Transmission mechanisms of NMR long-range J(13C, 19F) scalar couplings in 1-F,4-X-cubanes:: a DFT and experimental study

In order to get insight into transmission mechanisms of the title spin-spin coupling constants, (n)J(C, F-1) couplings (n=4, 5) were calculated at the DFT- B3LYP- 6-311-G**/EPR-III level, and (5)J(C alpha, F-1) couplings were measured in 9 members of the series 1-F, 4-X-cubanes, where the alpha atom of the X group is a carbon atom. Hyperconjugative interactions were evaluated within the NBO approach, using the same level of theory as that employed for calculating spin-spin coupling constants. The unusual (4)J(C-4, F-1) spin-spin coupling constants known in 1-F, 4-X-cubanes are rationalized in this work as originating mainly due to two factors, namely, (i) the strong sigma-hyperconjugative interactions involving as donors the cage C-C bonds; (ii) the particular arrangement of cage bonds not involving either the C-1 or the C-4 carbon atoms, C-i and C-j. For linear X substituents, the direction F-1-C-1 (.) (.) (.) C-4-X is a three-fold symmetry axis and there are six C-i-C-j equivalent cage bonds, which are involved in (C-i-C-j)->(C-1-F-1)* and (Ci-Cj)->(C-4-X)* hyperconjugative interactions. This means that coupling pathways involving such interactions are amplified six times, rendering the substrate very efficient for transmitting 'trans-cage' coupling constants. The large halogen substituent effects observed for (4)J(C-4, F-1) spin-spin coupling constants in 1-F, 4-X-cubanes (X=halogen atom) are rationalized in terms of interactions between sigma-hyperconjugative (involving C-C bonds) and negative hyperconjugative interactions involving the halogen lone-pairs.