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

NMR spectroscopy in solids: A comparison to NMR spectroscopy in liquids. Mojca Rangus Mentor: Prof. Dr. Janez Seliger Comentor: Dr. Gregor Mali. Introduction. NMR specrtometer Basics of NMR Types of interaction (concentrate on: spin ½ nuclei, diamagnetic compounds) Solution-state spectra

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

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  1. NMR spectroscopy in solids: A comparison to NMR spectroscopy in liquids Mojca Rangus Mentor: Prof. Dr. Janez Seliger Comentor: Dr. Gregor Mali

  2. 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

  3. Magnetization and magnetism • Nuclear magnetic moment • When magnetic field is applied, starts to precess around it’s 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:

  4. 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

  5. NMR periodic table

  6. 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

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

  8. Types of interaction • Zeeman interaction • Chemical shift • Direct dipole coupling • Indirect dipole coupling or J-coupling • Quadrupolar interaction

  9. 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:

  10. 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

  11. 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

  12. 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

  13. 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

  14. Solution-state NMR spectrum

  15. 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

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

  17. Magic angle spinning • Spinning the sample under the magic angle considerably narrows the resonance lines static MAS solution

  18. 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

  19. 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

  20. 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

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