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Chapter 13 - Spectroscopy

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Chapter 13 - Spectroscopy. YSU 400 MHz N uclear M agnetic R esonance Spectrometer(s). Techniques used to find structures of organic molecules. NMR spectroscopy : Based on the response of magnetic nuclei to an external magnetic field and an energy source (Radio frequency)

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Chapter 13 - Spectroscopy
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  1. Chapter 13 - Spectroscopy YSU 400 MHz Nuclear Magnetic Resonance Spectrometer(s)

  2. Techniques used to find structures of organic molecules NMR spectroscopy: Based on the response of magnetic nuclei to an external magnetic field and an energy source (Radio frequency) IR spectroscopy: Response of bonds within organic molecules to externally applied Infra Red light UV/Vis spectroscopy: Response of electrons within bonds to externally applied UV or Visible light Mass spectrometry: Response of molecules to being bombarded with high energy particles such as electrons

  3. 13.1 The Electromagnetic spectrumFigure 13.1

  4. 13.2 Two (quantized) energy states Figure 13.2

  5. 13.2 Physics Concepts • E = hni.e. Energy of the radiation is directly proportional to its frequency (n = Planck’s constant) • = c/li.e. Frequency of the radiation is inversely proportional to its wavelength (c = speed of light) E = hc/l i.e. Energy of the radiation is inversely proportional to its wavelength Take home :Longer wavelength, lower energy Higher frequency, higher energy

  6. 13.3 Introduction to 1H NMR – Nuclear Spin Nuclear spins of protons (1H nucleus) Figure 13.3

  7. Energy difference between states increases with field strength (Fig. 13.4)

  8. Schematic diagram of a nuclear magnetic resonance spectrometer

  9. Basic operation of a Fourier Transform (FT) NMR Instrument (Fig. 13.5)

  10. 13.4 NMR Spectrum Characteristics – Chemical Shift Position of signal is the chemical shift downfield upfield downfield upfield

  11. 13.4 NMR Spectrum Characteristics – Chemical Shift Chemical shift (d) = position of signal – position of TMS peak x 106 spectrometer frequency Enables us to use same scale for different size spectrometers (60 MHz, 400 MHz, 850 MHz, etc.) TMS = (CH3)4Si, signal appears at 0 Hz on spectrum, therefore used as reference Chemical shifts are reported as ppm (parts per million) relative to TMS and usually occur in the 0-12 ppm range for 1H spectra

  12. 13.5 Effect of molecular structure on 1H Chemical Shift CH3F CH3OCH3 (CH3)3NCH3CH3 4.3 3.2 2.2 0.9 i.e. electronegativity of other atoms plays a role in shift ~0.9 ppm

  13. ~2.2 ppm

  14. ~3.2 ppm

  15. ~4.3 ppm

  16. 13.5 Effect of structure on 1H Chemical Shift 7.3 5.3 0.9 Pi electrons reinforce external field and signals show downfield

  17. ~0.9 ppm “R3C-H – alkyl” ~5.3 ppm “C=C-H alkene” ~7.3 ppm “Ar-Hbenzene”

  18. 13.5 Effect of structure on 1H Chemical Shift Spectra typically have multiple signals the number depending on the number of unique types of protons

  19. 13.5 Typical 1H NMR Spectra Simple alkane protons – R2CH2 From spectroscopy sheet – chemical shift ~ 0.9-1.8 ppm

  20. 13.5 Typical 1H NMR Spectra Ether protons -O-C-H From spectroscopy sheet – chemical shift ~ 3.3-3.7 ppm

  21. 13.5 Typical 1H NMR Spectra Two types of ether protons -O-C-H From spectroscopy sheet – chemical shift ~ 3.3-3.7 ppm CH2 further downfield (two neighbouring O atoms)

  22. 13.5 Typical 1H NMR Spectra Aldehyde proton -CHO From spectroscopy sheet – chemical shift ~ 9-10 ppm 3 types of Ar-H proton – chemical shift ~6.5-8.5 ppm

