EMPIRICAL CALCULATIONS OF H-1-NMR SHIELDING IN CYCLOPHANES

Available ring current models for the calculation of proton shifts are evaluated on the;basis of cyclophanes, which provide many protons above the aromatic plane in well-defined conformations. The latter were simulated with the help of MM2 and CHARMm force fields. Agreement between calculated and observed anisotropy effects approaching Delta delta(av) = 0.17 ppm average deviation is found for 39 signals ranging over 6.6 ppm with the Johnson-Bovey model and the parameterization of double loop geometries p=0.64 Angstrom and r=1.39 Angstrom. The calculations include cyclophanes with protons in extremely close contact with the it-surface, as reported recently by Pascal et al. and by Vogtle et al. The calculations clarify some reported discrepancies but demonstrate, in one and two cases, the presence of severe additional van der Waals (vdW) compression shifts which amount to as much as 1.6 ppm. The consequences of the extreme sensitivity of those proton locations near the shielding cone edges are discussed, in particular with respect to application to supramolecular structures or to biopolymers.