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1 H NMR Spectroscopy:

1 H NMR Spectroscopy:. Nuclear Magnetic Resonance. Nuclear Shielding and 1 H Chemical Shifts. What do we mean by "shielding?" What do we mean by "chemical shift?". The electrons surrounding a nucleus affect the effective magnetic field sensed by the nucleus. C. H. Shielding.

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1 H NMR Spectroscopy:

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  1. 1H NMR Spectroscopy: Nuclear Magnetic Resonance

  2. Nuclear Shieldingand1H Chemical Shifts What do we mean by "shielding?" What do we mean by "chemical shift?"

  3. The electrons surrounding a nucleus affect the effective magnetic field sensed by the nucleus

  4. C H Shielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. H0

  5. C H Shielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. The direction of the induced magnetic field is opposite to that of the applied field. H0

  6. C H Shielding The induced field shields the nuclei (in this case, C and H) from the applied field. A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation. H0

  7. C H Chemical Shift Chemical shift is a measure of the degree to which a nucleus in a molecule is shielded. Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts. H0

  8. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 DownfieldDecreased shielding UpfieldIncreased shielding (CH3)4Si (TMS) Chemical shift (d, ppm)measured relative to TMS

  9. Cl Cl H C Cl 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 d 7.28 ppm The chemical shift, d ppm, is a measure of how far the signal is from the TMS reference signal which is set to 0ppm Chemical shift (d, ppm)

  10. The common scale for chemical shifts = d (ppm) distance downfield from TMS (Hz) d = operating frequency of the spectrometer (MHz) Chemical Shift: CH3

  11. Effects of Molecular Structureon1H Chemical Shifts protons in different environments experience different degrees of shielding and have different chemical shifts

  12. The chemical shift is independent of the operating frequency of the spectrometer

  13. 1456 Hz - 0 Hz position of signal - position of TMS peak x 106 d = x 106 d = spectrometer frequency 200 x 106 Hx Chemical Shift Example: The signal for the proton in chloroform (HCCl3) appears 1456 Hz downfield from TMS at a spectrometer frequency of 200 MHz. d = 7.28

  14. Question • What is the chemical shift of a proton that appears at 2065 Hz when recorded on a 300 MHz NMR spectrometer? • A) 6.88 ppm • B) 0.14 ppm • C) 2.07 ppm • D) 1.40 ppm

  15. Electron withdrawal produces NMR signals downfield at higher frequency (at larger d values)

  16. Electronegative substituents decreasethe shielding of methyl groups CH3Fd 4.3 ppm CH3OCH3d 3.2 ppm CH3N(CH3)2d 2.2 ppm CH3CH3d 0.9 ppm CH3Si(CH3)3d 0.0 ppm

  17. Electronegative substituents decreasethe shielding of methyl groups CH3Fd 4.3 ppm least shielded H CH3OCH3d 3.2 ppm CH3N(CH3)2d 2.2 ppm CH3CH3d 0.9 ppm CH3Si(CH3)3d 0.0 ppm most shielded H

  18. Effect is cumulative CHCl3d 7.3 ppm CH2Cl2d 5.3 ppm CH3Cld 3.1 ppm

  19. Question • Which proton is most shielded? • A) CHCl3 • B) CH2Cl2 • C) CHBr3 • D) CBr4

  20. d 0.9 d 0.9 CH3 CH3 d 1.2 d 0.8 H3C H d 1.6 C C H3C CH2 CH3 CH3 CH3 Methyl, Methylene, and Methine CH3 more shielded than CH2 ; CH2 more shielded than CH

  21. Question 3 • Select the most shielded proton in 1,1,2-trichlorobutane. • A) 1 • B) 2 • C) 3 • D) 4

  22. H H H H H C C H H H H H Protons attached to sp2 hybridized carbonare less shielded than those attachedto sp3 hybridized carbon CH3CH3 d 7.3 ppm d 5.3 ppm d 0.9 ppm

  23. Diamagnetic Anisotropy The p electrons are less tightly held by the nuclei than are s electrons; they are more free to move in response to a magnetic field Causes unusual chemical shifts for hydrogen bonded to carbons that form p bonds

  24.  5.3 H CH2OCH3  2.4 C C H H C C H H But Protons Attached to sp Hybridized Carbonare More Shielded than those Attachedto sp2 Hybridized Carbon

  25. d 0.9 d 1.3 d 0.9 H3C CH3  0.8  1.5  1.2  2.6 H3C CH2 Protons Attached to Benzylic and AllylicCarbons are Somewhat Less Shielded than Usual H3C—CH2—CH3

  26. Question • Assign the chemical shifts d 1.6, • d 2.2, and d 4.8 to the appropriate protons of methylene cyclopentane. • A) x = 1.6; y = 2.2; z = 4.8 • B) x = 4.8; y = 1.6; z = 2.2 • C) x = 1.6; y = 4.8; z = 2.2 • D) x = 2.2; y = 1.6; z = 4.8

  27. O d 2.4 H d 9.7 H3C C C H d 1.1 CH3 Proton Attached to C=O of Aldehydeis Most Deshielded C—H

  28. Question • Assign the chemical shifts d1.1, • d2.4, and d9.7 to the appropriate protons of propanal. • A) x = 2.4; y = 1.1; z = 9.7 • B) x = 1.1; y = 9.7; z = 2.4 • C) x = 9.7; y = 2.4; z = 1.1 • D) x = 1.1; y = 2.4; z = 9.7

  29. H C C C H C H H C C C C O C C H C Chemical Shift Table Type of proton Chemical shift (d),ppm Type of proton Chemical shift (d),ppm 2.5 R 0.9-1.8 Ar 2.3-2.8 1.6-2.6 H 4.5-6.5 2.1-2.5 C

  30. H Cl C H Ar O H C C H H O C H NR C Chemical Shift Table Type of proton Chemical shift (d),ppm Type of proton Chemical shift (d),ppm 3.1-4.1 6.5-8.5 Br 2.7-4.1 9-10 3.3-3.7 2.2-2.9

  31. H NR H OR H OAr O C HO Chemical Shift Table Type of proton Chemical shift (d),ppm 1-3 0.5-5 6-8 10-13

  32. Characteristic Values of Chemical Shifts

  33. 1H NMR spectrum / Integration 1-bromo-2,2-dimethylpropane

  34. Integration The area under each signal is proportional to the number of protons that give rise to that signal. The height of each integration step is proportional to the area under a specific signal. The integration tells us the relative number of protons that give rise to each signal, not absolute number.

  35. 1H NMR spectrum / Integration 1-bromo-2,2-dimethylpropane 4.5 (9) (2) 1

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