A 31P dynamic NMR study of the bond shift rearrangement in solid Li3P7

Sen T., Poupko R., Fleischer U., Zimmermann H., Luz Z.

Journal of the American Chemical Society, cilt.122, sa.5, ss.889-896, 2000 (Scopus)

Özet

Phosphorus-31 NMR measurements are reported on solid Li3P7(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 P7-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 105 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.