Optimizing NMR Methods for Structure Elucidation: Characterizing Natural Products and Other Organic Compounds
Chapter 2: Basics of the NMR Experiment
Published:26 Sep 2018
Special Collection: 2018 ebook collectionSeries: New Developments in NMR
2018. "Basics of the NMR Experiment", Optimizing NMR Methods for Structure Elucidation: Characterizing Natural Products and Other Organic Compounds, Darcy C Burns, William F Reynolds
Download citation file:
While this chapter contains more equations than all of the rest of the chapters combined, we have endeavored to keep it as non-mathematical as possible. It begins with a discussion of spin quantum numbers of nuclei and their magnetic moments, stressing why spin ½ nuclei are of particular importance for structure elucidation. It follows with the interaction of nuclear magnetic moments with magnetic fields. The energy level separation between nuclear spin levels in a magnetic field is small, resulting in a very small difference in nuclear spin populations in the different levels. This accounts for the intrinsically low sensitivity of NMR compared to other widely used spectroscopic techniques such as infrared spectroscopy. However, this is set off by the fact that NMR peaks are many orders of magnitude narrower than IR peaks, providing much more detailed molecular structure information. This is followed by two different simplified ways of describing the NMR experiment, one quantum mechanical and the other classical. Next is a discussion of four key NMR parameters: chemical shifts, coupling constants, spin–lattice (T1) and spin–spin (T2). Finally, a non-mathematical explanation of the origins of nuclear Overhauser enhancements is presented.