A study of NMR chemical shielding in 5-coordinate phosphorus compounds (phosphoranes)

The first ab initio calculations, using scaled DFT and EMPI methods, of the 31P NMR chemical shift of phosphoranes have given satisfactory values with a root-mean-square-error of 15-20 ppm from experimental shifts. The calculations are useful in predicting shifts for new compounds. Of special interest are calculations for the cyclic compounds illustrated above. The shifts are δ −97.7 and δ −89.8, respectively, and are quite unlike shifts around δ +19 reported for some P-phenyl derivatives purported to have the five-membered ring system. These shifts suggest instead 4-coordinate (phosphonium) phosphorus.