03 - NMR 2 - Other shielding effects H C C H CH C H a...

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1 Other shielding effects a) Effective for acetylene b) not as effective for acetylene Therefore, shifts for alkyne protons are only ~2.9-1.9. H C C H CH C H Hax is shielded by 2-3 and 5-6 c-c bonds; upfield by ~0.5 ppm Compare methyl, methylene, and methine protons 0.87 0.88 1.45 1.20
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2 Chemical shift table for 1 H examples O O 6.37 4.65 O 5.93 6.88 O N O O 8.22 7.48 7.61 N O O
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3 Despite the electron withdrawing ability of the oxygen, this resonance form of the electron donating methoxy “prevents” a downfield shift ortho to the group OCH 3 6.79 7.24 6.73 OCH 3 Chemical shift table for 13 C
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4 NMR spectra of methyl acetate 1 H chemical shifts
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6 13 C Chemical shifts
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9 Empirical methods for chemical shift calculation There are a number of methods used to calculate the approximate chemical shift for both 1 H and 13 C. Shoolery’s rule for sat. aliphatics δ = 0.23+ x + y See table 3-1 Tobey-Simon Rule: δ = 5.28+Z gem +Z cis +Z trans See table 3-2 for Z values For example, styrene: 1.35, 0.37, -0.10, respectively styrene H H H predicted/found 6.63/6.80 5.18/5.31 5.65/5.81
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03 - NMR 2 - Other shielding effects H C C H CH C H a...

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