Fundamental Concepts of NMR Spectroscopy

The nucleus of each isotope has the following intrinsic properties  

This determines the number of quantum states available for the nucleus. 

For example, a spin- /2 nucleus can be viewed as having two quantum states one with the spin axis at a 45° angle to the external magnetic field and one with the spin axis at a 135° angle to the external field. A spin-1 nucleus can be viewed as having three possible states 45°, 90°, and 135°. In this book we will be concerned primarily with spin- /2 nuclei. 

Here are some examples showing the composition of the nucleus (p = protons, n = neutrons)  

Note that there is a pattern Nuclei with an even number of protons and neutrons (even-even) have spin zero odd-even and even-odd nuclei tend to be spin-Vi and odd-odd nuclei tend to have a spin greater than 1/2. This is just a rule of thumb (e.g., 170 violates the rule ). Nuclei with spin greater than 1/2 are more difficult to observe than spin-Vi nuclei because they have a nuclear quadrupole moment that makes their NMR peaks very broad. For this reason, most NMR work is focused on the spin-Vi nuclei. Because NMR is usually done in deuterated solvents (D2O, CD3OD, etc.), we will have to occasionally consider the effects of a spin-1 (three quantum states) nucleus. 

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Fourier Transform NMR. In Fourier transform NMR (FTNMR), a repetitive radio frequency (RF) pulse is applied in order to excite all of the nuclei of the particular nuclear species being studied. The sum of the free induction decay (FID) curves from each pulse is analyzed by a Fourier transform method in order to generate the familiar frequency domain spectra. Fundamentally, parameters such as the frequency, intensity, application time of the appropriate RF pulse, and time intervals between these pulses are important variables when using this technique. The principle of the pulsed Fourier transform technique can be found in books covering the fundamental concepts of NMR spectroscopy (58,59). 

There are a large number of structural parameters for NMR of different nuclei and many examples of how they can be applied to the analysis of hydroxylamines, oximes and hydroxamic acids. Fortunately though, there are many very clear, meticulously written descriptions of INEPT, DEPT, INADEQUATE, COSY, NOESY and the like, in one- and two-dimensional NMR spectroscopy, that are cited in the references. Since their content is beyond the scope of the present chapter, a brief mention of some of the fundamental concepts that are essential for its understanding by the nonspecialist is in order. 

No two instructors teach organic chemistry exactly the same way. This book covers all the fundamental topics in detail, building each new concept on those that come before. Many topics may be given more or less emphasis at the discretion of the instructor. Examples of these topics are C NMR spectroscopy, ultraviolet spectroscopy, conservation of orbital symmetiy, amino acids and proteins, nucleic acids, and the special topics chapters, lipids and synthetic polymers. 

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