WSolids1 - MAS: Quadrupolar Nucleus
Description
This picture shows an example for the succesful simulation of a MAS spectrum of a quadrupolar nucleus that shows the combined effect of quadrupolar interaction and spin-spin coupling to a spin-1/2 nucleus in a powder sample. It is the 95Mo NMR MAS spectrum of pentacarbonyl-5-methyldibenzophosphole molybdenum(0), Mo(CO)5(MeDBP), and the results have been published in:
K. Eichele, R. E. Wasylishen, J. H. Nelson:
Solid-State 95Mo NMR Studies of Some Prototypal Molybdenum Compounds: Sodium Molybdate Dihydrate, Hexacarbonylmolybdenum, and Pentacarbonyl Phosphine Molybdenum (0) Complexes;
J. Phys. Chem. A 1997, 101, 5463-5468; DOI: 10.1021/jp9712415.
Background
If the nuclear quadrupolar coupling for a quadrupolar nucleus is sufficiently large, MAS cannot remove its effect on the line shape of the central transition and causes second-order broadening with characteristic lineshapes as well as a second-order shift. In order to obtain correct chemical shifts for the quadrupolar nucleus, simulation of the spectra is required. Optionally, indirect coupling to a heteronucleus can be added (note: quadrupolar interaction, if any, is neglected for the coupled heteronucleus).
Examples
The SVG images shown below were produced using the following tools: my own SpecPlot to plot the spectra, Platon or Ortep 3 for Windows to plot the molecular structures from X-ray data, and Inkscape to compose the picture.
Mo-95 MAS NMR spectrum of Mo(CO)5(MeDBP)
This example is also shown above. The experimental spectrum was acquired using two spectrometers simultaneously: a 4.7 T spectrometer provided the probe head and pneumatic unit, with tubing running across the room, whereas a 9.4 T spectrometer was collecting the spectrum. Usually, the frequency of Mo-95 is so low that it is outside the range of regular broadband probe heads, hence the low-field components. MAS was sufficient to suppress dipolar couplings to H-1.
The line shape here depends on the second-order quadrupolar interaction and the indirect spin-spin coupling to P-31 (122 Hz). In the simulation, you can set the coupling to zero and generate a regular second-order line shape.