Is the ³¹P chemical shift anisotropy of aluminophosphates a useful parameter for NMR crystallography?

Abstract

The 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 Al and/or 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, δ , is related to the length of the shortest P─O bond, whereas the more deshielded components, δ and δ can be related more readily to the mean P─O bond lengths and P─O─Al angles. Using the case of Mg-doped STA-2 as an example, the CSA is shown to be much larger for P(OAl) (OMg) environments, primarily owing to a much shorter P─O(Mg) bond affecting δ , however, because the mean P─O bond lengths and P─O─T (T = Al, Mg) bond angles do not change significantly between P(OAl) and P(OAl) (OMg) 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 P CSAs in AlPOs, in some cases (particularly doped materials), the experiments can still provide valuable additional information for spectral assignment.

Ab initiocomputation for solid-state31P NMR of inorganic phosphates: revisiting X-ray structures

“…This has the potential to be useful for spectral assignment for compounds which contain multiple crystallographic 31 P sites with similar isotropic shifts when computed isotropic shifts for the sites fall within the bounds of uncertainty, as has previously been done for some aluminophosphate framework materials. 58…”

Single-Crystal 31P and 7Li NMR of the Ionic Conductor LiH2PO4

“…One example is the correlation of the isotropic chemical shift of 207 Pb in lead-bearing minerals to the distance of oxygen atoms in the lead coordination sphere [11]. However, for 31 P in phosphates, where the first coordination sphere is always formed by covalently bound oxygen atoms, it is notoriously difficult to establish such relations [17]. One structural parameter that is promising in this context is the dimensionless distortion index (DDI) of the O-P-O bond angles in the PO 4 tetrahedra, i.e., their deviation from the ideal tetrahedral angle, 109.47 o .…”

“…For comparison, the phosphate groups in the mineral pyromorphite, Pb 5 (PO 4 ) 3 Cl, exhibit a smaller distortion with DDI = 0.0146, and accordingly also a smaller CS anisotropy of ∆δ = 20.2 ppm [19]. However, this apparent correlation between the distortion of the tetrahedral geometry and the anisotropy of the 31 P chemical tensor has only limited validity [17].…”

“…33), depicted in Figure 1b Berlin), with the crystallographic c axis and ab plane indicated by arrows. [10] (b) Wurtzite structure of AlN, according to Reference [9], viewed down the crystallographic (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) direction. The aluminum atoms (grey) and the nitrogen atoms (yellow), both located at Wyckoff position 2b, are tetrahedrally coordinated by each other with one Al-N bond directed parallel to the crystallographic c axis.…”

Exploring Accuracy Limits of Predictions of the 1H NMR Chemical Shielding Anisotropy in the Solid State

“…This analysis of accuracy limits of the PW DFT technique is the initial step towards incorporation of the 1 H CSA data into the NMR crystallography approaches for structural elucidation/refinement of compounds in the condensed phase [16]. It is easy to envision investigations similar to those which very recently adopted the 13 C [17,18,19,20], 15 N [21] or 31 P [22] CSA in NMR crystallography studies. Moreover, since the eigenvalues of the 1 H chemical shielding tensors can be particularly sensitive to structure, they could potentially be employed in methods for selecting the suitable candidate(s) from among the generated crystal structure predictions [23,24,25,26].…”