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Nuclear Magnetic Resonance

Nuclear Magnetic Resonance. Yale Chemistry 800 MHz Supercooled Magnet. B o. Atomic nuclei in the presence of a magnetic field  spin - with the field  spin - opposed to the field. Atomic nuclei in The absence of a magnetic field. resonance.  spin.  spin. resonance.

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Nuclear Magnetic Resonance

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  1. Nuclear Magnetic Resonance Yale Chemistry 800 MHz Supercooled Magnet

  2. Bo Atomic nuclei in the presence of a magnetic field  spin - with the field  spin - opposed to the field Atomic nuclei in The absence of a magnetic field

  3. resonance  spin  spin resonance Ho noresonance noresonance Rf The Precessing Nucleus

  4. The Precessing Nucleus Again The Continuous Wave Spectrometer The Fourier Transform Spectrometer

  5. Observable Nuclei • Odd At. Wt.; s = ±1/2 • Nuclei 1H113C615N719F931P15 …. • Abundance (%) 99.98 1.1 0.385 100 100 • Odd At. No.; s = ±1 • Nuclei 2H114N7 … • Unobserved Nuclei • 12C616O832S16…

  6. E = h h = Planck’s constant:1.58x10-37kcal-sec • E = h/2(Bo) = Gyromagnetic ratio: sensitivity of nucleus to the magnetic field. 1H = 2.67x104 rad sec-1 gauss-1 • Thus: = /2(Bo) • For a proton, • if Bo = 14,092 gauss (1.41 tesla, 1.41 T), • = 60x106 cycles/sec = 60 MHz • and • EN = Nh= 0.006 cal/mole

  7. E 60 MHz 500 MHz E = h/2(Bo) 100 MHz 11.74 T 1.41 T 2.35 T Bo Rf Field vs. Magnetic Field for a Proton

  8. Nuclei Rf (MHz) Bo (T) /2 (MHz/T) 500.00 11.74 42.58 1H 202.51 76.78 470.54 125.74 11.74 11.74 11.74 11.74 40.08 6.54 17.25 10.71 13C 2H 19F 31P Rf Field /Magnetic Field for Some Nuclei

  9. at 1.41 T 60 MHz 60,000,000 Hz 60,000,600 Hz upfield shielded downfield deshielded } Me4Si 60 Hz 240 Hz } 10 9 8 7 6 5 4 3 2 1 0  scale (ppm) at 500 MHz 500 Hz chemical shift Fortunately, all protons are not created equally!  = (obs - TMS)/inst(MHz) = (240.00 - 0)/60 = 4.00

  10. 12 ppm 2H 12 ppm 1H (TMS) RCO2H ~380 ppm 31P (H3PO4) ~220 ppm CH4 76.78 220 ppm 13C 19F (CFCl3) 500.00 PX3 CPH2 -SO2F -CF2-CF2- 470.54 202.51 100 200 300 400 500 MHz 125.74 RCH3 R2C=O Where Nuclei Resonate at 11.74T

  11. Chemical Shifts of Protons

  12. TMS TMS CHCl3 =7.26 CH2Cl2 =5.30 CH3Cl =3.05 CH4 =-0.20 10 10 9 9 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 0 0   CHBr3 =6.83 CH2Br2 =4.95 CH3Br =2.68 The Effect of Electronegativity on Proton Chemical Shifts

  13.  2.2 TMS area = 3  4.0 area = 2 10 9 8 7 6 5 4 3 2 1 0  homotopic protons: chemically and magnetically equivalent enantiotopic protons: chemically and magnetically equivalent Chemical Shifts and Integrals

  14.  = 1.2 area = 9 TMS  = 6.4 area = 1  = 4.4 area = 1 10 9 8 7 6 5 4 3 2 1 0  Spin-Spin Splitting “anticipated” spectrum

  15.  = 1.2 area = 9 TMS  = 6.4 area = 1  = 4.4 area = 1 H H H H 10 9 8 7 6 5 4 3 2 1 0  Jcoupling constant = 1.5 Hz Spin-Spin Splitting observed spectrum J is a constant and independent of field

  16. H H H local H local observed observed 8 7 6 5 4 3  applied applied Spin-Spin Splitting  = 6.4 J = 1.5 Hz • = 4.4 J = 1.5 Hz This spectrum is not recorded at ~6 MHz! and J are not to scale.

