On going through barriers

Professor Dean Tantillo of the UC Davis visited us last Friday and gave a thought-provoking talk that made me consider, once again, the way reactants traverse barriers on their way to products. No process is more fundamental to our understanding of chemical reactivity than the one that describes how energy barriers are being overcome by activated molecules. In a departure from textbooks, several of Dean’s recent works describe bifurcations of reaction paths, wherein the selectivity-determining step occurs after the transition state. I mentioned this on my blog in the past. Now that I heard the details in Dean’s talk, I really marvel at what computational tools offer these days. I think we have all witnessed the evolution of quantum chemistry approaches from mere explanation of experimental facts to prediction and uncovering some of the “unknown unknowns.” Bifurcation of reaction pathways is just that.

As I was listening to Dean’s lecture I noted a certain analogy between bifurcation – that “fork” in the downhill slope – to tunnelling. The analogy does not have anything to do with any causes of these clearly different phenomena, but there is something to be said about how Nature “cheats us” in both cases. Tunnelling describes how particles slip through energy barriers rather than surmount them. There are no (to my knowledge) enabling applications of tunneling in chemistry (I mean none of the “Hey, why don’t I use tunnelling to design my new catalyst…” sort of stuff), but tunnelling certainly helps understand some processes, especially when it comes to light atom transfer.

Below is an example of tunneling in organic chemistry. Without going into the details, let’s just say that it is possible to spectroscopically differentiate the syn and anti forms of the hydroxyl-substituted pyridine derivative shown below. The authors have found that the rate constant for the syn-to-anti conversion in the deuterated case is several orders of magnitude smaller than that of the protio derivative. Thus, this process is very likely a tunneling reaction.