Multinuclear NMR: Easy or Difficult? (Tips for Solid-state NMR)

  • Summary

NM140009E

  • How easy or difficult NMR measurements are depends on various factors. Sensitivity(S/N ratio) may be estimated in terms of receptivity given by the following formula.
    Large gyromagnetic ratio γ and natural abundance Nabd lead to large receptivity for 19F and 27Al, for example.
    Receptivity
  • Low-γ nuclei exhibiting low NMR frequencies (∝ γ ), such as 25Mg and 47,49Ti, may suffer from ringing effects, making their measurements difficult.
  • In solid-state NMR, it is easy to obtain high-resolution spectra for the nuclei with spin quantum number of 1/2, like 13C and 31P, while difficult for those having larger quantum number than 1/2, in general, because of quadrupolar broadening. Half integer spins (I=3/2, 5/2, 7/2, 9/2), such as 11B and 23Na, may yield high-resolution spectra via special techniques like MQMAS. These techniques are not applicable to integer spins (I=1, 3, …), such as 10B and 14N.
  • Although 1H high-power decoupling is widely utilized in solid-state NMR, the nuclei experiencing weak 1H dipolar interactions like 29Si require not so strong decoupling. It is difficult to decouple 1H-1H interactions, rendering solid-state 1H measurements difficult.
  • In solids, chemical shifts vary depending on the nuclear orientations (CSA: chemical shift anisotropy). For the nuclei having large CSA, such as 195Pt and 207Pb, magic angle sample spinning with realistic speeds cannot remove spinning sidebands.
  • Another factor of long longitudinal relaxation times T1 makes measurements difficult.

Reference

T. Nakai, New Glass, 28(2),17-28 (2013) ” (in Japanese) isavailable.

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