Nuclear Magnetic Resonance  

These NMR spectra are representative of NMR spectroscopy from its origin to present.

HO-CH2-CH3

Ethanol

1951 NMR spectrum of Ethanol

The first NMR spectrum of ethanol, taken at Stanford University in 1951.
Courtesy Varian Associates, Inc.

 

Low Res EtOH spectrum

Detail of the chemical characteristics represented by the first spectrum taken , the low resolution NMR spectrum at top. (Roberts)

High Res EtOH spectrum

Modern, high resolution spectrum of the characteristic ethyl triplet quartet in dilute ethanol.
The OH peak is not shown, but woud be a singlet farther downfield. (Roberts)

Complete, modern FT-NMR spectrum of ethanol. (Pouchert)

Modern EtOH NMR spectrum

 


NMR Spectroscopy

NMR spectroscopy measures the magnetic characteristics of a specific atom, usually hydrogen, in a molecule. Thus, hydrogens in different local environments of the sample molecule will give different peaks according to the electronic character of the environment. If the local environment of the hyrogen atom nucleus has a high electron density then the peak appears "upfield," or towards the right of the spectrum. If the local environment of a hydogen has a low electron density (eg the hydrogen atom is attached to an electronegative element) , the peak appears farther "downfield," or towards the left of the spectrum.

The spectrum measures the ease with which the nuclei of the hyrogen atoms can be changes from one nuclear spin state to another (hydtrogen can have two 'spin' states) One of the spin states may be thought of as being aligned with the magnetic field and the other (higher spin) against the magnetic field.

When electromagnetic radiation is appied to the nuclei when they are in the magnetic field they can be changed to the higher spin state by absorbing radiation. This radiation can be measured and recorded.

When the local electrons are high in density they 'sheild' the nucleus from the effect of the magnetic field.

NMR peaks are also split, as shown in Figure d. The splitting pattern is a function of the number of hydogens adjacent to the peak hydrogens. For example, in ethanol (Figure a), the three hydrogens of the CH3 group are adjacent to two hydrogens of the CH2 group. The number of peaks when split is equal to the number of adjacent peaks plus one. For the CH3 group of ethanol there are two adjacent hydrogens, so we expect the CH3 peak to be split into 3 peaks (2 adjacent hydrogens + 1). This is exactly what is observed in Figure d. In NMR nomenclature, splitting of a peak into one is a singlet, two is a doublet, three is a triplet, four is a quartet, and so on.

The first NMR spectrum of a fluid sample was taken of ethanol at Stanford University in 1951. Even from this early spectrum, the three hydrogen peaks can be resolved.

The splitting pattern described above is clearly shown in this figure. The OH peak, not shown, would be a singlet, not a triplet. This happens because the sample is contamintated by water. The OH hydrogen exchanges with hydrogens from water, preventing a peak from being recorded. As a result, the OH hydrogen does not split the CH2 peak.

The modern day spectrum

This type of modern, full-scale NMR spectrum is easily recognizable by any modern chemist. The line that rises above the peaks of the spectrum is the integration, or the area under the peaks. The integration of each peak is proportional to the number of hydrogens that peak represents.

The second, smaller spectrum at top is a 13C spectrum. The 13C spectrum uses a slightly different form of analysis to probe the electronic environments of the carbons in a sample. Because there are two carbons in ethanol, two main peaks are seen. The rest of the peaks are the solvent (CDCl3) or impurities.

Carbon-13 also can have a more complicated spectrum as its nucleus has more than two possible spin states.

 

 


 
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