Is the 31 P chemical shift anisotropy of aluminophosphates a useful parameter for NMR crystallography?
Magn Reson Chem
; 57(5): 176-190, 2019 05.
Article
em En
| MEDLINE
| ID: mdl-30105879
ABSTRACT
The 31 P chemical shift anisotropy (CSA) offers a potential source of new information to help determine the structures of aluminophosphate (AlPO) framework materials. We investigate how to measure the CSAs, which are small (span of ~20-30 ppm) for AlPOs, demonstrating the need for CSA-amplification experiments (often in conjunction with 27 Al and/or 1 H decoupling) at high magnetic field (20.0 T) to obtain accurate values. We show that the most shielded component of the chemical shift tensor, δ33 , is related to the length of the shortest PâO bond, whereas the more deshielded components, δ11 and δ22 can be related more readily to the mean PâO bond lengths and PâOâAl angles. Using the case of Mg-doped STA-2 as an example, the CSA is shown to be much larger for P(OAl)4-n (OMg)n environments, primarily owing to a much shorter PâO(Mg) bond affecting δ33 , however, because the mean PâO bond lengths and PâOâT (T = Al, Mg) bond angles do not change significantly between P(OAl)4 and P(OAl)4-n (OMg)n sites, the isotropic chemical shifts for these species are similar, leading to overlapped spectral lines. When the CSA information is included, spectral assignment becomes unambiguous, therefore, although the specialist conditions required might preclude the routine measurement of 31 P CSAs in AlPOs, in some cases (particularly doped materials), the experiments can still provide valuable additional information for spectral assignment.
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MEDLINE
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En
Ano de publicação:
2019
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Article