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

d) 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 upfield.(Roberts)

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

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

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 is electron-poor, the peak appears "upfield," or towards the right of the spectrum. If the local environment of a hydogen is electron-rich, the peak appears farther "downfield," or towards the left of the spectrum.

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.

b) 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.

c) This figure more clearly shows the peaks from Figure b.

d) 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.

e) 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.

References

Pouchert, Charles J. The Aldrich library of FT NMR spectra 2nd ed. Aldrich Chemical Co.: Milwaukee, 1983.

Roberts, J. D. Nuclear Magnetic Resonance: Applications to Organic Chemistry. McGraw-Hill: New York, 1959.