C-3-M-ALPHA BOND CONTRIBUTION TO POLARIZABILITY TENSOR AND (3)J(C1M-ALPHA) NMR COUPLING-CONSTANT IN 1-X-3-M-BICYCLO[1.1.1]PENTANES

In the present work, the relationship between the large substituent ef fects on (3)J(C1H) in 1-X-3-M-bicyclo[1.1.1]pentanes, I, and the polar izability of the bridgehead C-3--M-alpha bond is investigated. The exi stence of such a relationship is suggested by the finding that the eff ect of an electronegative substituent X on (3)J(C1Malpha) couplings in I (M = H) is due to a distortion of the C-3-H bond toward the C-1 cen ter, which enhances the Fermi contact interaction. If such distortion originates in an electrostatic effect, then in other members of this s eries it can be expected that the substituent effects on (3)J(C1Malpha ) couplings should depend strongly on the C-3-M-alpha bond polarizabil ity. Two approaches are followed. First, the ab initio CLOPPA-IPPP met hod is applied to study the C-3-M-alpha bond contribution to the molec ular static polarizability tensor in I (M = H, F, CH3). Such bond pola rizabilities are found to follow the same trend as calculated as well as experimentally determined substituent effects on (3)J(C1Malpha) cou plings, which were measured as part of this work in I [ X = H, Cl; M = F, CH3 and X = OCH3; M = Sn(CH3)(3)]. Second, (3)J(C1Malpha) coupling s (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) i f the calculation is carried out in the presence of an inhomogeneous e lectric field. (C) 1998 John Wiley & Sons,Inc.