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

2. 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…..

3. MC 13.1 Spectroscopy, Pt I 3 The Visible Electromagnetic Spectrum 400 nm750 nm Visible Light Longer Wavelength ( )Shorter Wavelength ( ) Higher Frequency ( )Lower Frequency ( ) Higher Energy (E)Lower Energy (E) continue….

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

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

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

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

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

9. MC 13.1 Spectroscopy, Pt I 9 continue….. Introduction to 1 H NMR Spectroscopy (cont) Nuclear Spin:  A spinning charge, such as the nucleus of 1 H or 13 C, 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 + +

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

11. 11 An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field + + + + + H0H0 continue…..

12. MC 13.1 Spectroscopy, Pt I 12 There is a slight excess of nuclear magnetic moments aligned parallel to the applied field + + + + + H0H0 continue…..

13. 13 Introduction to 1 H 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 + + EE  E ' Increasing Field Strength No Magnetic Field continue…..

14. MC 13.1 Spectroscopy, Pt I 14 Introduction to 1 H 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…..

15. MC 13.1 Spectroscopy, Pt I 15 Introduction to 1 H NMR Spectroscopy (cont) Nuclear Shielding and 1 H 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…..

16. MC 13.1 Spectroscopy, Pt I 16 Introduction to 1 H 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…..

17. MC 13.1 Spectroscopy, Pt I 17 Introduction to 1 H NMR Spectroscopy (cont) Chemical Shift (cont):  Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard Si CH 3 H3CH3C  = Position of Signal - Position of TMS Signal Spectrometer Frequency x 10 6 Calculating Chemical Shift Numbers: continue…..

18. MC 13.1 Spectroscopy, Pt I 18 Introduction to 1 H NMR Spectroscopy (cont) The Proton NMR Chemical Shift Scale: 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm) - measured relative to TMS Downfield Decreased shielding Upfield Increased shielding (CH 3 ) 4 Si (TMS) continue…..

19. 19 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm) (CH 3 ) 4 Si (TMS) Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H Chemical Shifts:  Protons in different environments experience different degrees of shielding and have different chemical shifts  7.28 ppm H C Cl continue…..

20. MC 13.1 Spectroscopy, Pt I 20 Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H 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 Decreased ShieldingIncreased shielding CH 3 FCH 3 OCH 3 (CH 3 ) 3 NCH 3 (CH 3 ) 4 Si  4.3  3.2  2.2  0.9  0.0 continue…..

21. MC 13.1 Spectroscopy, Pt I 21 Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H Chemical Shifts (cont):  Electronegative substituents decrease shielding H 3 C — CH 2 — CH 3  0.9  1.3 O 2 N — CH 2 — CH 2 — CH 3  1.0  4.3  2.0 continue…..

22. MC 13.1 Spectroscopy, Pt I 22 Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H Chemical Shifts (cont):  Electronegative substituents decrease shielding  The electronegativity effect is cumulative continue….. CHCl 3  7.3 CH 2 Cl 2  5.3 CH 3 Cl  3.1

23. MC 13.1 Spectroscopy, Pt I 23 Methyl ( CH 3 ), Methylene ( CH 2 ), and Methine ( CH ), : Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H Chemical Shifts (cont):  CH 3 more shielded than CH 2  CH 2 more shielded than CH H3CH3C C CH 3 H  0.9  1.6  0.8  0.9 H3CH3C C CH 3 CH 2 CH 3  1.2 continue…..

24. MC 13.1 Spectroscopy, Pt I 24 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm) (TMS) Example: continue….. Interpreting Proton NMR Spectra (cont) 4 lines; quartet 2 lines; doublet Cl 2 CH CH 3

25. 25 Interpreting Proton NMR Spectra (cont)  CH 3 CH 2 X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet) 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm) (TMS) 3 lines; triplet CH 3 BrCH 2 CH 3 4 lines; quartet CH 2 Splitting Patterns: The Ethyl Group BrCH 2 CH 3 continue…..