Ch_322a_9.03

Ch_322a_9.03 - magnetic field, B o , the electrons...

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The Chemical Shift: Position of the Resonance The location in a spectrum of an nmr resonance or signal is characteristic of the type of hydrogen: aromatic, vinylic, aliphatic, adjacent to electronegative group,etc. Chemical shift is the term for the shift in the location of the resonance relative to a standard resonance. Local magnetic influences determine the chemical shift.
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Do not confuse this induced magnetic field with the intrinsic magnetic dipole, μ . + μ nuclear magnetic dipole the nucleus experiences an effective magnetic field of B o - b . This screening effect is called "shielding." Shielding When a molecule is placed in a magnetic field, the electrons begin circulating. The circulating electrons generate a local magnetic field. .. an induced magnetic field , b . In the presence of an external
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Unformatted text preview: magnetic field, B o , the electrons circulate around the nucleus resulting in an induced magnetic field, b , with lines of force opposing B o at the nucleus. B o b Influence of Shielding on the Chemical Shift B o A resonance of H with no electrons B "upfield" shift in resonance due to b C in presence of an electronegative group The extent of the "upfield" shift due to shielding depends on the electron density around the H. If there is a nearby electronegative group that decreases the electron density, the resonance will shift "downfield." The induced magnetic field requires the applied (external) magnetic field to be stronger than what is needed to achieve resonance in a proton where there are no circulating electrons....
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Ch_322a_9.03 - magnetic field, B o , the electrons...

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