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Spectrocopy

Spectrocopy. Nuclear Magnetic Resonance (NMR)spectroscopy Infrared (IR) Spectroscopy Ultraviolet-Visible (UV-VIS) Spectroscopy Mass Spectrometry (MS). continue…. Principles of Molecular Spectroscopy: Electromagnetic Radiation. Electromagnetic Radiation travels at the speed of light

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Spectrocopy

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  1. Spectrocopy • Nuclear Magnetic Resonance (NMR)spectroscopy • Infrared (IR) Spectroscopy • Ultraviolet-Visible (UV-VIS) Spectroscopy • Mass Spectrometry (MS) continue….. MC 13.1 Spectroscopy, Pt I

  2. Principles of Molecular Spectroscopy:Electromagnetic Radiation • Electromagnetic Radiation travels at the speed of light • Has both particle and wave properties • A single packet of energy is a photon • The energy of a photon is proportional to its frequency continue….. MC 13.1 Spectroscopy, Pt I

  3. The Visible Electromagnetic Spectrum Shorter Wavelength () Longer Wavelength () 400 nm 750 nm Visible Light Higher Frequency () Lower Frequency () Higher Energy (E) Lower Energy (E) continue…. MC 13.1 Spectroscopy, Pt I

  4. Shorter Wavelength () Longer Wavelength () Higher Frequency () Lower Frequency () Higher Energy (E) Lower Energy (E) The Electromagnetic Spectrum (cont) Ultraviolet Infrared continue….. MC 13.1 Spectroscopy, Pt I

  5. Energy The Electromagnetic Spectrum (cont) Cosmic rays  Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves continue….. MC 13.1 Spectroscopy, Pt I

  6. E = h Quantized Energy States • Electromagnetic radiation is absorbed when theenergy of the photon corresponds to difference in energy between two states State #2 State #1 continue….. MC 13.1 Spectroscopy, Pt I

  7. Quantized Energy States (cont) What Kind of States are of Interest? StateSpectral RegionType of Spectroscopy Electronic Nuclear spin Vibrational Rotational UV-Vis Radiofrequency Infrared Microwave Ultraviolet-Visible Nuclear Magnetic Resonance Infrared Raman continue….. MC 13.1 Spectroscopy, Pt I

  8. Introduction to 1H NMR Spectroscopy The nuclei that are most useful to organic chemists are: • 1H and 13C • Both have spin = + or - 1/2 • 1H is 99% natural abundance • 13C is 1.1% natural abundance continue….. MC 13.1 Spectroscopy, Pt I

  9. + + Introduction to 1H NMR Spectroscopy (cont) 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 continue….. MC 13.1 Spectroscopy, Pt I

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

  11. + + + H0 + + An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field continue…..

  12. + + + H0 + + There is a slight excess of nuclear magnetic moments aligned parallel to the applied field continue….. MC 13.1 Spectroscopy, Pt I

  13. No Magnetic Field E ' E Increasing Field Strength Introduction to 1H NMR Spectroscopy (cont) Energy Differences Between Nuclear Spin States: • There is no difference in the absence of a magnetic field • The difference is proportional to the strength of the external magnetic field + + continue…..

  14. Introduction to 1H NMR Spectroscopy (cont) Some important relationships in NMR: • The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1H) depends on its molecular environment This is why NMR is such a useful tool for structure determination continue….. MC 13.1 Spectroscopy, Pt I

  15. Introduction to 1H NMR Spectroscopy (cont) Nuclear Shielding and 1H Chemical Shifts: What do we mean by "shielding?“ What do we mean by "chemical shift?" • 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 C H H 0 continue….. MC 13.1 Spectroscopy, Pt I

  16. Introduction to 1H NMR Spectroscopy (cont) Chemical Shift: • What do we mean by "shielding?“ • What do we mean by "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 C H H 0 continue….. MC 13.1 Spectroscopy, Pt I

  17. CH3 Si CH3 H3C CH3 Calculating Chemical Shift Numbers: Position of Signal - Position of TMS Signal x 106 d = Spectrometer Frequency Introduction to 1H NMR Spectroscopy (cont) Chemical Shift (cont): • Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard continue….. MC 13.1 Spectroscopy, Pt I

  18. UpfieldIncreased shielding (CH3)4Si (TMS) 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Chemical shift (, ppm) - measured relative to TMS Introduction to 1H NMR Spectroscopy (cont) The Proton NMR Chemical Shift Scale: DownfieldDecreased shielding continue….. MC 13.1 Spectroscopy, Pt I

  19. Cl  7.28 ppm Cl H (CH3)4Si (TMS) C Cl 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Chemical shift (, ppm) Introduction to 1H NMR Spectroscopy (cont) Effects of Molecular Structure on 1H Chemical Shifts: • Protons in different environments experience different degrees of shielding and have different chemical shifts continue…..

  20. CH3F CH3OCH3 (CH3)3N CH3CH3 (CH3)4Si d 4.3 d 3.2 d 2.2 d 0.9 d 0.0 Decreased Shielding Increased shielding Introduction to 1H NMR Spectroscopy (cont) Effects of Molecular Structure on 1H Chemical Shifts (cont): • Protons in different environments experience different degrees of shielding and have different chemical shifts • Electronegative substituents decrease the shielding of hydrogen atoms on methyl groups continue….. MC 13.1 Spectroscopy, Pt I

  21. d 0.9 d 1.3 d 0.9 H3C — CH2 — CH3 d 4.3 d 2.0 d 1.0 O2N — CH2 — CH2 — CH3 Introduction to 1H NMR Spectroscopy (cont) Effects of Molecular Structure on 1H Chemical Shifts (cont): • Electronegative substituents decrease shielding continue….. MC 13.1 Spectroscopy, Pt I

  22. Introduction to 1H NMR Spectroscopy (cont) Effects of Molecular Structure on 1H Chemical Shifts (cont): • Electronegative substituents decrease shielding • The electronegativity effect is cumulative CHCl3 7.3 CH2Cl2  5.3 CH3Cl  3.1 continue….. MC 13.1 Spectroscopy, Pt I

  23. CH3 CH3 CH3 CH2 H H3C C C H3C CH3 CH3 Introduction to 1H NMR Spectroscopy (cont) Effects of Molecular Structure on 1H Chemical Shifts (cont): Methyl ( CH3 ), Methylene ( CH2 ), and Methine ( CH ), : • CH3 more shielded than CH2 • CH2 more shielded than CH d 0.9 d 0.9 d 1.2 d 1.6 d 0.8 continue….. MC 13.1 Spectroscopy, Pt I

  24. Cl2CH CH3 Cl2CH CH3 Cl2CH CH3 (TMS) 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Chemical shift (, ppm) Interpreting Proton NMR Spectra (cont) Example: continue….. 2 lines; doublet 4 lines; quartet MC 13.1 Spectroscopy, Pt I

  25. BrCH2CH3 Splitting Patterns: The Ethyl Group BrCH2 CH3 (TMS) 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Br CH2 CH3 Chemical shift (, ppm) Interpreting Proton NMR Spectra (cont) • CH3CH2X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet) 3 lines; triplet CH3 4 lines; quartet CH2 continue…..

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