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Chapter 13 Spectroscopy

Chapter 13 Spectroscopy. Infrared spectroscopy Ultraviolet-Visible spectroscopy Nuclear magnetic resonance spectroscopy Mass Spectrometry. Chapter 13; Spectroscopy. Principles of Spectroscopy NMR Theory NMR Active Nuclei Resonance Obtaining a NMR Spectrum Analysis of 1 H-NMR Spectrum

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Chapter 13 Spectroscopy

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  1. Chapter 13Spectroscopy Infrared spectroscopy Ultraviolet-Visible spectroscopy Nuclear magnetic resonance spectroscopy Mass Spectrometry

  2. Chapter 13; Spectroscopy • Principles of Spectroscopy • NMR Theory • NMR Active Nuclei • Resonance • Obtaining a NMR Spectrum • Analysis of 1H-NMR Spectrum • Number of Peaks • Area of Peak • d value • Spin- spin splitting (n+1) rule

  3. The Electromagnetic Spectrum Cosmic rays  Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves Energy

  4. Shorter Wavelength () Longer Wavelength () Ultraviolet Infrared Higher Frequency () Lower Frequency () Higher Energy (E) Lower Energy (E) The Electromagnetic Spectrum 400 nm 750 nm Visible Light

  5. Sample Detector E = h Electromagnetic Radiation Electromagnetic radiation is absorbed when theenergy of photon corresponds to difference in energy between two states.

  6. 13.3Introduction to 1H NMR Spectroscopy Proton Nuclear Magnetic Resonance Spectroscopy 1H and 13C are the most useful nuclei to organic chemists.

  7. Mass Number Z Atomic Number Mass Number; Number of Proton + Nuetrons in Nucleus Atomic Number; Number of Protons in Nucleus Only nuclei with either odd atomic # or odd mass # are NMR active

  8. + + Nuclear Spin A spinning charge, such as the nucleus of 1H or 13C, generates a magnetic field. The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin –1/2.

  9. + + + + The distribution of nuclear spins is random in the absence of an external magnetic field. +

  10. + + + + There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. + H0

  11. + E ' E + Energy Differences Between Nuclear Spin States no difference in absence of magnetic field proportional to strength of external magnetic field increasing field strength

  12. + + E The two nuclei are in resonance when the amount of energy (in the radiofrequency range) applied equals the energy difference between the two spin states

  13. 13C 35Cl 19F 2H 37Cl 1H 90 22.6 84.6 13.7 8.82 The frequency of absorbed electromagneticradiation is different for different elements, and for different isotopes of the same element. (Ho = 21150 Gauss) MHz

  14. 13C 35Cl 2H 37Cl 1H 90 22.6 84 13.7 8.82 22,600,000 90,000,900 90,000,000 22,600,600

  15. Some important relationships in NMR The frequency of absorbed electromagneticradiation for a particular nucleus (such as 1H)depends on its molecular environment (bonds). This is why NMR is such a useful toolfor structure determination.

  16. C H Nuclear Shielding and 1H Chemical Shifts Hlocal An external magnetic field affects the motion of the electrons (in bonds) in a molecule, (in addition to protons in nucleus) inducing a magnetic field within the molecule. The direction of the induced magnetic field is opposite to that of the applied field. H0

  17. Hlocal Heffective H0 or Happlied Heffective = Happlied - Hlocal • Strength of Hlocal • depends on types of • bonds in molecule • Nucleus is “shielded” • from full effect of • applied (external) • magnetic field due to • the different types of • bonds in molecule.

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

  19. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Increasing Ho; RF Constant Increasing RF; H0 Constant UpfieldIncreased shielding DownfieldDecreased shielding (CH3)4Si (TMS) Chemical shift (, ppm)measured relative to TMS

  20. CH3 Si CH3 H3C CH3 position of signal - position of TMS peak x 106 d = spectrometer frequency Chemical Shift Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard. Units are ppm (parts per million) d Is defined as 0 for TMS

  21. Interpreting Proton (1H) NMR Spectra 1. number of signals 2. their intensity (as measured by area under peak) • Peak Position, d value • splitting pattern (multiplicity)

  22. ClCH2CH2CH3 CH3CH2CH2Cl H3CCH2CH3 chemically equivalent Number of Signals is Number of Chemically Nonequivalent protons (H’s) replacement test: replacement by some arbitrary "test group" generates same compound Replacing protons at C-1 and C-3 gives same compound (1-chloropropane)

  23. Peak area depends on the number Of chemically equivalent protons 2 peaks; 6:2 or 3:1 ratio H3CCH2CH3

  24. 1H-NMR Spectrum of p-dimethylbenzene

  25. N CCH2OCH3 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Figure 13.11 (page 533) OCH3 NCCH2O Chemical shift (, ppm)

  26. Br H C C H3C H Diastereotopic protons replacement by some arbitrary test group generates diastereomers diastereotopic protons can have differentchemical shifts  5.3 ppm  5.5 ppm

  27. 13.5Effects of Molecular Structureon1H Chemical Shifts protons in different environments experience different degrees of shielding and have different chemical shifts

  28. Peak Position (d value) • Two Items That Deshield (increase d value) are.. • Electronegative Atoms • Multiple Bonds/ Aromaticity

  29. CH3F CH3OCH3 (CH3)3N CH3CH3 (CH3)4Si d 4.3 d 3.2 d 2.2 d 0.9 d 0.0 least shielded H most shielded H Electronegative substituents decreasethe shielding of methyl groups

  30. d 4.3 d 2.0 d 1.0 O2N—CH2—CH2—CH3 Electronegative substituents decrease shielding (or increase d); Distance Effect • CHCl3d 7.3 • CH2Cl2d 5.3 • CH3Cl d 3.1 Effect is cumulative

  31. 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  7.3  5.3  0.9

  32. H attached to sp2 C is deshielded H lies in region where H local reinforces Ho So the proton is deshielded

  33. C H H attached to an sp3 H is shielded Hlocal The direction of the induced magnetic field is opposite to that of the applied field. H0

  34.  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

  35. Hlocal Fig. 13.9 Where p electrons are H lies in region where H local opposes Ho so the proton is shielded

  36. 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

  37. H H C C N C C C H R C H H C C C C O Ar H C C Table 13.1 (p 528) Type of proton Chemical shift (),ppm Type of proton Chemical shift (),ppm 2.1-2.3 0.9-1.8 2.5 1.5-2.6 2.3-2.8 2.0-2.5

  38. H H Cl C H Ar O H C Br C C H O C C H H NR C Table 13.1 (p 528) Type of proton Chemical shift (),ppm Type of proton Chemical shift (),ppm 2.2-2.9 4.5-6.5 3.1-4.1 6.5-8.5 2.7-4.1 9-10 3.3-3.7

  39. H NR H OR H OAr O C HO Table 13.1 (p 528) Type of proton Chemical shift (),ppm 1-3 0.5-5 6-8 10-13

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