What would you say if I told you that a reaction rate could be influenced by the shape of the container the reaction is being run in? Those of you who do larger-scale reactions will have no trouble believing it, since mixing effects start to become prominent in larger vessels. But that’s not where I’m going here: what if I told you that a particular simple reaction ran five times slower, compared to the bulk liquid, when it was confined between two walls that are six microns apart? How about that system still running five times slower than the one where it’s being confined between two walls that are eight microns apart?
Right. But we’re going to have to get our heads around this, because this paper demonstrates that it’s exactly what happens, and for a good reason, too. This is a followup to some work that I blogged about here, from the Ebbesen group. They’ve been investigating the intersection of organic chemistry with physics, in the form of the coupling of vibrational modes with the vacuum state and inside optical cavities. In this case, it’s a Fabry-Pérot cavity (two parallel reflective surfaces), and the reaction is the deprotection of trimethylsilyl phenylacetylene, using TBAF in methanol. That’s a simple one, and well-suited to the experiment, since there’s a prominent C-Si band in the vibrational (IR) spectrum that can be targeted. The liquid reactants are mixed and injected into the optical cavity, and the reaction is monitored by FTIR. For controls, they use the bulk reaction, and also in another cavity whose spacing is not tuned to that particular vibrational peak.
A Fabry-Pérot cavity has a regular series of resonance modes, depending on both the cavity size and the refractive index of the material inside it, and this one gets tuned so that one of these lands right on top of that C-Si peak. In this situation, you have vibrational strong coupling, VSC, which splits that original peak into two hybrid states, separated by Rabi splitting.(Dang it, I told you that this was physics, but fear not, we are not going to go on to Consider the Hamiltonian). That splitting is proportional to the strength of the original absorption, so the more concentrated you make the sample, the bigger an effect you should see. As long as the starting material(s) and the product have slightly different refractive indices, you can monitor a product peak by IR without affecting anything about the reaction itself. The thing is, all this VSC and Rabi splitting happens whether there’s any IR hitting the system or not – it’s a consequence of an interaction between the optical and vibrational zero-point energies, fundamental properties of the molecules themselves.
This system works exactly as predicted. The reaction rate changes noticeably, going down about fivefold inside the cavity, and it depends on the Rabi splitting just as expected. Running the reaction at different temperatures and breaking down the thermodynamics shows some interesting stuff: the transition state of the reaction has clearly changed (both the enthalpy and entropy terms show large differences). Normally, this reaction goes by attack of the fluoride on the silicon, forming a pentavalent species, but under VSC conditions, it looks like the carbon-silicon bond starts breaking first, a dissociative mechanism rather than an associative one. There’s actually a higher energy barrier in this new landscape, which is why the reaction slows down.
What’s also weird to think about is that this effect depends on the orientation of each individual molecule with respect to the cavity. At any given instant, only a given fraction of them are really experiencing the strong coupling, so the rate change of the whole sample is actually an averaged-out value of what (for the molecules involved) must be a much greater effect. A number of things might be possible in other systems:
While we found that in our case, the reaction was slowed down, it is possible that depending on the chemical landscape, reaction rates can also be accelerated. When the reaction leads to multiple products, it is likely that the product ratios are modified under VSC, providing a way to optimize the yield of a given product. Site-selective chemical reactions are another possibility that should be explored. Finally, chemistry under VSC has the advantage that it works in the dark and at room temperature.
Picture the landscape of organic chemistry as we know it, all the reactions and catalysts that we have now – and then picture being able to twang the rubber-sheet landscape of all their thermodynamics, altering ground state and transition state energies just by running things between properly sized gaps. I’m imagining a (rather expensive) flow machine that sends its reaction mixtures through precisely formed optical cavities, each of which is tunable to the vibrational modes of the particular bonds in the particular molecules you’re trying to affect. Not all of those are going to be accessible (the Rabi splitting has to be large), but enough of them surely are so that entirely new reactions are possible. What a thought – Clarke’s third law, for sure.
Update: here’s another post on this work, worth a look since it’s hard to find comment on this area of research.