Wednesday, July 16, 2008

Turning Point

The turning point in dynamic NMR was the article "DNMR: the program" by Gerhard Binsch (JACS, March 12, 1969). The article said: if we monitor chemical exchange by the coalescence of two singlets, a large variation of the rate is reflected into a small change of the spectrum. Therefore our estimate will be inaccurate.

If, keeping A and B as they are, we simply add a third nucleus C, coupled with both A and B, see what happens:

Now a small variation in the rate of exchange is the cause of a large change in the spectrum. Therefore we can estimate the rate (by simulation) with higher accuracy and confidence.
40 years later the lesson has not been learned yet and there's people who prefer to add a methyl group to their compounds to monitor the exchange rate by the coalescence of the singlets. I can understand this choice if the reason is to maximize the intensity of the signals.
I suspect, however, that the true reason is a different one. There is an approximate formula from which you can calculate the rate of exchange from the temperature of coalescence.
It's very approximate, but much easier to put in practice. A single spectrum and a simple formula instead of collecting 10 spectra and fitting each element of the series, then plotting the data to extrapolate the Eyring equation... The accurate method not only is much faster, it also avoids using a computer. The problem, today, is never the computer but always the software. Is the 40 years old DNMR difficult to obtain? You don't need DNMR: today you have WinDNMR, Spinworks, Mexico, iNMR....


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