Ch_322a_9.05

Ch_322a_9.05 - b produces shielding and deshielding sectors...

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Shielding by !"# lectrons The highly polarizable ! -electrons circulate in the presence of a magnetic field which produces an induced magnetic field, b . Aromatics benzene a "ring current" is produced from the circulating electrons b lines of force of b are perpendicular to plane of ring Because of the orientation of the induced magnetic field relative to the planar benzene ring, there are defined "shielding" ( B o - b ) and "deshielding" ( B o + b ) sectors. Shielding means the resonance is shifted to higher field and deshielding means the resonance is shifted to lower external magnetic field. B o + + + + + + + + + - - - - - - - The " anisotropy " of the induced magnetic field.
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Additional ! -Electron Systems Alkynes R-C C-R b the lines of force of b oppose B o along the molecular axis The anisotropy of
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Unformatted text preview: b produces shielding and deshielding sectors. The R groups are in a shielding sector , but another factor also influences the chemical shift. C C R R + + + + _ _ _ _ _ _ _ _ _ B o C C R R In the presence of an external magnetic field (B o ) , the !-electrons circulate around the molecular axis. b The lines of force of b oppose B o perpendicular to the molecular plane. The anisotropy of b produces shielding and deshielding sectors. The R groups are in a deshielding sector . C C + + + + _ _ _ _ _ _ R R R R Additional !-Electron Systems Alkenes C=C R R R R C C B o In the presence of an external magnetic field (B o ) , the !-electrons circulate above and below the molecular plane....
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This note was uploaded on 07/22/2009 for the course CHEM 322AL taught by Professor Jung during the Summer '07 term at USC.

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Ch_322a_9.05 - b produces shielding and deshielding sectors...

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