31P CP/MAS NMR of polycrystalline and immobilized phosphines and catalysts with fast sample spinning

Abstract

AbstractCross‐polarization (CP) at fast magic angle spinning (MAS) frequencies leads to a splitting of the Hartmann–Hahn (HH) matching profile into a centerband and additional bands of higher orders. The matching profiles differ with the substance categories. Therefore, signal intensity is usually lost, when e.g. the routine standard NH4H2PO4 is used for optimizing the 1H–31P HH match prior to measuring phosphines and their metal complexes in polycrystalline or immobilized form. Here, a variety of model compounds, such as Ph2PCH2CH2PPh2 and (CO)2Ni(PPh3)2, which can be used as 31P CP standards for analogous substances or materials are presented. Investigating the influences of MAS frequency, contact time, 1H pulse power and sample volume on the matching profiles of the model compounds leads to general trends. Thereby, a new strategy for measuring difficult samples with CP at high MAS rates has been developed: their optimum CP parameters are derived from the most intense maxima in the HH matching profiles of the corresponding model compounds. This new strategy is compared with variations of a conventional ramp sequence. Although the latter generally provide smaller signal half‐widths, the new strategy leads to higher signal intensities. The new method was successfully applied to polycrystalline and immobilized phosphines and catalysts. Copyright © 2003 John Wiley & Sons, Ltd.