25-30 August 2019
Henry Ford Building
Europe/Berlin timezone

Improving bis-nitroxides' geometry for MAS-DNP

29 Aug 2019, 12:30
30m
Lecture Hall A (Henry Ford Building)

Lecture Hall A

Henry Ford Building

Talk Hyperpolarization techniques Hyperpolarization in Materials

Speaker

Frederic Mentink-Vigier (National High Magnetic Field Laboratory, Florida State University)

Description

Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP) is becoming a mainstream method to increase the sensitivity of solid-state NMR experiments. By irradiating a sample with a strong microwave ($\mu$w) source, nitroxide biradicals are used to enhance the proton polarization via the cross-effect (CE) mechanism.$^1$
To increase the enhancement factor (the NMR signal ratio with/without $\mu$w irradiation, $\mathrm{\epsilon_{on/off}}$), the biradical structures underwent extensive optimization.$^{2, 3}$ In particular, the nitroxides’ g-tensors relative orientation ($\alpha,\beta,\gamma$) is essential for efficient CE under MAS.$^4$
From the early biradical design the role of $\beta$ was clearly identified experimentally and was recently confirmed theoretically by scanning the complete angular space.$^5$
Nonetheless both approaches (experimental and theoretical) are incomplete. First, improving $\mathrm{\epsilon_{on/off}}$ may not correlate with increasing proton polarization as biradical induces nuclear depolarization without $\mu$w. Biradicals with lower $\mathrm{\epsilon_{on/off}}$ can provide equal or better polarization performance.$^6$ Second, the simulations neglected the relative orientation’s impact on the enhancement vs magnetic field.$^7$ Therefore, the question “Can bis-nitroxide be improved?” is still debatable.
In this presentation, using a new quantitative theoretical approach,$^7$ this question is discussed and an ideal orientation is presented. A single parameter is sufficient to evaluate the potential performance of a bis-nitroxide structure. Finally, the method is illustrated on the bTurea series to explain recent experimental observation of AMUPol,$^2$ bcTol-M,$^3$ and HydrOPol.$^8$

  1. Rosay et al., Phys. Chem. Chem. Phys. 12, (2010).
  2. Sauvée et al., Chem. - A Eur. J. 22, (2016).
  3. Geiger et al., Chem. - A Eur. J. 24, (2018).
  4. Mentink-Vigier et al., J. Magn. Reson. 258, (2015).
  5. Perras et al.,ChemPhysChem. 18, (2017).
  6. Mentink-Vigier et al., J. Am. Chem. Soc. 140, (2018).
  7. Mentink-Vigier et al., Phys. Chem. Chem. Phys. 21, (2019).
  8. Stevanatoa et al., 60th ENC, Asilomar (2019).

Primary author

Frederic Mentink-Vigier (National High Magnetic Field Laboratory, Florida State University)

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