Efficiency of the Refocused ³¹P−²⁹Si MAS-J-INEPT NMR Experiment for the Characterization of Silicophosphate Crystalline Phases and Amorphous Gels

Abstract

One- and two-dimensional refocused MAS-J-INEPT NMR experiments in the solid state (through-bond polarization transfer) involving the highly abundant 31P spin and the rare 29Si spin are described for the crystalline silicophosphate phase Si5O(PO4)6 and complex mixtures of SiP2O7 polymorphs. The evaluation of the 2JP-O-Si coupling constants for all 29Si sites is obtained by the careful analysis of the INEPT build-up curves under fast MAS. The results are in agreement with the crystallographic data, taking into account the various J coupling paths. The efficiency of the experiment is demonstrated by its application to more complex systems such as silicophosphate amorphous gels (obtained by the sol-gel process).

New perspectives in the PAW/GIPAW approach: JP-O-Si coupling constants, antisymmetric parts of shift tensors and NQR predictions

“…Both AIII and AIV monoclinic forms are distinguished through P1 and P2 chemical shifts ( δ = |δ P1 − δ P2 | approximately 8 ppm for AIII and 2 ppm for AIV). [23,24] Again, the calculated values are in excellent agreement with the experimental ones, demonstrating the intrinsic precision of the 31 P (and 29 Si) GIPAW approach. Moreover, it is important to notice that δ is not sufficient to characterize properly the symmetry of a given SiP 2 O 7 phase.…”

“…Full experimental assignments for 29 Si and 31 P resonances have been obtained recently by extensive use of one-and two-dimensional (2D) 31 P- 29 Si solid-state NMR experiments. [23,24] The crystallographic data for all phases and the NMR data for α-Ca(PO 3 ) 2 were taken from the literature. [36] The 31 P (MAS) and 29 Si (static) experimental and calculated spectra (based on the GIPAW data; Table 1) of Si 5 O(PO 4 ) 6 are presented in Fig.…”

“…By 31 P → 29 Si MAS-J-INEPT experiments, it has been possible to study the 29 Si evolution versus the second delay of the INEPT sequence (τ ). [24] The INEPT curves were fitted by using the multispin formula derived from the product operator (PO) formalism. The INEPT curves for Si1 and Si2 were shown to be different and correspond indeed to different spin systems (SI 6 for Si1 involving a unique P-O3-Si1 chemical path and SI 3 I 3 for Si2 involving P-O2-Si2 and P-O5-Si2 chemical paths; see the inserts in Fig.…”

“…Silicophosphate phases, including Si 5 O(PO 4 ) 6 and SiP 2 O 7 polymorphs [23] are fully characterized by 29 Si, 31 P NMR and GIPAW calculations. | 2 J P -O -Si | coupling constants are measured (with reasonable precision) by 31 P- 29 Si MAS-J-INEPT experiments [24] and compared with GIPAW calculations. It has to be stressed that Si 5 O(PO 4 ) 6 acted as a standard for the implementation of J calculations in PAW.…”

“…Polymorphs of SiP 2 O 7 (Figs 1c and 5): 31 P MAS, 7.04 T, ν 0 ( 31 P) = 121.50 MHz, Ø = 4 mm, ν rot = 14 kHz, NS = 8, RD = 10 s, t 90 • ( 31 P) = 3.9 µs. 31 P- 29 Si MAS-J-INEPT, [24] 7.04 T, ν 0 ( 31 P) = 121.50 MHz, ν 0 ( 29 Si) = 59.63 MHz, Ø = 4 mm, ν rot = 14 kHz, triple resonance probe, NS = 1776, dummy scans (DS) = 16, RD = 7 s, t 90 • ( 31 P) = 3.9 µs, t 90 • ( 29 Si) = 4.5 µs, fixed τ = 12.85 ms, variable τ (see Ref. [24] for pulse scheme and experimental details).…”