CHARACTERIZATION OF NMR DESHIELDING IN PHOSPHOLE AND THE PHOSPHOLIDE ION

Ab initio chemical shielding calculations employing the gauge includin g atomic orbtial (GIAO) method have been carried out on phosphole, the phospholide ion, and related thiophene, pyrrole, and furan compounds, to provide an understanding of the NMR shieldings observed in the pho sphole systems. Structure and energy considerations in phosphole const rained to be fully planar (as opposed to being nonplanar and pyramidal ) show that the system is conjugated but that this conjugation is in i tself not responsible for the observed deshielding effect. Rather, it is shown that removing the stabilizing proton from the PH bond in phos phole to form the phospholide ion allows the system to become essentia lly fully conjugated and creates a lone pair only weakly coupled to th e ring, and that this in-plane lone pair is responsible for the large downfield (paramagnetic) shifts seen in the ion relative to the parent compound.