CHARACTERIZATION OF NMR SHIELDING IN 7-PHOSPHANORBORNENES

Ab initio chemical shielding calculations employing the gauge-including atomic orbital (GIAO) method have been carried out on a series of phosphorus bicyclic and monocyclic molecules in order to provide an understanding of the unusual chemical shifts seen in 7-phosphanorbornene compounds. It is shown that the strong interaction between the phosphorus substituent (hydrogen, in the models studied here) and the carbon-carbon double bond is responsible for the large deshielding seen in these systems. The localized nature of the phosphorus lone pair, the carbon-carbon double bond, a phosphorus p orbital perpendicular to the molecular C(s) plane of symmetry, and their energy proximity to the HOMO-LUMO energy gap give rise to large calculated deshieldings in good agreement with experimental trends. Model calculations on the monocyclic systems as a function of flap angle correlate well with results from the bicyclic molecules, and it is shown that the calculated shieldings in the present study relate in a simple way to the HOMO-LUMO energy gap.