OK, it’s been a few years since I blogged about this particular weirdness, so let’s do some more. There have been a couple more recent reports of the effects of “vibrational strong coupling” on chemical reaction rates. What the heck is that? There’s some background in that link, but we can hand-wave our way through by saying that under certain conditions you can form what would have to be called hybrid states of light and matter (“polaritons”). These conditions obtain, for example, with molecules that have particularly strong IR absorption bands, when at high concentration inside a very small cavity that confines light to particular wavelengths. It’s a quantum mechanical effect and has been observed and validated many times by experiment in simple model systems. But we’re only in recent years finding ourselves able to apply such effects to organic molecules.
Here’s one from last fall, showing enzyme rate changes under the influence of strong coupling of water vibrational modes. The model system was pepsin, where a water molecule is involved in the enzyme’s active site. Coupling of OH stretching band, a whopper familiar to anyone who’s ever taken an IR spectrum, decreased the rate constant by nearly five-fold, whereas strong coupling to the bending vibrational mode had no effect. The mechanistic basis for this difference is frankly not clear: there’s something about the detailed mechanism of water in that active site that can distinguish these effects, and it’s hard to imagine another analytical technique that would have revealed it.
And here’s a paper that just came out on the Prins reaction, familiar to most synthetic organic chemists. It’s certainly familiar to me! In this case, as in the water one above, running the reaction under conditions of vibrational strong coupling to the carbonyl stretching vibration slow down the reaction rate by up to 70%. The rate change follows the amount of VSC very closely; you can actually scan along the CO absorption band wavenumber by wavenumber and see it max out right where the absorption does.
As the authors say, “While broadly attributed to the re-shaping of the reaction potential energy landscape by hybrid light-matter states, a clear picture of the effect of VSC on organic chemistry is still lacking.” Given the mechanism of the Prins, the slower rate is what you would expect if you had somehow made the carbonyl bond less polar than before (less of a positive charge on the carbonyl carbon). It’s interesting that the earlier example linked in the first paragraph, the deprotection of TMS-acetylene, also involved a slowdown of the reaction rate, and that also could (in theory) be ascribed to making the C-Si bond less polar. In both cases, there’s no evidence of a larger change in the mechanistic pathway, just a weird tuning of the one that’s already in effect.
This work, for now, has no practical application whatsoever. But it represents a tool for both changing reaction dynamics and investigating reaction mechanisms that we’ve never really investigated before. I’ve always had a weakness for physical organic chemistry (as my old grad school professor Ned Arnett figured out when I took his course at Duke), a weakness that fortunately rarely extends so far as actually wanting to do any of it in the lab. But I’m a big fan of the stuff from afar, and I will be interested to see what comes of VSC.