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

Abstract

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/π 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 π 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.