Tuesday, 23 April 2013

Chemistry Classics: NMR Spectroscopy

Day two of #RealTimeChem week, and here's post two of five in my series about classic chemistry papers. This is meant to be a quick read to give you an idea of where the techniques organic chemists use every day originated.

Today: James T. Arnold, Srinivas S. Dharmatti, and Martin E. Packard, "Chemical Effects On Nuclear Induction Signals From Organic Compounds", J. Phys. Chem. 1951, 19, 507.

NMR spectroscopy hardly needs introducing. It's a routine technique for probing chemical structure, most commonly using 1H and 13C nuclei. The theory of NMR developed over several decades, and picking one point as the "first NMR paper" is somewhat misleading. Not that I'd let that stop me, of course! For a bit of context on the development of NMR, I recommend this short piece by Stuart Cantrill.

The paper I've picked is not the first NMR spectrum to be published, nor the first observation of chemical shift. I think it represents a turning point, though: the first time NMR was used to characterise the structure of a typical organic molecule, pointing the way to the powerful tool we now have at our disposal.

In less than one page, Arnold et al. report the 1H spectra of the first five alkyl alcohols: methanol, ethanol, n-propanol, n-butanol and n-pentanol. The only spectrum that is printed is that of ethanol (typical chemists...). The authors integrated the peaks (relative to the -OH peak) and showed that they are proportional to the number of protons in the signal. In addition to reporting the spectra of these compounds, the authors allude to measurements of secondary and tertiary isomers, as well as carboxylic acids.

Their final comment is prescient: "It seems to us that there may be certain chemical problems besides analysis, such as the study of chemical reactions and equilibria, which can be investigated by this method."

Within a couple of years, significantly improved spectra of the oh-so-important ethyl alcohol had appeared, allowing chemists to see the fine structure in each peak, and within a couple of decades NMR had been commercialised and widely adopted.

If you're interested, here are a few good articles about the development of NMR which provide context for this paper.
Stuart Cantrill: NMR for chemical analysis, 1950-1951
James N. Shoolery: NMR spectroscopy in the beginning
David Ellard: history of MRI

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