Organic Spectroscopy Lecture 5 Notes

Cook ndsu thursday january 17 13 7 inversion recovery

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Unformatted text preview: n-Recovery Experiment ©2013 Gregory R. Cook, NDSU Thursday, January 17, 13 8 13C ©2013 Gregory R. Cook, NDSU Thursday, January 17, 13 NMR pulse sequences 9 13C ©2013 Gregory R. Cook, NDSU Thursday, January 17, 13 NMR pulse sequences 10 nOe - nuclear Overhauser effect ‣ ‣ Through space interaction between nuclei whether or not they are coupled Magnitude decreases as inverse of distance to the sixth power nOe ∝ 1/L 6 C C HA HB L ~3-4 Å ‣ Strongly irradiate one peak making the equilibrium for that peak with greater number of spins in excited state. Nearby nuclei adjust to lower energy state to compensate and peak intensity increases. ‣ Best with rigid systems. ‣ Enhancement typically small. Need to take a difference spectrum to see it. ©2013 Gregory R. Cook, NDSU Thursday, January 17, 13 11 nOe - nuclear Overhauser effect ‣ Also called “Cross Polarization” ‣ Maximum enancement ‣ NOEmax = 1/2 (γirr/γobs) ‣ For 1H --> 13C enhancement 1/2 (267.5/67.28) = 1.988 ~200% enhancement possible ‣ For 1H --> 1H enhancement 1/2 (267.5/267.5) = 0.5 ~50% enhancement possible ‣ Generally much smaller than maximum possible enhancement ‣ Depends on rate of relaxation and distance ©2013 Gregory R. Cook, NDSU Thursday, January 17, 13 12 nOe - nuclear Overhauser effect ©2013 Gregory R. Cook, NDSU Thursday, January 17, 13 13 nOe - nuclear Overhauser effect ‣ nOe is very sensitive to relaxation so it is best to exclude all paramagnetic materials (iron particles) and deoxygenate your solvents B H H3C C C C A H3C CO2H irr A: 17% enhancement of C irr B: no enhancement of C H3C OH H geraniol H ©2013 Gregory R. Cook, NDSU Thursday, January 17...
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