AB-INITIO NMR CHEMICAL SHIELDINGS IN THE NEUTRAL AND CHARGED 7-PHOSPHABICYCLO[2.2.1]HEPTANE, 7-PHOSPHABICYCLO[2.2.1]HEPTENE, AND 7-PHOSPHABICYCLO[2.2.1]HEPTADIENE SYSTEMS - THE LARGEST PREDICTED DOWNFIELD SHIFT FOR A CONVENTIONAL ORGANOPHOSPHORUS MOLECULE

Ab initio quantum mechanical calculations of phosphorus NMR chemical s hieldings have been performed at the MP2 level of theory on the neutra l and singly charged 7-phosphabicyclo[2.2.1]heptane, -heptene, and -he ptadiene neutral, cationic, and anionic molecules using a P:tz2p/C:tzp /H:dz locally dense basis set. Inclusion of correlation at the MP2 lev el is quantitatively important for the dienes and all the anionic spec ies. The presence of two lone pairs in the anions leads to drastically reduced HOMO-LUMO energy gaps and corresponding large deshielding eff ects. The absolute isotropic shielding of -756.6 ppm (delta + 1085.0) calculated for the anionic diene is by far the largest downfield shiel ding predicted for phosphorus in a conventional organophosphorus compo und. (C) 1997 Elsevier Science B.V.