How would you tell the difference between each of

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The following regioisomers are indistinguishable by elemental analysis and mass spectrometry. How would you tell the difference between each of them using only 1 H NMR? N H 3 CH 3 vs. N 3 H 3 C H vs. N H 3 C OCH 3 OCH OCH
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Misc. NMR questions Transition metal complexes of Arduengo carbenes are becoming increasingly important in organometallic catalysis. Consider the following Arduengo carbene copper complex: N N Cu CH 3 1. The 1 H NMR resonance for the copper-bound methyl group comes in at -0.491 ppm. In general, most alkyl resonances fall between 0 and 3 ppm. Qualitatively, why does this unusual methyl group exhibit a resonance with a chemical shift less than 0 ppm? 2. The 1 H NMR resonances for the isopropyl groups are as follows: 1.11 ppm (doublet, 12H), 1.45 ppm (doublet, 12H), and 2.69 ppm (septet, 4H). Normally, the methyl groups in an isopropyl group are chemically equivalent, and so one only observes two resonances attributable to the isopropyl group. Why, in this case, are there three distinct isopropyl resonances? (I.e., why are all three carbons in each of the isopropyl groups chemically inequivalent from each other?) 3. Represent the above molecule as LCuCH 3 , and pretend that NMR is your only tool of characterization. Using 1 H and 13 C NMR, the above structure would be indistinguishable - from the ion-paired structure [L 2 Cu] + [Cu(CH 3 ) 2 ] . If 63 Cu NMR were a viable method, - how could you tell the difference between the ion-paired [L 2 Cu] + [Cu(CH 3 ) 2 ] structure and the covalent LCuCH 3 structure shown above?
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  • Spring '05
  • NoProfessor
  • Chemistry, Nuclear magnetic resonance, Proton NMR, Methane, Carbon-13 NMR

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