Through-space spin-spin coupling in van der Waals dimers and CH/pi interacting systems. An ab initio and DFT study.

The through-space J(HH) and J(CH) spin-spin coupling constants of model van der Waals dimers (involving methane, ethylene, and benzene), and of selected compounds showing the CH/pi interaction, have been investigated by means of DFT and ab initio calculations. In the range of intermolecular separations for which the interaction is stabilizing, weak couplings (0.1-0.3 Hz) are predicted for J(CH), while the corresponding J(HH) 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 J(CH) for pi systems stem largely from an incomplete cancellation of spin-orbit terms. The results obtained for the larger molecules, that is, 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.