Investigating Relationships between the Crystal Structure and ³¹P Isotropic Chemical Shifts in Calcined Aluminophosphates

Abstract

Solid-state NMR spectra have historically been assigned using simple relationships between NMR parameters, e.g., the isotropic chemical shift, and aspects of the local structure of the material in question, e.g., bond angles or lengths. Density functional theory (DFT) calculations have effectively superseded these relationships in many cases, owing to the accuracy of the NMR parameters typically able to be calculated. However, the computational time required for DFT calculations may still be prohibitive, particularly for very large systems, where structure-spectrum relationships must still be used to interpret the NMR spectra. Here we show that, for calcined aluminophosphates (AlPOs), structurespectrum relationships relying on either the mean P-O-Al angle or the mean P-O distance, both suggested in previous literature, provide a poor prediction of the 31 P isotropic shielding, iso, calculated by DFT. However, a relationship dependent on both parameters yields predicted iso in excellent agreement with DFT, with a mean error of ~1.6 ppm. The predictive ability of the relationship is not improved by introducing further parameters (many used in previous work) describing the local structure, suggesting that the twoparameter relationship is close to an optimum balance between accuracy and overparameterisation. The ability to predict accurately the outcome of DFT-level calculations will be of particular interest in cases where the actual calculations would be impractical or even impossible with current computational hardware, or where many such calculations are required quickly.

A Multinuclear NMR Study of Six Forms of AlPO-34: Structure and Motional Broadening

“…This relationship is different from that reported for calcined AlPOs, 54 but similar to that derived from computational results as discussed in the Supporting Information (S9).…”

“…54 The work presented here demonstrates an extension of our earlier work, focused on calcined AlPOs, to materials containing disordered SDAs and frameworkbound anions, suggesting that the method may be more generally applicable to AlPOs containing guest molecules; further investigation of this is ongoing.…”

“…As was observed for 13 The 31 P MAS NMR spectra of the six samples of AlPO-34 are shown in Figure 3 and, in each case, three resonances are observed, corresponding to the three crystallographically-distinct P sites (or the three sets of pseudo-equivalent P sites, where local disorder leads to more than three formally distinct P). Assignment of the P sites can be made based on the relationship between 31 P chemical shift and the mean P-O-Al bond angle, POAl, and the mean P-O bond length, rPO, determined using computation by Dawson and Ashbrook,54 in an extension to much earlier work. While this relationship was only demonstrated for calcined AlPOs in the original work, further investigation shows, perhaps surprisingly, that a similar relationship holds for as-made AlPOs.…”

Perspective: Current advances in solid-state NMR spectroscopy

“…Approaches that have been used successfully in NMR studies include quench-cooling of MD simulations of molten solids, 65 as well as approaches that avoid the need for large-scale DFT calculations, such as training neural networks to predict chemical shifts in glasses 66 or deriving empirical correlations from related compounds. 67,68 In contrast to "static disorder," dynamics tend to average NMR parameters, and so, depending on the time scale of the dynamics, will narrow rather than broaden NMR line shapes. This is illustrated in Fig.…”

Recent advances in computational 31P NMR: Part 1. Chemical shifts

“…In a continuation of their earlier studies, recently reviewed by the principal authors, Dawson, et al performed a systematic DFT investigation of the solid‐state 31 P NMR isotropic chemical shifts of the local structure of aluminophosphates, capable of providing information on the number of crystallographic phosphorus sites, their relative populations, and the positions of any dopant atoms in the framework. Based on the recently demonstrated simple relationship between the local structure around phosphorus atom (first of all, the mean P─O bond length and P─O─Al bond angle) and calculated at the DFT level 31 P NMR isotropic chemical shift, δ iso , for a series of calcined aluminophosphates, the authors extended this approach to "as‐made" aluminophosphates, illustrated in Figure . It was demonstrated that the presence of the framework‐bound anions and/or guest species within the pores of aluminophosphates could be translated directly to a distortion of the local framework geometry without considering any additional structural parameters.…”