1 3 Nuclear Magnetic Resonance Spectroscopy Nuclear magnetic resonance spectroscopy (NMR) is the most powerful tool available for organic structure determination. Like IR spectroscopy, NMR can be used with a very small sample, and it does not harm the sample. The NMR spectrum provides a great deal of information about the structure of the compound, and some structures can be deter-mined using only the NMR spectrum. More commonly, however, the NMR spectrum is used in conjunction with other forms of spectroscopy and chemical analysis to determine the structures of complicated organic molecules. NMR is used to study a wide variety of nuclei, including IH, l3C, 15N, 19F, and 3 1p. Organic chemists find proton (IH) and carbon-13 (l3e) NMR to be most use-ful because hydrogen and carbon are major components of organic compounds. His-torically, NMR was first used to study protons (the nuclei of hydrogen atoms), and proton magnetic resonance ( I H NMR) spectrometers are the most common. "Nuclear magnetic resonance" is assumed to mean "proton magnetic resonance" unless a differ-ent nucleus is specified. We begin our study of NMR with proton magnetic resonance and conclude with a discussion of l 3C NMR. A nucleus with an odd atomic number or an odd mass number has a nuclear spin that can be observed by the NMR spectrometer. A proton is the simplest nucleus, and its odd atomic number of 1 implies it has a spin. We can visualize a spinning proton as a rotating sphere of positive charge (Figure 13-1). This movement of charge is like an electric current in a loop of wire. It generates a magnetic field (symbolized by B), called the magnetic moment, that looks like the field of a small bar magnet. spinnjng proton loop of current bar magnet circulation � (� �\ H H \ J � � induced magnetic field 1 3-1 I ntroduction 1 3-2 Theory of Nuclear Magnetic Resonance ... Figure 1 3-1 The magnetic moment. A spinning proton generates a magnetic field, called jts magnetic moment. Thjs magnetic field (8) resembles that of a small loop of current or bar magnet. 559
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