The PI4+ cation has an extremely large negative P-31 nuclear magnetic resonance chemical shift, due to spin-orbit coupling: A quantum-chemical prediction and its confirmation by solid-state nuclear magnetic resonance spectroscopy

We have used density-functional methods including explicit spin-orbit corre ctions, to calculate the P-31 nuclear magnetic resonance (NMR) chemical shi fts of the tetrahalophosphonium cations PX4+ (X = F, Cl, Br, I). The agreem ent between theory and experimental literature data for PF4+, PCl4+, and PB r4+ is good. For PI4+, the calculations predict an extremely negative (high -field) shift of approximately -520 ppm, due to particularly large spin-orb it contributions from the four heavy iodine substituents, transmitted to th e phosphorus nucleus by a very effective Fermi-contact mechanism. No experi mental data were available for PI4+. We have, therefore, prepared the compo unds PI4AsF6, PI4SbF6, PI4AlI4, and PI4GaI4 and recorded their solid-state P-31 NMR spectra, both with and without magic-angle spinning of the sample. Using the noncoordinating AsF6- and SbF6- anions, the measured isotropic s hifts are -519 and -517 ppm, respectively, in good agreement with the predi cted extreme value for the isolated cation. In contrast, delta(31)P values of only -304 and -295 ppm are found for PI4AlI4 and PI4GaI4, respectively. The large deviation from the isolated-cation limit in the latter two compou nds is probably related to significant I ... I secondary bonding interactio ns, as found in the solid-state structure of PI4AlI4. The observed solid-st ate shift tensors are discussed. The present results disagree clearly with previous claims for the synthesis of PI5. (C) 1999 American Institute of Ph ysics. [S0021-9606(99)30706-6].