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

Abstract

In order to get insight into transmission mechanisms of the title spin–spin coupling constants, ⁿJ(C, F₁) couplings (n = 4, 5) were calculated at the DFT-B3LYP-6-311-G**/EPR-III level, and couplings were measured in 9 members of the series 1-F,4-X-cubanes, where the α 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 ⁴J(C₄,F₁) 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 σ-hyperconjugative interactions involving as donors the cage C–C bonds; (ii) the particular arrangement of cage bonds not involving either the C₁ or the C₄ carbon atoms, C _i and C _j . For linear X substituents, the direction F₁–C₁··· C₄–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 )→(C₁–F₁)* and (C _i –C _j )→(C₄–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 ⁴J(C₄,F₁) spin–spin coupling constants in 1-F,4-X-cubanes (X = halogen atom) are rationalized in terms of interactions between σ-hyperconjugative (involving C–C bonds) and negative hyperconjugative interactions involving the halogen lone-pairs.

Acknowledgements

RHC gratefully acknowledges financial support from CONICET (PIP 2140) and UBACyT (X-222). EWD and NJH are grateful to the Australian Research Council for financial support. ED and ALE gratefully acknowledge support from the Dirección General de Enseñanza Superior e Investigación Científica (BQU2000-0211-CO2).