bear with me for a paragraph - I always thought that someone should write a textbook titled "Quantum Mechanics: A Hand-Waving Approach", and that's what you're about to get here. The theory tells us, among many other weird things, that the vacuum between molecules is not what we might think it is. That's more properly the quantum electrodynamic vacuum, the ground state of electromagnetic fields. Because the Planck constant is not zero - tiny, but crucially not zero - the QED vacuum is not the empty nothingness that we think of classically. It's a dielectric, it's diamagnetic, and its properties can be altered. The theory that tells us such odd things is to be taken very seriously indeed, though, since it has made some of the most detailed and accurate predictions in the history of science.
And the vacuum-field fluctuation part of the theory has to be taken very seriously, too, because these effects have actually been measured. This was first accomplished via the Lamb shift and the Casimir effect is the latest poster child. That relates to the properties of the vacuum between two very closely spaced physical plates, and we're now to the point, technologically, where we actually make structures of this kind, measure their sizes and compositions, and determine what's going on inside them.
So what, those few of you who are still reading would like to know, does this have to do with chemistry? Well, when a real molecule is placed between such plates, its energy levels behave in strange ways. And this latest paper demonstrates that with a photochemical rearrangement - the reaction rates change completely depending on whether or not the starting material is confined in the right sort of space, and they change exactly as the cavity is tuned more closely to the absorption taking place. In effect, the molecule is now part of a completely new system (molecule-plus-cavity), and this new system has different energy levels - and can do different chemistry.
The photochemistry shown is not exciting per se, but the fact that it can be altered just by putting the molecule in a very tiny box is exciting indeed:
"The rearrangement of the molecular energy levels by coupling to the vacuum field has numerous important consequences for molecular and material sciences. As we have shown here, it can be used to modify chemical energy landscapes and in turn the reaction rates and yields. Strong coupling can either speed up or slow down a reaction depending on the reorganization of specific energy levels relative to the overall energy landscape. Both rates and the thermodynamics of the reaction will be modified. . .The coupling was done here to an electronic transition but it could also be done to a specific vibrational transition for instance to modify the reactivity of a bond. In this way it can be seen as analogous to a catalyst which changes the reaction rate by modifying the energy landscape."
I look forward to seeing how this field develops. If we end up being able to make reactions go the way we want them to by coupling our starting materials to actual fabric of space, I will officially decide that I am, in fact, living in someone's science fiction novel, and I will be very happy about that. I can picture a vacuum-field flow chemistry machine, pumping reactants through various ridiculously small and convoluted lattices, as someone turns a chrome-plated crank on the side to adjust the geometry of the cavities to change the product distributions. OK, there are perhaps a couple of engineering challenges there, but you get the idea.
And speaking as an organic chemist, I have a few other questions: can these vacuum field effects occur in some of the other confined spaces that we're more used to thinking about? The insides of zeolites, for example? The interior of a cyclodextrin? Between sheets of graphene? Inside the active site of an enzyme? I'm sure that there are reasons why not all of these would be able to show such an effect (irregular geometry, just to pick one), but it does make you wonder.