Nuclear Magnetic Resonance (NMR) spectroscopy
Most people are aware of the medical magic called MRI (magnetic resonant imaging) scanning. By this methodology, one can get detailed pictures of the interior of the human body, often with selective visualization of damaged or unhealthy tissue. This method is based on a spectroscopy technique called nuclear magnetic resonance (NMR) - the word "nuclear" was dropped in the term MRI, lest the layperson mistake that ionizing radiation is associated with the method, which it is not. NMR spectroscopy is obtained in the radio frequency energy range of 60-750 Megahertz (106 sec-1).
NMR spectroscopy is also much sensitive than UV-vis, although it is easily possible to obtain spectra on 1 mg of compound. It is quite easy and convenient for quantitative measurements, using spectrometers that presently cost in the range of US$80,000-900,000. In addition, the spectrometers are sufficiently complex that almost all are interfaced to computers, and require dedicated technical support staff to maintain their functionality. Spectra must be obtained in special tubes (typically 5 or 10 mm diameter) as solutions, although recently it has become more easy to obtain spectra on at least 100 mg of pure, pressed powder samples (no solvent).
A Bruker 500 MHz FT-NMR spectrometer
NMR spectroscopy is perhaps the most powerful of the techniques we have discussed, although it has limitations. A variety of atomic isotopes have magnetic properties and will absorb energy. This is an absorption by the atomic nucleus when the appropriate input energy causes a "resonant absorption" at the natural frequency of the isotope, hence nuclear magnetic resonance. The absorption is specific for a particular atomic isotope.
A few of the abundant isotopes have strong NMR absorptions and give strong spectra. 1H, the most abundant isotope of hydrogen, gives good spectra readily. 31P, an important atom in biological molecules, likewise give good spectra. However, 12C has no magnetic moment, and gives no NMR spectrum. But, the 13C gives good spectra under special conditions that enhance sensitivity (important, since 13C is only 1.1% of natural abundance of carbon on earth). Many other nuclei also give NMR spectra, and can be used for special, selected studies. If necessary, chemists can synthesize special molecules with enrichment of NMR-active isotopes, in order to increase the sensitivity of their experiments.
NMR would not be very useful if one obtain only one peak for one type of isotope. But, the position of a NMR peak depends not only on the atom observed, but also upon the local environment of the atom. The atoms that are attached to the observed atom will influence its position and appearance, and in most cases atoms that are even a bit further away will have some influence. The result is that one typically gets a separate peak for each NMR-active atom that is not related to another by symmetry. For example, one can identify the number of different types of hydrogen atoms in a molecule by 1H-NMR.
In addition, by NMR one can identify the number of a type of atom that is attached nearby to a particular atom. This cannot be done for all nuclei, but can be done for magnetically active nuclei. As a result, one can recognize certain patterns in a spectrum as representing different ways of connecting atoms together. This is a tremendously powerful capability that is unique to NMR spectroscopy. Whereas one must determine UV-vis and IR spectral assignments by comparison to known molecules, one may understand NMR spectra from first principles. As a result, for molecules with NMR-active nuclei, NMR alone can obtain a tremendous amount of information.
Two examples of 1H-NMR spectra are shown. The molecules, propanol and isopropanol, have the same numbers of hydrogen atoms, but different structures. A full discussion of the patterns is beyond the scope of this page, but it should be obvious that these molecules -- which have extremely similar UV-vis and IR spectra, as well as the same molecular formulae, C3H8O -- have very different spectra. As a result, NMR allows their easy differentiation.
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Overall, NMR spectra give information about the types of atoms that are in a molecule, how the atoms are connected together, and the relative number of each type of atom that is in the molecule. In many cases, a complete or near-complete identification of molecular structure can be done by using a combination of NMR spectroscopic techniques looking at different nuclei.