2012 BFY Abstract Detail Page
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||What's missing from the traditional explanation of NMR experiments?
||How do NMR experiments work? This question is addressed at the undergraduate level in both chemistry and physics lectures and laboratories. In most descriptions, it is taken for granted that Hydrogen NMR is proton NMR, without discussion of why this is the case, and how for Hydrogen, an external magnetic field creates a two state quantum system with an energy gap simply and directly proportional to the value of the nuclear magnetic moment. No mention is made of the magnetic moment of the two electrons that make up the covalent bond connecting the Hydrogen atom to the (most often) carbon backbone of the molecule. No mention is made of the complete cancelation of the electron-induced magnetic field at the Hydrogen nucleus that is required for this technique to yield easily interpretable data. We think a brief discussion of the implications of the Pauli Exclusion Principle in connection with the physics of covalent bonds would clarify the context in which NMR experiments with the hydrogens can be simply understood, especially with regard to the necessary disappearance of the hyperfine splitting of the ground state of Hydrogen.
See for example, A.C. Melissinos and J. Napolitano, ``Experiments in Modern Physics'', (Academic Press, San Diego, 2nd. Ed. 2003), Chap.7, and D.A. McQuarrie and J.D. Simon, ``Physical Chemistry: A Molecular Approach'', (University Science Books, Sausilito, CA, 1997) Chap.14. These are excellent texts which we use in our courses. We allege no errors in them about NMR experiments.
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Dr. Greg Severn
Department of Physics, University of San Diego
5998 Alcala Park
Phone: 619 260 6845
Dr. Jim Bolender
Dept. of Chemistry and Biochemistry, University of San Diego