Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is extremely useful for identification and analysis of organic compounds. The principle on which this form of spectroscopy is based is simple. The nuclei of many kinds of atoms act like tiny magnets and tend to become aligned in a magnetic field. In NMR spectroscopy, we measure the energy required to change the alignment of magnetic nuclei in a magnetic field. To illustrate the procedure with a simple example, consider the behavior of a proton (1H) in a magnetic field. There are two possible alignments of this magnetic nucleus with respect to the direction of the applied field, as shown in Figure 9-21. The nuclear magnets can be aligned either with the field direction, or opposed to it. The two orientations are not equivalent, and energy is required to change the more stable alignment to the less stable alignment.

Figure 9-21: Schematic representation of the possible alignments of a magnetic nucleus (here hydrogen) in an applied magnetic field. Transitions between the two states constitute the phenomenon of nuclear magnetic resonance. The arrows through the nuclei represent the average component of their nuclear magnetic moment in the field direction.

A schematic diagram of an NMR instrument is shown in Figure 9-22. When a substance such as ethanol, CH3−CH2−OH, the hydrogens of which have nuclei (protons) that are magnetic, is placed in the transmitter coil and the magnetic field is increased gradually, at certain field strengths radio-frequency energy is absorbed by the sample and the ammeter indicates an increase in the flow of current in the coil. The overall result is a spectrum such as the one shown in Figure 9-23. This spectrum is detailed enough to serve as a useful "fingerprint" for ethanol, and also is simple enough that we will be able to account for the origin of each line. It is the purpose of this section to explain how the complexities of spectra such as that of Figure 9-23 can be interpreted in terms of chemical structure.

Figure 9-22: Essential features of a simple NMR spectrometer

For what kinds of substances can we expect nuclear magnetic resonance absorption to occur? Magnetic properties always are found with nuclei of odd-numbered masses, 1H, 13C, 15N, 17O, 19F, 31P, and so on, as well as for nuclei of even mass but odd atomic number, 2H, 10B, 14N, and so on. Nuclei such as 12C, 16O, and 32S, which have even mass and atomic numbers, have no magnetic properties and do not give nuclear magnetic resonance signals. For various reasons, routine use of NMR spectra in organic chemistry is confined to 1H, 19F, 13C, and 31P. We shall be concerned in this chapter only with NMR spectra of hydrogen (1H) and of carbon (13C).

Figure 9-23: Proton NMR spectrum of ethanol (containing a trace of hydrochloric acid). Chemical shifts are relative to tetramethylsilane (CH4)4Si, that is, TMS =0.00ppm. The stepped line is an integral of the areas under each of the resonance lines.

The kind of NMR spectroscopy we shall discuss here is limited in its applications because it can be carried out only with liquids or solutions. Fortunately, the allowable range of solvents is large, from hydrocarbons to concentrated sulfuric acid, and for most compounds it is possible to find a suitable solvent.

Nuclear magnetic resonance spectra may be so simple as to have only a single absorption peak, but they also can be much more complex than the spectrum of Figure 9-23. However, it is important to recognize that no matter how complex an NMR spectrum appears to be, in involves just three parameters: chemical shifts, spin-spin splittings, and kinetic (reaction-rate) processes. We shall have more to say about each of these later. First, let us try to establish the relationship of NMR spectroscopy to some of the other forms of spectroscopy we have already discussed in this chapter.