Wednesday, April 23, 2008
Satellite Transitions in the Solid State NMR Spectra of Spin I= 5/2 Nuclei
The solid state MAS NMR spectra of spin I = n/2 (n = 1, 3, 5 ....) quadrupolar nuclei are more complicated than many people realize. There are chemical shielding anisotropy effects and both first order and second order quadrupolar effects which must be taken into account. Magic angle spinning averages out the first order quadrupolar effects and the chemical shielding anisotropy however, these interactions may show up as spinning sidebands. The field dependant second order quadrupolar effects are only partially averaged by magic angle spinning. In an MAS spectrum, each transition of the spin I = n/2 nucleus will have both a frequency shift and line shape characterized by the partially averaged second order quadrupolar interaction. The frequency shifts and lineshapes can provide valuable information about the quadrupolar coupling constant and the asymmetry parameter. The figure below shows the centerband region of the 27Al (I = 5/2) MAS NMR spectrum of aluminum-tris-acetylacetonate (Al(acac)3) acquired at 11.75 Tesla. There is only one aluminium site in the asymmetric unit of this compound. The major component of the spectrum is the central transition (m=1/2 - m= -1/2). It is the most intense as it is unaffected by the first order quadrupolar interaction and therefore does not have its intensity spread over multiple spinning sidebands. The first satellite transitions (m=3/2 - m= 1/2 and m= -1/2 - m= -3/2) are affected equally by the second order quadrupolar interaction and appear as a narrow resonance to higher frequency than the central transition. The second satellite transitions (m=5/2 - m= 3/2 and m= -3/2 - m= -5/2) are also affected equally by the second order quadrupolar interaction and appear as a weak broad resonance to higher frequency than both the first satellite transition and the central transition. Both satellite transitions are weak as their intensity is spread over many spinning sidebands (not shown). The second satellite transition is weaker than the first as it has a much broader line shape.