C₃(SINGLE BOND)M_α Bond contribution to polarizability tensor and ³J(C₁M_α) NMR coupling constant in 1-X-3-M-bicyclo[1.1.1]pentanes

In the present work, the relationship between the large substituent effects on 3J(C1H) in 1-X-3-M-bicyclo[1.1.1]pentanes, I, and the polarizability of the bridgehead C3(SINGLE BOND)Mα bond is investigated. The existence of such a relationship is suggested by the finding that the effect of an electronegative substituent X on 3J(C1Mα) couplings in I (M=H) is due to a distortion of the C3(SINGLE BOND)H bond toward the C1 center, which enhances the Fermi contact interaction. If such distortion originates in an electrostatic effect, then in other members of this series it can be expected that the substituent effects on 3J(C1Mα) couplings should depend strongly on the C3(SINGLE BOND)Mα bond polarizability. Two approaches are followed. First, the ab initio CLOPPA-IPPP method is applied to study the C3(SINGLE BOND)Mα bond contribution to the molecular static polarizability tensor in I (M=H, F, CH3). Such bond polarizabilities are found to follow the same trend as calculated as well as experimentally determined substituent effects on 3J(C1Mα) couplings, which were measured as part of this work in I [X=H, Cl; M=F, CH3 and X=OCH3; M=Sn(CH3)3]. Second, 3J(C1Mα) couplings (M=H, CH3) are calculated at an ab initio level for X=H, F, and they are compared with those obtained in the parent compound (X=H) if the calculation is carried out in the presence of an inhomogeneous electric field.