NMR spectroscopy in solids: A comparison to NMR spectroscopy in liquids

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NMR spectroscopy in solids A comparison to NMR
spectroscopy in liquids

• Mojca Rangus
• Mentor Prof. Dr. Janez Seliger
• Comentor Dr. Gregor Mali

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Introduction

• NMR specrtometer
• Basics of NMR
• Types of interaction (concentrate on spin ½
  nuclei, diamagnetic compounds)
• Solution-state spectra
• Solid-state spectra
• Methods for improving solid-state NMR spectra

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Magnetization and magnetism

• Nuclear magnetic moment
• When magnetic field is applied, starts
  to precess around its direction with Larmor
  frequency
• We describe the movement of the magnetic moment
  with equation
• It is conventent to go from lab. frame to
  rotating frame, which rotates around
  direction with the same frequency that
  precesses

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NMR

• We observe the total magnetization of the sample
• Magnetization is fliped in xy plane with the aid
  of rotating radiofrequency (rf) field
• Projection of the magnetization on the xy plane
  is then recorded
• The nuclei with nonzero spin (nonzero magnetic
  moment) can be observed

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NMR periodic table
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Single pulse

• The same coils are used for excitation and
  recording
• After a pulse a delay is needed before the start
  of the recording
• From recorded signal (FID) a spectrum is obtained
  with Fourier transormation
• This means that the magnetic moments are alredy
  scattered in xy plane

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Spin echo

• With a special pulse sequence the magnetic
  moments are gathered before the recording starts
• An efficient method to avoid dead time problem

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Types of interaction

• Zeeman interaction
• Chemical shift
• Direct dipole coupling
• Indirect dipole coupling or J-coupling
• Quadrupolar interaction

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Zeeman interaction

• It can be described with a Hamiltonian
• or in ternsor form
• In the magnetic field the two spin states have
  different energies
• It is far the strongest interaction and all other
  types of interaction can be considered as
  corrections
• Order of the magnitude

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Chemical shift

• Indirect interaction of the nuclear spins with
  the external magnetic field through the
  surrounding electrons
• If the electronic environment of nuclei differ,
  the local mag. fields differ and therefore the
  resonance frequencies are different
• Contains information about electronic states
• Chemical shifts also depend on the orientation of
  the molecule in the magnetic field

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Direct dipole coupling

• Two neighbouring nuclei are coupled through their
  magnetic dipole moments
• Useful for molecule structure studies and
• provides a good way to estimate distances
• between nuclei and hence the geometrical
• form of the molecule

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J-coupling

• Nuclear spins are coupled with the help of the
  molecular electrons
• It is exclusively intramolecular
• The mechanism responsible for the multiplet
  structure
• It can be viewed only in solution-state NMR
  spectra where the spectral lines are narrow
  enough to observe the interaction

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Electric quadrupole coupling

• Nucleus with the electric quadrupole moment
  intarects strongly with the electric field
  gradients generated by surrounding electron
  clouds
• Quadrupole interaction is totaly averaged
• in liquids, but in solids is the strongest after
  
• Zeeman
• In solids we often need to take into account
• second order contributions

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Solution-state NMR spectrum
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Single crystal spectra

• All interactions are orientation dependent
• Therefore it is possible to conduct NMR
  experiments in similar way as X-ray diffraction
• From single crystal spectra it is possible to
  reconstruct the interaction tensors and from
  there the electronic and geometric
  characteristics of the compound

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Powder spectra

• All orientations of the molecules are presented
  equally
• Resonance lines become extremely broad
• Anisotropic nature of the interactions comes
  fully into account

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Magic angle spinning

• Spinning the sample under the magic angle
  considerably narrows the resonance lines

static
MAS
solution
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Decoupling
static

• In the mechanism of decoupling a strong rf field
  is applied so that magnetic moments are flipped
  randomely back and forth to narrow the
  anisotropic broadeneng of the resonance lines

static with low power decoupling
static with high power decoupling
decoupling MAS
solution-state spectrum
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Cross polarization (CP)

• Cross polarization (CP) is one of the most
  important techniques in solid-state NMR
• Polarization from abundant spins is transferred
  to dilute ones via the direct diploe coupling

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Summary

• Basic principles of NMR
• Viewed the most important type of interactions
  that are encountered in a compound
• Similarities and differences of solution-state
  and solid-state spectra
• The most important techniques used to improve
  powder NMR spectra