Through-Space Spin-Spin Coupling in van der Waals Dimers and CH/p Interacting Systems. An Ab Initio and DFT study

The through-space JHH and JCH spin-spin coupling constants of model van der Waals dimers (involving methane, ethylene and benzene), and of selected compounds showing the CH/p interaction, have been investigated by means of DFT and ab initio calculations. In the range of intermolecular separations for which the interaction is stabilising, weak couplings (0.1- 0.3 Hz) are predicted for JCH, while the corresponding JHH couplings are much smaller. The relative contributions (Fermi-contact, spin-orbit and spin-dipole) are strongly dependent on the geometry of the dimers and on the distance; the non-negligible values of JCH for p systems stem largely from an incomplete cancellation of spin-orbit terms. The results obtained for the larger molecules, i.e. acetonitrile@calix[4]arene 5, the imine 6 and the aryl ester 7 are consistent with those on the model dimers. For 7, the occurrence of a through-space mechanism for the transmission of coupling is established by examining trends in the magnitude of couplings as a function of the number of intervening covalent bonds.