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 Solid-state spectra Methods for improving solid-state NMR spectra

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:

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

NMR periodic table

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

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

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

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:

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

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

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

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

Solution-state NMR spectrum

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

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

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

Decoupling static static with low power decoupling static with high power decoupling decoupling + MAS solution-state spectrum 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

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

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