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 abo ve the aromatic plane in well-defined conformations. The latter were s imulated with the help of MM2 and CHARMm force fields. Agreement betwe en 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 doubl e loop geometries p=0.64 Angstrom and r=1.39 Angstrom. The calculation s 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 Waal s (vdW) compression shifts which amount to as much as 1.6 ppm. The con sequences of the extreme sensitivity of those proton locations near th e shielding cone edges are discussed, in particular with respect to ap plication to supramolecular structures or to biopolymers.