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Chem-805

Chem-805. Solid State NMR. Solution State NMR. Rapid Motion average anisotropic interactions Isotropic Chemical Shift d Average scalar coupling J Dipolar interactions affect only relaxation Easy sample handling High signal to noise Advance pulse sequence easy to perform

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Chem-805

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  1. Chem-805 Solid State NMR

  2. Solution State NMR • Rapid Motion average anisotropic interactions • Isotropic Chemical Shift d • Average scalar coupling J • Dipolar interactions affect only relaxation • Easy sample handling • High signal to noise • Advance pulse sequence easy to perform • Chemical system can be fully analyzed

  3. Solid State NMR • Slow motion result in anisotropic interactions and broaden heavily the spectra • Chemical Shift, Scalar Coupling as well as Dipolar interactions are orientation dependant • In order to simplify the spectra several techniques must be used: • MAS (Magic Angle Spinning) • CP (Cross Polarization) • High power decoupler • Multiple-Pulse experiments (CRAMPS ...)

  4. Interactions in Solid State NMR • Chemical Shift Anisotropy (CSA) • Couplings Interactions (Interactions that split the NMR Signal) • J : scalar coupling (small interaction) ~ 0 – 10 kHze.g. 1JCH 125 Hz • D: Dipolar coupling (large interaction) ~ 50-100 KHz (e.g. DCH ~ 125 KHz) • Q: Quadrupolar Coupling: Very large interaction

  5. Interactions in Solid State NMR:CSA • Chemical Shift depends on the orientation of the molecule. That dependence comes from the fact that the charge distribution is rarely spherically symmetrical. The shielding depends on the orientation of the electron cloud. • In solution NMR, rapid tumbling of the molecules causes averaging of the shielding yielding an average isotropic chemical shift. • CSA depends on the factor (3Cos2θ - 1), where θis the angle between the long axis of the ellipsoid with the field Bo • To eliminate CSA, we can spin rapidly the sample at an angle that would eliminate the factor (3Cos2θ - 1). If the angle is set at 54.74oThe orientation dependence becomes = 0. This is called the magic Angle

  6. Magic Angle spinning : MAS Rapid Spinning    Average angle <  > = 54.74o 

  7. CSA: Chemical shift Anisotropy CH3COOH

  8. Anisotropic interactions: Chemical shift Chemical Shift depends on orientation: Figure represent the Chemical shift vs Orientation in Single Crystal

  9. CSA: Chemical shift Anisotropy The spectra in powder sample represent the sum of the various orientations

  10. Solution vs Solid (Static)

  11. MAS: Magic Angle Spinning

  12. MAS: Magic Angle Spinning The Isotropic shift do not change with different spinning speed If the spinning speed is not enough to cover the CSA pattern spinning side bands appear around the isotropic chemical shift at multiple of the spinning speed.

  13. Dipolar Coupling • For proton, Homonuclear dipolar coupling is very large (> 100 KHz). It depends on the orientation and on the distance between the nuclei (r-3) • To remove such coupling the MAS spinning frequency must be larger than the coupling (Spinning> 100 KHz for proton coupling!!!). This spinning frequency cannot be realized: our probe is limited to 15 KHz The presence of several Dipolar coupling result in very broad spectra D

  14. Dipolar Coupling: dilute nuclei • For Carbon-13 or other nuclei not very abundant, the Homonuclear dipolar coupling is not a problem as the nuclei are more distant. MAS can eliminate this factor. • Heteronuclear Dipolar interaction with other nuclei (like proton) can be reduce significantly with MAS and residual dipolar coupling can be further eliminate using high power decoupling (HD)

  15. Cross-Polarization: CP • The problem for detecting a low-g nuclei comes from their low abundance, their long relaxation time and their low spin polarization. • A solution to the low sensitivity and long relaxation problem is to use an experiment called Cross Polarization, which involves transferring polarization from the “abundant, high sensitivity” nuclei (1H) to the “dilute” nuclei (like 13C, 15N, 29Si …). • CP consist of bringing the heteronuclei into a dipolar thermal contact by adjusting their precession rate (Hartman-Hahn match), during a time called the “contact time”. During this time the low frequency nuclei gets polarized. The CP contact time is followed by the detection time of the heteronuclei in presence of HPD (High Power Decoupler)

  16. Cross-Polarization: CP 1H In the Lab Frame: 13C n = B0 H ~ 4 gC Frequencies (energy) are very different nH nC Hartman-Hahn HB1H = CB1C In the Rotating Frame: By adjusting the power, The frequency of proton can match frequency of Carbon nH nC

  17. Cross-Polarization: CP 1H nH = HB1H nC nH = 90o B1H 13C CT nC= CB1C High Power Decoupler 1H Mixing Time Hartman-Hahn Contact during mixing time (CT) C13 magnetization grows 13C Contact Time

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