1. Chapter 13 Nuclear Magnetic Resonance Spectroscopy

2. Infrared Spectroscopy
Mass Spectrometry

3. Nuclear Magnetic Resonance Spectroscopy

4. Maps the carbon-hydrogen framework
NMR Spectroscopy
Maps the carbon-hydrogen framework
Probes molecular structure in greater detail than IR or MS
A complete molecular structure can oftentimes be determined by NMR alone
NMR has revolutionized organic chemistry since its introduction in the 1950’s
Before the advent of NMR, structure elucidation may have taken months (or years!)
13.1 An Overview of Proton NMR Spectroscopy

5. NMR Machine

6. Sample + CDCl3 & (CH3)4Si

7. The NMR Spectrometer
13.11 The NMR Spectrometer

8. NMR Spectroscopy
Nuclei with an odd # of protons and/or an odd # of neutrons exhibit magnetic properties
called “spin”
Nuclei with even numbers of BOTH P’s and N’s = no magnetic phenomena
1H and 13C are the most important of the spin active nuclei

9. Nuclear spin causes the nuclei to behave like a tiny magnets

10. Nuclear Spin 1H (and 13C) nuclei can have one of two spin states
quantum numbers +½ or -½

11. Protons in a Magnetic Field
13.2 Physical Basis of NMR Spectroscopy

12. Protons in a Magnetic Field
This absorption is nuclear magnetic resonance and is detected by an NMR spectrometer

16. The 1H NMR Spectrum Absorptions detect the protons in the molecule
Separate resonance for each chemically non-equivalent set of nuclei
The size (area) of a peak is proportional to the number of contributing protons

17. Chemical Shift
The local magnetic field (Bp) “sensed” by a proton is different than the applied magnetic field (B0)
This is due to the electrons around the proton which oppose the external field
Bp = B0 – Be
The reduction of the local field is called shielding

18. Chemical Shift
Hence, the electronegativity of nearby atoms will affect the shielding around a proton
Electronegative neighbors = deshielding
H in question “feels” external magnetic field more
Electropositive neighbors = increase in shielding
H in question “feels” external magnetic field less

19. Chemical Shift

20. Chemical Shift and Structure
Electronegativities of nearby groups is one of the most important factors in chemical shift

22. Chemical Shift
The amount of alkyl substitution will also affect chemical shift

23. Chemical Shift
A proton near more than one functional group will be affected by both