Solid-state NMR connectivity studies in dipolarly coupled inorganic networks: Crystal structure and site assignments for the lithium polyphosphide LiP5

The crystal structure of LIP, has been refined from single-crystal X-ray da ta to a residual of R = 0.020 for 2357 structure factors and 56 variable pa rameters. The structure as determined in the acentric space group Pna2(1) b y H, G, von Schnering and W, Wichelhaus [Naturwissenschaften 59, 78 (1972)] is confirmed, However, the lithium position has been determined with consi derably greater accuracy, The polyanionic network of the phosphorus atoms c an also be refined in the corresponding centrosymmetric group Pnam, where t wo adjacent lithium positions with partial occupancy are found. The phospho rus connectivities within this network are studied by means of advanced sol id-state NMR techniques. Crystalline polyphosphides represent a particular challenge for such experiments owing to the presence of strong homonuclear (31)p-(31)p dipode-dipole couplings within the network of phosphorus polyan ions, To meet this challenge a powerful strategy has been designed by combi ning magic-angle spinning with (31)p-(31)p double-quantum-filtered two-dime nsional exchange, rotational resonance, and heteronuclear cross-polarizatio n spectroscopies. Based on this information, the NMR results are discussed in connection with the local environments of the phosphorus sites. (C) 1999 academic Press.