Out West: good times and a paradox

I flew into Calgary last night and spent this Friday visiting the University of Calgary. I have not been to the Department of Chemistry here for over 10 years, the last time being for the PhD exam of one of Warren Piers’ students. Earlier today I gave a talk at the Department, met with the faculty and students, and just came back to the hotel after a great dinner with Warren, Todd Sutherland, and Thomas Baumgartner. Warren treated us to some awesome wine, which will be fondly remembered. To me, these kinds of visits always provide the best way to catch up on what goes on in other labs. I particularly enjoy hearing students talk about their research, and today was no exception.

I want to mention one interesting piece of research that made me think (a lot). Take a look at the figure below. “A” represents a diastereotopic pair of protons in a generic RCH2R1 molecule. I can name many cases where VT NMR measurements have been employed in efforts to reveal the so-called coalescence temperature, which provides a measure of conformational preferences of a given compound. Dimethylformamide (inset B) is a simple achiral molecule that serves as a relevant example. In it, the methyl groups have different 1H NMR chemical shifts at room temperature, but coalesce at higher temperature. This property has important ramifications as it enables one to measure barriers to rotation, rate constants, etc. The example shown in inset “C” comes from Tom Back at the University of Calgary and teaches a peculiar behaviour seen in the 1H NMR of the depicted selenium compound. Apparently, the CH2 group appears as a AB quartet at 213K. Slowly but surely, this set of signals is transformed into a singlet at 291K. We might all be inclined to say: “Gotcha… This must be coalescence”. However, upon further heating, another AB quartet emerges at 377K! This appears to be a paradox, but only if we assume that coalescence of NMR signals of diastereotopic protons in variable-temperature experiments must be due to dynamic exchange processes. The danger of default assumptions… The re-appearance of the AB quartet at higher temperature (inset C) suggests that coalescence must attributed to coincidental chemical shift equivalence. In other words, the two diastereotopic protons that are shown display temperature-dependent chemical shifts that change in opposite directions, and whose coalescence is not to be equated with dynamic exchange. This is a fascinating finding suggesting that a lot of papers out there need to be reexamined (just heat beyond coalescence, baby, and see what happens).

aaa

http://pubs.acs.org/doi/abs/10.1021/jo301846a?m=102ivao&af=R&pageSize=20&searchText=PES

On this note, I am off to bed – I am catching a 7:30 plane back to Toronto. I hope not to see any nightmares related to overinterpretation of coalescence in our own work (as I write this note, I don’t think we did it, but the night is still young…).

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