Transcript CH 9-3 Power Point

CH 9-3: Nuclear Magnetic Resonance (NMR)
Spectroscopy
•The most important instrumental technique used by
organic chemists to determine the structure of organic
compounds.
•NMR spectroscopy helps to identify functional groups, and
the C-H framework of an organic compound.
•The technique is based on the spinning nuclei of atoms
behaving as if they were tiny magnets. These nuclei
include: 1H, 13C, 15N, 19F, 31P.
Nuclear Magnetic Resonance (NMR)
•In the absence of an
external magnetic
field the N-S poles of
hydrogen nuclei are
randomly oriented.
Nuclear Magnetic Resonance (NMR)
•In the presence of a very
strong magnetic field, the
hydrogen nuclei may align
themselves with or
opposite to the external
field, at different energy
levels.
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Nuclear Magnetic Resonance (NMR)
•If the sample is irradiated
with electromagnetic energy
(radio frequency energy), the
nuclei absorb energy, the
poles “flip,” and the energy
of the nuclei are at the same
level.
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Nuclear Magnetic Resonance (NMR)
•When the excited nuclei
“relax”, the absorbed
energy is released and
measured, giving a signal
on the NMR spectrum.
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BSU 400 MHz NMR
Spectrometer
•Modern “high field”
NMR spectrometers
have superconducting
magnets. BSU 400MHz
NMR obtained with a
$350,000 from the NSF.
1 MHz = 106 Hz
(measure of frequency
of EMR)
AM radio ~ 1 MHz
FM radio ~ 100 MHz
Information obtained from NMR
(1) Chemical equivalence of H’s and C’s
(2) Types of functional groups – “Chemical Shift” (ppm)
scale.
(3) “Signal splitting” gives us structural fragments: puzzle
pieces.
(4) Identification of acidic H’s by acid-base exchange with
D2O.