  17. Multiplicity of Spin-Spin Splitting for s = ±1/2 multiplicity (m) = 2s + 1

  18.  = 1.68 area = 3  = 3.43 area = 2 90 Hz/46 pixels = 3J/12 pixels : J= 7.83 Hz 1H NMR of Ethyl Bromide (90 MHz) CH3CH2Br for H spins     for Hspins   

  19. 6H, d, J = 7.5 Hz 1H, s 4.25 3.80 1H NMR of Isopropanol (90 MHz) (CH3)2CHOH 1H septuplet; no coupling to OH (4.25-3.80)x90/6 = 7.5 Hz

  20. + D2O (CH3)2CHOD Proton Exchange of Isopropanol (90 MHz) (CH3)2CHOH

  21. homotopic sets of protons pro-R R Diastereotopic protons pro-S Diastereotopic Protons: 2-Bromobutane

  22. d1.03 (3H, t) d1.70 (3H, d) d1.82 (1H, m) d1.84 (1H, m) d4.09 (1H, m (sextet?)) Diastereotopic Protons: 2-Bromobutane at 90MHz

  23. d1.13 (3H, t, J = 7.3 Hz) Is this pattern at d2.46 a pentuplet given that there are two different values for J? d9.79 (1H, t, J = 1.4 Hz) 1H NMR of Propionaldehyde: 300 MHz No!

  24. d2.46 7.3 Hz 7.3 Hz 7.3 Hz 1.4 Hz 7.3 Hz quartet of doublets 7.3 Hz 1H NMR of Propionaldehyde: 300 MHz d9.79 (1H, t, J = 1.4 Hz) d1.13 (3H, t, J = 7.3 Hz)

  25. 3H, t, J = 7.0 Hz J = 14.4 Hz J = 1.9 Hz J = 6.9 Hz d3.96 (1H, dd, J = 6.9, 1.9 Hz) 2H, q, J = 7.0 Hz d4.17 (1H, dd, J = 14.4, 1.9 Hz) d6.46 (1H, dd, J = 14.4, 6.9 Hz) 1H NMR of Ethyl Vinyl Ether: 300 MHz

  26. d6.46 14.4 6.9 d4.17 1.9 1.9 14.4 d3.96 6.9 ABX Coupling in Ethyl Vinyl Ether d3.96 (1H, dd, J = 6.9, 1.9 Hz) d4.17 (1H, dd, J = 14.4, 1.9 Hz) d6.46 (1H, dd, J = 14.4, 6.9 Hz) Another example

  27. Dependence of J on the Dihedral Angle The Karplus Equation

  28. triplet of triplets He 3.0 3.0 He 4.03, J(Ha) = 3.0 Hz J(He) = 2.7 Hz 2.7 2.7 11.4 Hz 400 Hz 1H NMR (400 MHz): cis-4-t-Butylcyclohexanol He4.03, J(Ha) = 3.0 Hz J(He) = 2.7 Hz

  29. triplet of triplets Ha3.51, J(Ha) = 11.1 Hz J(He) = 4.3 Hz Ha 11.1 11.1 4.3 4.3 30.8 Hz 1H NMR (400 MHz): trans-4-t-Butylcyclohexanol

  30.  J  >> J  ~= J 10 10 10 9 9 9 8 8 8 7 7 7 6 6 6 5 5 5 4 4 4 3 3 3 2 2 2 1 1 1 0 0 0     = 0 Peak Shape as a Function of  vs. J

  31. H H H H H H Downfield shift for aromatic protons ( 6.0-8.5) Bo Magnetic Anisotropy

  32. R C C H Upfield shift for alkyne protons ( 1.7-3.1) Magnetic Anisotropy Bo

  33. 13C Nuclear Magnetic Resonance 13C Chemical Shifts

  34. Where’s Waldo?

  35. One carbon in 3 molecules of squalene is 13C What are the odds that two 13C are bonded to one another? ~10,000 to 1

  36. JCH = 151 Hz JCH = 3 Hz JCH = 126 Hz C1 C2 TMS JCH = 118 Hz JCH = 5 Hz 30 20 10 0 ppm () 26.6 18.3 13C NMR Spectrum of Ethyl Bromide at 62.8 MHz

  37. JCH = 151 Hz JCH = 3 Hz JCH = 126 Hz TMS JCH = 118 Hz JCH = 5 Hz 13C NMR Spectrum of Ethyl Bromide at 62.8 MHz Off resonance decoupling of the 1H region removes small C-H coupling. C1 Broadband decoupling removes all C-H coupling. C2 30 20 10 0 ppm () 26.6 18.3

  38. 13C Spectrum of Methyl Salicylate (Broadband Decoupled)

  39. 13C Spectrum of Methyl p-Hydroxybenzoate (Broadband Decoupled)

  40. The 13C Spectrum of Camphor

  41. The End F. E. Ziegler, 2004

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