19: Nuclear Magnetic Resonance Spectroscopy

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19.1: Theory of Nuclear Magnetic Resonance
As is the case with other forms of optical spectroscopy, the signal in nuclear magnetic resonance (NMR) spectroscopy arises from a difference in the energy levels occupied by the nuclei in the analyte. In this section we develop a general theory of nuclear magnetic resonance spectroscopy that draws on quantum mechanics and on classical mechanics to explain these energy levels.
19.2: Environmental Effects on NMR Spectra
In this section we will consider why the location of a nucleus within a molecule—what we call its environment—might affect the frequency at which it absorbs and why a particular absorption line might appear as a cluster of individual peaks instead of as a single peak.
19.3: NMR Spectrometers
Earlier we noted that there are two basic experimental designs for recording a NMR spectrum. One is a continuous-wave instrument in which we scan through the range of frequencies over which the nucleus of interest absorbs, exciting them sequentially. Most instruments use pulses of RF radiation to excite all nuclei at the same time and then use a Fourier transform to recover the signals from the individual nuclei. Our attention in this chapter is limited to instruments for FT-NMR.
19.4: Applications of Proton NMR
Proton NMR finds use for both qualitative analyses and quantitative analyses; in this section we briefly consider each of these areas.
19.5: Carbon-13 NMR
In this chapter we consider C-13 NMR, which was slower to develop than proton NMR because it has less sensitivity.
19.6: Two-Dimensional Fourier Transform NMR
The NMR spectra consider up to this point are shown in one dimension (1D), which is the frequency absorbed by the analyte's nuclei expressed in ppm. In addition to 1D experiments, there are a host of 2D experiments in which we apply a sequence of two or more pulses, recording the resulting FID after applying the last pulse. In this section we will consider one example of a 2D NMR experiment in some detail: 1H – 1H correlation spectroscopy, or 1H – 1H COSY.