NMR-3 Spin-Sping Coupling

NMR-3 Spin-Sping Coupling - NMR Spin-Spin Coupling Further...

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NMR Spin-Spin Coupling Further Discussion
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Signs and Mechanisms • Most important mechanism is Fermi contact mechanism (see below). • J can be either positive or negative. • Signs of J usually DO NOT affect the NMR spectra. Positive J Negative J Many exceptions 1 J HC = 120 ~ 200 Hz H C H
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One bond Coupling and s- Character 1 J HC (Hz) 144 160 179 S-character 29% 32% 36% sp2.4 sp2 sp1.8
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Signs of J (Another View) β α E β α E α α β β Positive J Negative J C C H b H a H a H b
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Two Bond Couplings H H 2 J HH = 12 ~ 18 Hz H H 2 J HH = 0 ~ 3 Hz H H 2 J HH = 5 Hz C H H ! 2 J HH α 109 120 114 C H H C H D J HH = J HD ! H ! D = 6.51 x J HD Geminal Coupling
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Three Bond Coupling C C H H 3 J HH Vicinal coupling φ 0 90 180 “Karplus relationship”
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Long Range Coupling 4 J, 5 J, 6 J etc. Coupling between protons that are separated by more than 3 bonds. • In saturated systems, 4 J and 5 J are commonly observed when C-H and C-C bonds exist in the zig-zag arrangement. H C C C C H H C C C H M or W arrangement
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Long Range Coupling in Saturated Systems H H H H H H J = 3 ~ 4 Hz H H J = 7 ~ 8 Hz CH 3 H J = 1.5 Hz H H J = 18 Hz
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Long Range Coupling in Unsaturated Systems C C H H C H H H 4 J = 1.33Hz 4 J = 1.75Hz H H H H 5 J = 9.63Hz 5 J = 8.04Hz H CH 3 6 J = 0.8Hz 6 J = 0.2Hz H H H H O 5 J = 0.4Hz H 3 C C C CH 3 5 J = 2.7Hz C C C CH 3 7 J = 1.3Hz C H 3 C C C C CH 2 OH 9 J = 0.4Hz (!) C C C H 3 C
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Trans-3-methoxyacrylonitrile H A H B H C “A 1st order spectrum”
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Bromostilbene Br H b H a H a = 7.024 ppm H b = 7.072 ppm “A 2nd order spectrum”
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This note was uploaded on 01/18/2012 for the course CHEM 460AB taught by Professor Sasaki during the Fall '11 term at University of Washington.

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NMR-3 Spin-Sping Coupling - NMR Spin-Spin Coupling Further...

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