Application of advanced NMR spectroscopy

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Application of advanced NMR spectroscopy to the
structure determination
(NMR short course) Hongjun Pan

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Introduction Since the discovery of the Nuclear
Magnetic Resonance (NMR) sixty year ago (1945),
the NMR technology and its application in science
have been explosively developed. At nowadays, the
NMR become a routine research method in many
science fields such as chemistry, physics,
material research, biology and medical disease
diagnosis. For organic synthesis, natural product
identification and drug design, the NMR is
probably the most important technique for
structure elucidation. The modern NMR
spectroscopy is based on the pulsed Fourier
Transform (FT) methods. The theory of modern
FT-NMR technology is complicate, and strong
knowledge in quantum mechanics and mathematics is
required to fully understand the impacts of
pulses on the nuclear spin systems. Ironically,
regular chemists and biologists can still use
FT-NMR technology very well in their research
without knowing the detail of the FT-NMR
theory. The Department of Chemistry
Instrumentation Facilities (DCIF) is equipped
with six modern FT-NMR spectrometers (Varian
Mercury 300, two Bruker Avance 400, two Varian
Inova 500 and Bruker Avance 600), three Mass
spectrometers (Bruker FT-MS, Bruker MALDI,
GC-MS), Bruker EPR, FT-IR and polarimeter. About
400 researchers from several departments of MIT
and local companies use DCIF instruments. The aim
of this lecture is to illustrate the application
of advanced NMR spectroscopy to structure
elucidation for those regular NMR users without
going through the complicate quantum mechanics
and mathematics.
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The principle of NMR phenomenon
All nuclei carry a charge In some nuclei this
charge spin around the nuclear axis. This
spin generates a magnetic dipole along the
axis. The spin angular momentum is described in
terms of spin quantum number I
Spin Angular Momentum z-component in the
Magnetic field
m is the magnetic quantum number
m -I, -I1, I
I ½ 1H 13C 15N 31P 29Si I1 2H
6Li I3/2 23Na 39K I0 12C 16O No
NMR!!!!
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The magnetic dipole of a nucleus with I 1 has
three orientations in the magnetic field and
precess in the magnetic field with the frequency
Lamar frequency is the gyromagnetic ratio
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2I 1 possible spin states
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The population of the nuclei in each energy level
follows the Boltzmann distribution and the NMR
signal is proportional to the population
difference between the two states.
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Quantum mechanic model
Vector model
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Relationship between the time domain signal and
frequency domain signal
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Rotating frame
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The Bx rotates in the laboratory frame with
frequency ?0, which defines the rotating frame.
Description of NMR is relative to this frequency
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In NMR spectrum, each nucleus has a
characteristic frequency or chemical shift
defined as
Two spin coupled each other with a coupling
constant J. The chemical shift d is 60, 200 abd
500 Hz respect to the static field of 60, 200 and
500 MHz instruments. The J is a constant in
different field, but, d(in Hz) is proportional to
the field strength. So the resolution is
proportional to the field strength.
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Sample preparation 1. Use good NMR tubes,
not cheap tubes 2. Complete dissolved in
the solvent 3. High concentration, free
radicals and undissolved solids cause line
broadening and poor shimming. 4. The length
of the sample solution should be much longer than
the NMR detection coil Insert the NMR tube into
the probe at certain depth too long Break
the sample and damage the probe too short
cause shimming problem. Always use the gauge to
adjust the depth of the tube. 1H NMR 1 10
mg 13C NMR 25 mg 100 mg
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Locking and Shimming
Why need locking? Magnetic field of the
superconducting magnet fluctuates and drifts (1
10 Hz/hour), which can not be controlled by any
means. This causes line broadening, reduces the
resolution and S/N ratio. What can we do for
this? Find a way to compensate the fluctuation
and drifting. Since the magnetic field
fluctuation and drifting affect the NMR signal,
so the best way is to monitor the NMR signal
change, such NMR signal should be strong enough
and does not affect those NMR signals we want to
detect. Deuterium NMR should be a good choice for
monitoring, because most of NMR sample dissolve
in solvents, so deuterated solvents have strong
deuterium NMR signal which does not affect most
of other NMR signals. A compensation magnetic
field is added to or subtracted from the main
magnetic field to keep the total magnetic field
constant. This is locking Why need
shimming? The environment around the NMR sample
is not homogeneous due to different materials and
shapes, so the magnetic field around the sample
is not homogeneous, which makes the NMR peaks
broad. Shimming is to minimize the inhomogeniety
and improve the signal resolution and S/N.
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Z axis magnetic field gradient
Z1
Z2
Z3
Z Axis Top
Bottom
Z4
Total effect Z1 Z2 Z3 Z4
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Lineshape defects due to various shimming problems
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Sw1300 Too small A fold over peak at 0.94 ppm
which is from 0 ppm
Expansion of the normal spectrum
Normal window Sw is big enough sw6000Hz
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NT1
NT128
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Receiver gain is set too high Or too much
sample Clip the FID
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Acquisition time is too short Truncate the FID
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Increase acquisition time Bruker TD Varian at Ad
d line broadening or apodization
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Data processing
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Summary

• Nuclei have spins which have certain different
  orientations in the magnetic field or energy
  states, and precesses in the magnetic field with
  Lamar frequency.
• The frequency of the pulse is defined also as a
  rotating frame. All description of NMR signal are
  related to this rotating frame.
• Nuclei can jump to higher energy states by
  radiation, or bulk magnetization rotates by the
  magnetic field of radiation, and then precesses
  about the external magnetic field.
• In the solution NMR, any anisotropy of of
  interactions (couplings) will be averaged to
  zero, only isotropic portion affect the NMR
  spectra.
• Due to the field drifting and fluctuation,
  locking field is needed. Saturation of locking
  causes locking unstable and broaden the NMR
  signals
• Due to the inhomogeneity of the field around the
  sample, shimming is necessary.
• if the spectral width is set too small, cause
  folding over problem.
• Too much sample or receiver gain is set too
  high, cause clipping problem.
• Sensitivity enhancement and resolution
  enhancement