1. Chapter 13 Spectroscopy Infrared spectroscopy Ultraviolet-visible spectroscopy Nuclear magnetic resonance spectroscopy Mass spectrometry Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 13.1 Principles of Molecular Spectroscopy: Electromagnetic Radiation

3. Is propagated at the speed of light Has properties of particles and waves The energy of a photon is proportional to its frequency Electromagnetic Radiation

4. Figure 13.1: The Electromagnetic Spectrum 400 nm750 nm Visible Light Longer Wavelength ( )Shorter Wavelength ( ) Higher Frequency ( )Lower Frequency ( ) Higher Energy (E)Lower Energy (E)

5. Figure 13.1: The Electromagnetic Spectrum UltravioletInfrared Longer Wavelength ( )Shorter Wavelength ( ) Higher Frequency ( )Lower Frequency ( ) Higher Energy (E)Lower Energy (E)

6. Cosmic rays  Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves Figure 13.1: The Electromagnetic Spectrum Energy

7. 13.2 Principles of Molecular Spectroscopy: Quantized Energy States

8. Electromagnetic radiation is absorbed when the energy of photon corresponds to difference in energy between two states.  E = h

9. electronic vibrational rotational nuclear spin UV-Vis infrared microwave radiofrequency What Kind of States?

10. 13.3 Introduction to 1 H NMR Spectroscopy

11. 1 H and 13 C both have spin = ±1/2 1 H is 99% at natural abundance 13 C is 1.1% at natural abundance The Nuclei that are Most Useful to Organic Chemists are:

12. 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. + +

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

14. + + + + + An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. B0B0

15. + + + + + There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. B0B0

16. No difference in absence of magnetic field Proportional to strength of external magnetic field Energy Differences Between Nuclear Spin States + + EE  E ' increasing field strength

17. Some Important Relationships in NMR The frequency of absorbed electromagnetic radiation is proportional to the energy difference between two nuclear spin states which is proportional to the applied magnetic field. Units Hz kJ/mol (kcal/mol) tesla (T)

18. Some Important Relationships in NMR The frequency of absorbed electromagnetic radiation is different for different elements, and for different isotopes of the same element. For a field strength of 4.7 T: 1 H absorbs radiation having a frequency of 200 MHz (200 x 10 6 s -1 ) 13 C absorbs radiation having a frequency of 50.4 MHz (50.4 x 10 6 s -1 )

19. Some Important Relationships in NMR The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1 H) depends on its molecular environment. This is why NMR is such a useful tool for structure determination.

20. 13.4 Nuclear Shielding and 1 H Chemical Shifts What do we mean by "shielding"? What do we mean by "chemical shift"?

21. 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. C H B 0B 0

22. 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. B 0B 0 C H

23. 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. B 0B 0 C H

24. Chemical Shift Chemical shifts (  ) are measured relative to the protons in tetramethylsilane (TMS) as a standard. Si CH 3 H3CH3C  = position of signal - position of TMS peak spectrometer frequency x 10 6

25. 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm) measured relative to TMS Upfield Increased shielding Downfield Decreased shielding (CH 3 ) 4 Si (TMS)

26. Chemical Shift Example: The signal for the proton in chloroform (HCCl 3 ) appears 1456 Hz downfield from TMS at a spectrometer frequency of 200 MHz.  = position of signal - position of TMS peak spectrometer frequency x 10 6  = 1456 Hz - 0 Hz 200 x 10 6 Hz x 10 6  = 7.28

27. 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm)  7.28 ppm H C Cl

28. 13.5 Effects of Molecular Structure on 1 H Chemical Shifts Protons in different environments experience different degrees of shielding and have different chemical shifts.

29. Electronegative Substituents Decrease the Shielding of Methyl Groups least shielded Hmost shielded H 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

30. Electronegative Substituents Decrease Shielding H 3 C—CH 2 —CH 3 O 2 N—CH 2 —CH 2 —CH 3  0.9  1.3  1.0  4.3  2.0

31. Effect is Cumulative CHCl 3  7.3 CH 2 Cl 2  5.3 CH 3 Cl  3.1

32. Methyl, Methylene, and Methine CH 3 more shielded than CH 2 ; CH 2 more shielded than CH H3CH3C C CH3CH3 CH 3 H  0.9  1.6  0.8 H3CH3C C CH3CH3 CH 3 CH2CH2  0.9 CH 3  1.2

33. Protons Attached to sp 2 Hybridized Carbon are Less Shielded than those Attached to sp 3 Hybridized Carbon HH HH H H C C HH HH CH 3  7.3  5.3  0.9

34. But Protons Attached to sp Hybridized Carbon are More Shielded than those Attached to sp 2 Hybridized Carbon  2.4 CH 2 OCH 3 C C H C C HH HH  5.3

35. Protons Attached to Benzylic and Allylic Carbons are Somewhat Less Shielded than Usual  1.5  0.8 H3CH3CCH 3  1.2 H3CH3CCH 2  2.6 H 3 C—CH 2 —CH 3  0.9  1.3

36. Proton Attached to C=O of Aldehyde is Most Deshielded C—H  2.4  9.7  1.1 CC O H H CH 3 H3CH3C

37. Table 13.1 Type of proton Chemical shift (  ), ppm Type of proton Chemical shift (  ), ppm C HR 0.9-1.8 1.5-2.6 C H CC 2.0-2.5 C H C O 2.1-2.3 C H NC C HAr 2.3-2.8

38. Table 13.1 Type of proton Chemical shift (  ), ppm Type of proton Chemical shift (  ), ppm C HBr 2.7-4.1 9-10 C O H 2.2-2.9 C HNR 3.1-4.1 C HCl 6.5-8.5HAr C C H 4.5-6.5 3.3-3.7 C H O

39. Table 13.1 Type of proton Chemical shift (  ), ppm 1-3HNR 0.5-5HOR 6-8HOAr 10-13 C O HOHO