Introduction to Nuclear Magnetic Resonance (NMR)
IR helps identify functional groups, but does not help distinguish between, say, different
ketones, or between different esters, etc. This where NMR (and MS) comes in.
Carbon-13 and proton NMR are based on the same principles, but carbon-13 NMR is
easier to grasp as there is usually only one peak per type of carbon. Proton NMR usually
shows multiple peaks for each type of proton, due to magnetic coupling between
adjacent (up to three bonds apart) protons that results in peak splitting. (adjacent
carbon-13 atoms give similar coupling, but their low natural abundance of 1.1% means
this is rare, and the resulting split peaks are accordingly very very small).
The next slides illustrate the limitations of infrared spectroscopy to distinguish between
homologous molecules, and the power of NMR to reveal their molecular structure.
Chapter 2
10
Carbon-13 NMR
Proton NMR
four
protons
four protons
Carbon-13 NMR
Proton NMR
six
protons
four protons
Illustration for the need to characterize closely related compounds
Extract of stingers of africanized honey bee
O
G.J. Hunt et al,
DISCOVERY OF 3-METHYL-2-BUTEN-1-YL ACETATE, A
NEW ALARM COMPONENT IN THE STING APPARATUS
OF AFRICANIZED HONEYBEES Journal of Chemical
Ecology, Vol. 29, No. 2, February 2003 (°C 2003)
O
O
O
O
O
O
O
Retention time in gas chromatogram [minutes]
H2SO4
O
OH
HO
heat
O
O
Butyl acetate
H2SO4
O
OH
HO
O
O
Isopentyl acetate
heat
Carbon-13 NMR
O
O
Proton NMR
3 protons
Carbon-13 NMR
4 protons
3 protons
2 protons
Proton NMR
Carbon-13 NMR
O
O
Proton NMR
C8H10
C8H10
C7H9NO
C8H12O2