Introduction to Nuclear Magnetic Resonance Spectroscopy

The NMR phenomenon is dependent upon the magnetic properties of the nucleus. Certain nuclei possess spin angular momentum, which gives rise to different spin states in the presence of a magnetic field. Nuclei with 7=0, where I is the spin quantum number, including all nuclei with both an even atomic number and an even mass number, such as O and C, do not have a magnetic moment and therefore do not exhibit the NMR phenomenon. In theory, all other nuclei can be observed by NMR. 

NMR involves the absorption of radiowaves by the nuclei of some combined atoms in a molecule that is located in a magnetic field. NMR can be considered a type of absorption spectroscopy, not unlike ultraviolet/visible (UV/VIS) absorption spectroscopy. Radiowaves are low-energy electromagnetic radiation with frequencies on the order of 10 Hz. The SI unit of frequency, 1 Hz, is equal to the older frequency unit 1 cycle per second (cps) and has the dimension of inverse seconds, s . The energy of radiofrequency (RF) radiation can therefore be calculated from 

In analytical chemistry, NMR is a technique that enables us to study the shape and structure of molecules. In particular, it reveals the different chemical environments of the NMR-active nuclei preseut in a molecule, from which we can ascertain the structure of the molecule. NMR provides informatiou on the spatial orientation of atoms in a molecule. If we already know what types of compounds are present, NMR can provide a means of determining how much of each is in the mixture. It is thus a method for both qualitative and quantitative analysis, particularly of organic compounds. In addition, NMR is used to study chemical equilibria, reaction kinetics, motion of molecules, aud intermolecular interactions. 

Three Nobel Prizes have been awarded in the field of NMR. The first was in 1952 to the two physicists, E. Purcell and F. Bloch, who demonstrated the NMR effect in 1946. In 1991, R. Ernst and W. Anderson were awarded the Nobel Prize for developing pulsed Eourier transform (FT)-NMR and 2D NMR methods between 1960 and 1980. FT-NMR and 2D experiments form the basis of most NMR experiments run today, even in undergraduate instrumental analysis laboratories. We will use the acronym NMR to mean FT-NMR, since this is the predominant type of NMR instrument currently produced. The 2002 Nobel Prize in Chemistry was shared by three scientists for developing methods to use NMR and MS (MS is discussed in Chapters 9 and 10) in the analysis of large biologically important molecules such as proteins. K. Wiithrich, a Swiss professor of molecular biophysics, received the prize for his work in determining the 3D structure of proteins using NMR. 

Since the 1970s, the technology associated with NMR has advanced dramatically. The theory, instrnment design, and mathematics that make NMR so powerful are complex a good understanding of qnantnm mechanics, physics, and electrical engineering is needed to understand modern NMR experiments. Fortnnately, we do not need to completely understand the theory in order to make nse of the techniqne. This chapter will address NMR in a simplified approach using a minimum of mathematics. 

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Ault, A., and G. O. Dudek, NMR—An Introduction to Nuclear Magnetic Resonance Spectroscopy, Holden-Day, San Francisco, 1976. 

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