Introduction to NMR Spectroscopy

NMR is a spectroscopic technique that relies on the magnetic properties of the atomic nucleus. When placed in a strong magnetic field, certain nuclei resonate at a characteristic frequency in the radio frequency range of the electromagnetic spectrum. Slight variations in this resonant frequency give us detailed information about the molecular structure in which the atom resides. 

NMR Spectroscopy Explained Simplified Theory, Applications and Examples for Organic Chemistry and Structural Biology, by Neil E Jacobsen Copyright 2007 John Wiley Sons, Inc. 

The resonant frequencies of some important nuclei are shown below for the magnetic field strength of a typical NMR spectrometer (Varian Gemini-200)  

The spectrometer is a radio receiver, and we change the frequency to tune in each nucleus at its characteristic frequency, just like the stations on your car radio. Because the resonant frequency is proportional to the external magnetic field strength, all of the resonant frequencies above would be increased by the same factor with a stronger magnetic field. The relative sensitivity is a direct result of the strength of the nuclear magnet, and the effective sensitivity is further reduced for those nuclei that occur at low natural abundance. For example, 13C at natural abundance is 5700 times less sensitive (1/(0.011 x 0.016)) than H when both factors are taken into consideration. 

The resonant frequency is not only a characteristic of the type of nucleus but also varies slightly depending on the position of that atom within a molecule (the chemical environment ). This occurs because the bonding electrons create their own small magnetic field that modifies the external magnetic field in the vicinity of the nucleus. This subtle variation, on the order of one part in a million, is called the chemical shift and provides detailed information about the structure of molecules. Different atoms within a molecule can be identified by their chemical shift, based on molecular symmetry and the predictable effects of nearby electronegative atoms and unsaturated groups. 

If necessary, review the suggested sections to prepare for this chapter  

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Nudear mi netic resonance (NMR) spectroscopy is arguably the most powerful and broadly applicable technique for structure determination available to organic chemists. It provides the most information about molecular structure, and in some cases, the structure of a compound can be determined using only NMRspectroscopy. In practice, the structures of compUcated molecules are determined through a combination of techniques that include NMR and IR spectroscopy and mass spectrometry. 

NMR spectroscopy involves the study of the interaction between electromagnetic radiation and the nuclei of atoms. A wide variety of nuclei can be studied using NMR spectroscopy, including H, N, and In practice, NMR spectroscopy and NMR spec- 

When the nucleus of a hydrogen atom (a proton) is subjected to an external magnetic field, the interaction between the magnetic moment and the magnetic field is quantized, and the 

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Abraham R J, Fisher J and Loftus P 1988 Introduction to NMR Spectroscopy (Chichester Wiley)... 

R.J. Abraham, J. Fisher and P. Loltus, Introduction to NMR Spectroscopy, John Wiley Sons, Ltd, Chichester (1988). 

Almost every NMR book begins with a discussion of basic theory. Short, elementary accounts of theory are given in such books as Nuclear Magnetic Resonance by P. J. Hore28 and Introduction to NMR Spectroscopy by R. J. Abraham, J. Fisher, and P. Loftus,29 as well as in books mentioned later that are directed toward biological applications or molecular structure elucidation. 

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