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The NMR spectrum proves to be of great utility in structure elucidation because the properties it displays can be related to the chemist's perception of molecular structure. The chemical shift of a particular nucleus can be correlated with its chemical environment, the scalar coupling (or J-coupling) indicates an indirect interaction between individual nuclei, mediated by electrons in a chemical bond, and, under suitable conditions, the area of a resonance is related to the number of nuclei giving rise to it. Thus, when we speak of "assigning a spectrum" we are really trying to identify a chemical structure that is consistent with the spectrum; we may have a fairly good idea of what this structure is likely to be, or we may have very little.
Over the past fifty years nuclear magnetic resonance spectroscopy, commonly referred to as nmr, has become the preeminent technique for determining the structure of organic compounds. Of all the spectroscopic methods, it is the only one for which a complete analysis and interpretation of the entire spectrum is normally expected. Although larger amounts of sample are needed than for mass spectroscopy, nmr is non-destructive, and with modern instruments good data may be obtained from samples weighing less than a milligram.
A typical continuous wave (CW) -spectrometer is shown in the following diagram. A solution of the sample in a uniform 5 mm glass tube is oriented between the poles of a powerful magnet, and is spun to average any magnetic field variations, as well as tube imperfections. Radio frequency radiation of appropriate energy is broadcast into the sample from an antenna coil (colored red). A receiver coil surrounds the sample tube, and emission of absorbed rf energy is monitored by dedicated electronic devices and a computer. An nmr spectrum is acquired by varying or sweeping the magnetic field over a small range while observing the rf signal from the sample. An equally effective technique is to vary the frequency of the rf radiation while holding the external field constant.
Fig 1. Continuous wave (CW) -spectrometer
The general distribution of proton chemical shifts associated with different functional groups is summarized in the following chart.
Fig 2. Typical chemical shifts in 1H-NMR
Unlike proton nmr spectroscopy, the relative strength of carbon nmr signals are not normally proportional to the number of atoms generating each one. Because of this, the number of discrete signals and their chemical shifts are the most important pieces of evidence delivered by a carbon spectrum. The general distribution of carbon chemical shifts associated with different functional groups is summarized in the following chart. Bear in mind that these ranges are approximate, and may not encompass all compounds of a given class.
Fig 3. Typical chemical shifts in 13C-NMR