  23. 13.5 Typical 1H NMR Spectra Carboxylic acid proton -CO2H From spectroscopy sheet – chemical shift ~ 10-13 ppm 3 types of Ar-H proton – chemical shift ~6.5-8.5 ppm

  24. 13.6 Integration – Ratio of different types of H 5 1 Lines on spectra are curves Areas underneath each curve give a reliable ratio of the different numbers of each type of proton

  25. 13.6 Integration – Ratio of different types of H 3 3 2 Areas are given as a ratio, not an absolute number

  26. 13.6 Integration – Ratio of different types of H 3 3 2 2 2

  27. 13.7 Spin-Spin Splitting – Effect of neighbouring H on shape

  28. 13.7 Spin-Spin Splitting – Effect of neighbouring H on shape

  29. 13.7 Spin-Spin Splitting – Effect of neighbouring H on shape

  30. 13.7 Spin-Spin Splitting – Effect of neighbouring H on shape General rule for splitting patterns For simple cases, multiplicity for H = n + 1 Where n = number of neighbouring protons i.e 1 neighbour, signal appears as a doublet 2 neighbours, signal appears as a triplet 3 neighbours, signal appears as a quartet 4 neigbours, signal appears as a quintet, etc. Complex splitting patterns are referred to as multiplets

  31. 13.7-13.10 Basis of Splitting Patterns For red H : neighbouring H (blue) has two possible alignments, either with, or against, the external field (Ho). This effects the local magnetic environment around the red H and thus there are two slightly different frequencies (and thus chemical shifts) at which the red H resonates. Same applies to the blue H.

  32. 13.7-13.10 Basis of Splitting Patterns Red H will be a triplet Blue H’s will be a doublet

  33. 13.7-13.10 Basis of Splitting Patterns Red H will be split into a quartet, blue H’s will be split into a doublet

  34. 13.7-13.10 Basis of Splitting Patterns - Coupling Constants Gaps between lines (in Hz) will be the same for adjacent protons (here ~7.4 Hz). This is the coupling constant.

  35. Using CouplingConstants CH3CH2 but which one? CH3CH2O Find J and match signals

  36. CouplingConstants – Nonequivalent Neighbours If nonequivalent neighbours have same J value then n+1 applies for signal CH3CH2 CH2CH2Cl CH3CH2 CH3CH2CH2

  37. 13.11 Complex Splitting Patterns When nonequivalent neighbours have different J values then n+1 does not apply for signal Figure 13.20 Generally for alkene protons: J trans > J cis

  38. 13.11 Complex Splitting Patterns

  39. 13.12 1H NMR Spectra of Alcohols Figure 13.21 Acidic protons exchange with any H2O in sample

  40. Glycosyl amide structure from NMR - NOESY N-H N-H YSU

  41. Glycosyl amide structure from 2-D NMR - COSY H-2 H-4 H-3 H-5 N-H H1, H2, H3, and H4 hard to distinguish just from coupling constants (all t, J~9 Hz) David Temelkoff YSU

  42. 13.14 13C NMR Spectroscopy Figure 13.23

  43. 13.14 13C NMR Spectroscopy • Carbon 13 isotope and not 12C is observed in NMR • 13C very low abundance (<1%), integration not useful • Spectra usually “decoupled” and signals are singlets • Number of distinct signals indicates distinct carbons • Same ideas about shielding/deshielding apply • Spectra often measured in CDCl3 and referenced to • either the C in TMS (0 ppm) or the C in CDCl3, which shows • as a triplet at 77.0 ppm

  44. 13.15 13C NMR Chemical Shifts (see Sheet) 13C NMR (ppm) 21, 52, 121, 122, 120, 126, 132, 134, 148, 168, 169

  45. 13.15 13C NMR Chemical Shifts (see Sheet) 13C NMR (ppm) 23, 28, 32, 128, 151, 197

  46. 13.15 13C NMR Chemical Shifts (see Sheet)

  47. 13.15 13C NMR – Information on Symmetry

  48. Not covering 13.17-13.19