Determination of distances between aluminum and spin-1/2 nuclei using cross polarization with very weak radio-frequency fields

Abstract

In this work the possibility of using cross-polarization (CP) experiment for the determination of distances between spin-5/2 and spin-1/2 nuclei in polycrystalline and amorphous materials was investigated. The properties of the method were experimentally studied in an isolated Al2H spin system within the as-synthesized AlPO4-31 and in “infinite” aluminophosphate networks within the calcined AlPO4-31 and the hydrated VPI-5. In all three cases time-dependent oscillations due to coherent polarization transfer between aluminum and spin-1/2 nuclei were detected. The crucial parameters that influenced the visibility of dipolar oscillations were rotating-frame spin-lattice relaxation times of both nuclear species involved in a CP process and the homogeneity of rf fields. For a successful measurement the relaxation times, which varied remarkably with amplitudes of applied rf fields, had to be comparable to or larger than the period of time-domain oscillations. Radio-frequency field inhomogeneity was minimized when amplitudes of rf fields were adjusted to the Hartmann–Hahn sideband matching condition 3ν1I+ν1S=νR, the nutation frequency of spin-5/2 nuclei was limited to νR/2<3ν1I<νR, and the nutation frequency of spin-1/2 nuclei was limited to 0<ν1S<νR/2. Such adjustment assured also an efficient spin locking of quadrupolar spins. Experiments in the as-synthesized and calcined AlPO4-31 showed that in small isolated spin systems the time evolution of CP can elucidate the underlying geometry of the system whereas in “infinite” spin networks the splitting of a Pake-type doublet in the Fourier transform of a CP signal can yield an overall strength of the dipolar coupling. Although the use of weak rf fields reduced the robustness of the experiment it also introduced the selectivity of polarization transfer, which, as indicated by numerical simulation, in some cases allows the extraction of pairwise dipolar couplings in multispin systems. The possibility of a selective polarization transfer and a selective determination of distances between octahedrally coordinated Al1 nuclei and tetrahedrally coordinated P2 and P3 nuclei in the hydrated VPI-5 was demonstrated experimentally.