The effects of extraframework species on the structure-based prediction of 31P isotropic chemical shifts of aluminophosphates

Abstract

31P NMR spectroscopy is a valuable technique for the characterization of the local structure of aluminophosphates (AlPOs), capable of providing information on the number of crystallographic P sites, their relative populations, and the positions of any dopant atoms in the framework. Assigning the 31P spectra may, however, require multinuclear NMR experiments and/or density functional theory (DFT) calculations, which can be time consuming, computationally costly, and challenging in cases involving disorder or dynamics. To address the issue of computational cost, we recently demonstrated a simple relationship between the local structure around P (primarily in terms of the mean P-O bond length and P-O-Al bond angle) and the 31P isotropic chemical shift, δiso, calculated by DFT for a series of calcined AlPOs. Here, we extend this approach to as-made AlPOs where we show that, at least to a first approximation, the presence of framework-bound anions and/or guest species within the pores of AlPOs can be translated directly to a distortion of the local framework geometry without considering any additional structural parameters. This allows the prediction of a DFT-level 31P δiso even in cases where the structure may be highly disordered or partially incomplete (precluding the use of electronic structure calculations), and we investigate the minimal structural information required to provide meaningful results. The structure-spectrum relationship produced forms the basis for the geometry-based DIStortion COde (DISCO), which can rapidly (on the ms timescale) predict the outcome of a DFT calculation of 31P δiso to within 1.1 ppm.