nmr theory - Since the magnetic dipole of a given nucleus...

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Since the magnetic dipole of a given nucleus is a constant, you might predict that all nuclei of a given type would undergo the spin-flip transition at exactly the same applied frequency in a given magnetic field. Fortunately, in a typical organic molecule this is not the case. This is because the electrons in the molecule have small magnetic fields associated with them and these tend to oppose the applied field, screening the nuclei from the full strength of the applied field. The greater the electron density, the greater this 'shielding' will be, hence nuclei which are in electron rich environments will undergo transition at a higher applied field than nuclei in electron poor environments. The resulting shift in the NMR signal for a given nuclei is referred to as the chemical shift , and, in general, protons or carbons adjacent to electronegative atoms will be deshielded and moved to a higher chemical shift (undergo transition at a lower applied field; i.e. “downfield”). The scale utilized for measuring chemical shifts is defined by the equation shown below: Chemical Shift ( ) = (shift observed/oscillator frequency) x 10 6 = ppm Experimentally, for both 1 H and 13 C NMR, the scale is anchored at zero by the NMR absorption of the molecule tetramethyl silane ((CH 3 ) 4 Si) in which the carbons and protons are more highly shielded than those observed in most common organic molecules. For 1 H NMR, the scale generally extends from 0-12 ppm; the scale for 13 C nuclei, however, is much larger and covers the range 0-220 ppm. On the scale for 1 H NMR , simple hydrocarbon protons tend to absorb in the region 0.5-1.5, protons on a carbon adjacent to a carbonyl are shifted to 2, electronegative atoms (oxygen or halogens) move -protons to 3-4, alkene protons are shifted to 5- 6, aromatic protons to 7-8, aldehydic protons to 10, and the most highly shifted protons are generally those of carboxylic acids, with values of 12. Click here to see a table showing these correlations. Electronegative groups are "deshielding" and tend to move NMR signals from neighboring protons further "downfield" (to higher ppm values). Protons on oxygen or nitrogen have highly variable chemical shifts which are sensitive to concentration, solvent, temperature, etc. The -system of alkenes, aromatic compounds and carbonyls strongly deshield attached protons and move them "downfield" to higher ppm values. For 13 C NMR , simple methyl carbons tend to absorb in the region 15-30, simple methylene carbons are shifted to 20-65, electronegative atoms (oxygen or halogens) move attached carbons to 40-80, alkyne carbons are shifted to 70-90, alkene carbons to 100-150, aromatic carbons to 120-170, and the most highly shifted carbons are generally those of carbonyls, with values of 180-220. Click here to see a table showing these correlations. Electronegative groups are "deshielding" and tend to
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This note was uploaded on 03/31/2009 for the course CH 318N taught by Professor Willson during the Spring '08 term at University of Texas at Austin.

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nmr theory - Since the magnetic dipole of a given nucleus...

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