Cubane shaped clusters, precursors for aluminophosphate frameworks: a solid state multinuclear NMR study, in time and frequency domains

Abstract

A detailed solid state NMR study of molecular aluminophosphates is presented. The crystallographic structures of [Al₄(HPO₄)₄(C₂H₅OH)₁₂]Cl₄·4C₂H₅OH (compound 1) and [Al₄(HPO₄)₄(C₂H₅OH)₁₂]Br₄·4C₂H₅OH (compound 2) were obtained at low temperature. ¹³C CP MAS NMR dynamics were carefully studied: non-exponential growth of magnetisation was observed for ¹³C CP even for moderately coupled ¹³CH₃ groups. This approach was extended for the first time to ³¹P CP MAS NMR. Variable contact time as well as inversion recovery cross polarisation (IRCP) experiments showed unambiguously that the ¹H → ³¹P magnetisation transfer was also not exponential. The ³¹P IRCP MAS experiment proved to be a powerful tool for the study of complex amorphous materials, allowing editing of the spectra. Variable contact time experiments under fast MAS led to the observation of strong dipolar oscillations. The subsequent analysis of the CP data in the frequency domain led to the direct accurate determination of ³¹P–¹H distances. This approach can be used as an alternative method for the measurement of distances in solid state NMR. ²⁷Al NMR quadrupolar parameters for 1 and 2 were derived by using several static and MAS techniques, in both time and frequency domains. The time domain response to a two-pulse sequence led to the very unusual observation of multiple quadrupolar Solomon echoes. The spectroscopic data related to 1 and 2 are a good starting point and pertinent tools for the study of more complex derivatives.