A ³¹P Dynamic NMR Study of the Bond Shift Rearragement in Solid Li₃P₇

Phosphorus-31 NMR measurements are reported on solid Li 3 P 7 (monoglyme) 3 in the temperature range −90 to +70 °C, under both, nonspinning and magic angle spinning (MAS) conditions. At low temperatures (<−30 °C) the spectra correspond to a static situation, exhibiting a superposition of three subspectra due to the apical, equatorial, and basal phosphorus atoms in the P 7 -cage. Analysis of these spectra provided information on the principal values of the chemical shift tensors of the various P atoms in their respective principal axis systems. Their orientations in the molecular frame were obtained from quantum mechanical calculations. In the temperature range −30 to +70 °C the spectra exhibit dynamic effects, which at high temperatures result in a line shape corresponding to a single average axially symmetric chemical shift tensor. This is interpreted in terms of a bond shift rearrangement similar to the Cope rearrangement process in bullvalene. Analysis of the results yields approximate kinetic parameters for the reaction. At room-temperature the rate constant is about 10 5 s - 1, and the activation energy lies between 7 and 12 kcal/mol. Due to the amorphous nature of the sample used and the incomplete fit of the experimental and simulated spectra, the possibility of a heterogeneous reaction with a distribution of rates and an independent 3-fold jumps process, cannot be ruled out.