Ab initio NMR chemical shieldings in the neutral and charged 7-phosphabicyclo[2.2.1]heptane, -heptene, and -heptadiene systems; the largest predicted downfield shift for a conventional organophosphorus molecule Original Research Article

Ab initio quantum mechanical calculations of phosphorus NMR chemical shieldings have been performed at the MP2 level of theory on the neutral and singly charged 7-phosphabicyclo[2.2.1]heptane, -heptene, and -heptadiene neutral, cationic, and anionic molecules using a P:tz2p/C:tzp/H:dz locally dense basis set. Inclusion of correlation at the MP2 level is quantitatively important for the dienes and all the anionic species. The presence of two lone pairs in the anions leads to drastically reduced HOMO-LUMO energy gaps and corresponding large deshielding effects. The absolute isotropic shielding of −756.6 ppm (δ + 1085.0) calculated for the anionic diene is by far the largest downfield shielding predicted for phosphorus in a conventional organophosphorus compound.